Restoration of Procynosuchus, a member of the cynodont group, which includes the ancestors of mammals
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Evolutionary biology |
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Darwin's finches by John Gould |
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The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes.[1] Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.
After the Cretaceous–Paleogene extinction event wiped out the non-avian dinosaurs (birds being the only surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, by the end of which all modern orders had appeared.
Mammals are the only living synapsids.[2] The synapsid lineage became distinct from the sauropsid lineage in the late Carboniferous period, between 320 and 315 million years ago.[3] The sauropsids are today's reptiles and birds along with all the extinct animals more closely related to them than to mammals.[3] This does not include the mammal-like reptiles, a group more closely related to the mammals.
Throughout the Permian period, the synapsids included the dominant carnivores and several important herbivores. In the subsequent Triassic period, however, a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammaliaforms appeared during this period; their superior sense of smell, backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. The nocturnal lifestyle may have contributed greatly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced rauisuchians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.
Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were very rare and fragmentary; but, since the mid-1990s, there have been many important new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of important divergence points for modern species. When used carefully, these techniques often, but not always, agree with the fossil record.
Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the evolution of mammals centers on the shapes of the teeth, the hardest parts of the tetrapod body. Other important research characteristics include the evolution of the middle ear bones, erect limb posture, a bony secondary palate, fur, hair, and warm-bloodedness.
- 2The ancestry of mammals
- 3Therapsids
- 5From cynodonts to crown mammals
- 6Earliest crown mammals
- 8Evolution of major groups of living mammals
- 9Evolution of mammalian features
- 9.5Warm-bloodedness
Definition of 'mammal'[edit]
Figure 1:In mammals, the quadrate and articular bones are small and part of the middle ear; the lower jaw consists only of dentary bone.
While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
Steam download bytes 2017. One such feature available for paleontology, shared by all living mammals (including monotremes), but not present in any of the early Triassictherapsids, is shown in Figure 1 (on the right), namely: mammals use two bones for hearing that all other amniotes use for eating. The earliest amniotes had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian tetrapods use this system including amphibians, turtles, lizards, snakes, crocodilians, dinosaurs (including the birds), ichthyosaurs, pterosaurs and therapsids. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone, which carries the teeth) and the squamosal (another small skull bone). In the Jurassic, their quadrate and articular bones evolved into the incus and malleus bones in the middle ear.[4][5] Mammals also have a double occipital condyle; they have two knobs at the base of the skull that fit into the topmost neck vertebra, while other tetrapods have a single occipital condyle.[4]
In a 1981 article, Kenneth A. Kermack and his co-authors argued for drawing the line between mammals and earlier synapsids at the point where the mammalian pattern of molarocclusion was being acquired and the dentary-squamosal joint had appeared. The criterion chosen, they noted, is merely a matter of convenience; their choice was based on the fact that 'the lower jaw is the most likely skeletal element of a Mesozoic mammal to be preserved.'[6] Today, most paleontologists consider that animals are mammals if they satisfy this criterion.[7]
The ancestry of mammals[edit]
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Amniotes[edit]
The first fully terrestrial vertebrates were amniotes — their eggs had internal membranes that allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water (a few amphibians, such as the common Suriname toad, have evolved other ways of getting around this limitation). The first amniotes apparently arose in the middle Carboniferous from the ancestral reptiliomorphs.[8]
Within a few million years, two important amniote lineages became distinct: mammals' synapsid ancestors and the sauropsids, from which lizards, snakes, turtles/tortoises, crocodilians, dinosaurs, and birds are descended.[3] The earliest known fossils of synapsids and sauropsids (such as Archaeothyris and Hylonomus, respectively) date from about 320 to 315 million years ago. The times of origin are difficult to know, because vertebrate fossils from the late Carboniferous are very rare, and therefore the actual first occurrences of each of these types of animal might have been considerably earlier than the first fossil.[9]
Synapsids[edit]
The original synapsid skull structure has one hole behind each eye, in a fairly low position on the skull (lower right in this image).
Synapsid skulls are identified by the distinctive pattern of the holes behind each eye, which served the following purposes:
- made the skull lighter without sacrificing strength.
- saved energy by using less bone.
- probably provided attachment points for jaw muscles. Having attachment points further away from the jaw made it possible for the muscles to be longer and therefore to exert a strong pull over a wide range of jaw movement without being stretched or contracted beyond their optimum range.
The synapsid pelycosaurs included the largest land vertebrates of the Early Permian, such as the 6 m (20 ft) long Cotylorhynchus hancocki. Among the other large pelycosaurs were Dimetrodon grandis and Edaphosaurus cruciger.
Therapsids[edit]
Therapsids descended from pelycosaurs in the middle Permian and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including larger temporal fenestrae and incisors that are equal in size.[10]
The therapsid lineage that led to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with some that could easily be mistaken for mammals:[11]
- gradual development of a bony secondary palate. Most books and articles interpret this as a prerequisite for the evolution of mammals' high metabolic rate, because it enabled these animals to eat and breathe at the same time. But some scientists point out that some modern ectotherms use a fleshy secondary palate to separate the mouth from the airway, and that a bony palate provides a surface on which the tongue can manipulate food, facilitating chewing rather than breathing.[12] The interpretation of the bony secondary palate as an aid to chewing also suggests the development of a faster metabolism, because chewing reduces the size of food particles delivered to the stomach and can therefore speed their digestion. In mammals, the palate is formed by two specific bones, but various Permian therapsids had other combinations of bones in the right places to function as a palate.
- the dentary gradually becomes the main bone of the lower jaw.
- progress towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was erratic and very slow — for example: all herbivorous therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact, modern monotremes still have semi-sprawling limbs.
Therapsid family tree[edit]
(simplified from;[10] only those that are most relevant to the evolution of mammals are described below)
Therapsids |
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Only the dicynodonts, therocephalians, and cynodonts survived into the Triassic.
Biarmosuchia[edit]
The Biarmosuchia were the most primitive and pelycosaur-like of the therapsids.[13]
Dinocephalians[edit]
Dinocephalians ('terrible heads') included both carnivores and herbivores. They were large; Anteosaurus was up to 6 m (20 ft) long. Some of the carnivores had semi-erect hindlimbs, but all dinocephalians had sprawling forelimbs. In many ways they were very primitive therapsids; for example, they had no secondary palate and their jaws were rather 'reptilian'.[14]
Anomodonts[edit]
Lystrosaurus, one of the few genera of dicynodonts that survived the Permian–Triassic extinction event
The anomodonts ('anomalous teeth') were among the most successful of the herbivorous therapsids — one sub-group, the dicynodonts, survived almost to the end of the Triassic. But anomodonts were very different from modern herbivorous mammals, as their only teeth were a pair of fangs in the upper jaw and it is generally agreed that they had beaks like those of birds or ceratopsians.[15]
Theriodonts[edit]
The theriodonts ('beast teeth') and their descendants had jaw joints in which the lower jaw's articular bone tightly gripped the skull's very small quadrate bone. This allowed a much wider gape, and one group, the carnivorous gorgonopsians ('gorgon faces'), took advantage of this to develop 'sabre teeth'. But the theriodont's jaw hinge had a longer term significance — the much reduced size of the quadrate bone was an important step in the development of the mammalian jaw joint and middle ear.
The gorgonopsians still had some primitive features: no bony secondary palate (but other bones in the right places to perform the same functions); sprawling forelimbs; hindlimbs that could operate in both sprawling and erect postures. But the therocephalians ('beast heads'), which appear to have arisen at about the same time as the gorgonopsians, had additional mammal-like features, e.g. their finger and toe bones had the same number of phalanges (segments) as in early mammals (and the same number that primates have, including humans).[16]
Cynodonts[edit]
Artist's conception of the cynodont Trirachodon within a burrow
The cynodonts, a theriodont group that also arose in the late Permian, include the ancestors of all mammals. Cynodonts' mammal-like features include further reduction in the number of bones in the lower jaw, a secondary bony palate, cheek teeth with a complex pattern in the crowns, and a brain which filled the endocranial cavity.[17]
Multi-chambered burrows have been found, containing as many as 20 skeletons of the Early Triassic cynodont Trirachodon; the animals are thought to have been drowned by a flash flood. The extensive shared burrows indicate that these animals were capable of complex social behaviors.[18]
Triassic takeover[edit]
The catastrophic mass extinction at the end of the Permian, around 252 million years ago, killed off about 70 percent of terrestrialvertebrate species and the majority of land plants.
As a result,[19]ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, which were dominant predators in the late Permian,[20] the cynodonts' principal competitors for dominance of the carnivorous niches were a previously obscure sauropsid group, the archosaurs, which includes the ancestors of crocodilians and dinosaurs.
The archosaurs quickly became the dominant carnivores,[20] a development often called the 'Triassic takeover'. Their success may have been due to the fact that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea, as mammals do today; considerable water is required to keep urea dissolved.[21]
However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which has been tested) and the fact that desert-dwelling mammals are as well adapted in this department as archosaurs,[22] and some cynodonts like Trucidocynodon were large-sized predators.[23]
The Triassic takeover was probably a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches that had minimal competition from the archosaurs: the tritylodonts, which were herbivores, and the mammals, most of which were small nocturnal insectivores (although some, like Sinoconodon, were carnivores that fed on vertebrate prey, while still others were herbivores or omnivores).[24] As a result:
- The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
- As the body length of the mammals' ancestors fell below 50 mm (2 inches), advances in thermal insulation and temperature regulation would have become necessary for nocturnal life.[25]
- Acute senses of hearing and smell became vital.
- This accelerated the development of the mammalian middle ear.
- The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight.[26] Brain tissue requires a disproportionate amount of energy.[27][28] The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
- Probably as a side-effect of the nocturnal life, mammals lost two of the four cone opsins, photoreceptors in the retina, present in the eyes of the earliest amniotes. Paradoxically, this might have improved their ability to discriminate colors in dim light.[29]
This retreat to a nocturnal role is called a nocturnal bottleneck, and is thought to explain many of the features of mammals.[30]
From cynodonts to crown mammals[edit]
Fossil record[edit]
Mesozoic synapsids that had evolved to the point of having a jaw joint composed of the dentary and squamosal bones are preserved in few good fossils, mainly because they were mostly smaller than rats:
- They were largely restricted to environments that are less likely to provide good fossils. Floodplains as the best terrestrial environments for fossilization provide few mammal fossils, because they are dominated by medium to large animals, and the mammals could not compete with archosaurs in the medium to large size range. Tracks from the Early Cretaceous of Angola show the existence of raccoon-size mammals 118 Million years ago.[31]
- Their delicate bones were vulnerable to being destroyed before they could be fossilized — by scavengers (including fungi and bacteria) and by being trodden on.
- Small fossils are harder to spot and more vulnerable to being destroyed by weathering and other natural stresses before they are discovered.
In the past 50 years, however, the number of Mesozoic fossil mammals has increased decisively; only 116 genera were known in 1979, for example, but about 310 in 2007, with an increase in quality such that 'at least 18 Mesozoic mammals are represented by nearly complete skeletons'.[32]
Mammals or mammaliaforms[edit]
Some writers restrict the term 'mammal' to the crown group mammals, the group consisting of the most recent common ancestor of the monotremes, marsupials, and placentals, together with all the descendants of that ancestor. In an influential 1988 paper, Timothy Rowe advocated this restriction, arguing that 'ancestry.. provides the only means of properly defining taxa' and, in particular, that the divergence of the monotremes from the animals more closely related to marsupials and placentals 'is of central interest to any study of Mammalia as a whole.'[33] To accommodate some related taxa falling outside the crown group, he defined the Mammaliaformes as comprising 'the last common ancestor of Morganucodontidae and Mammalia [as he had defined the latter term] and all its descendants.' Besides Morganucodontidae, the newly defined taxon includes Docodonta and Kuehneotheriidae. Though haramiyids have been referred to the mammals since the 1860s,[34] Rowe excluded them from the Mammaliaformes as falling outside his definition, putting them in a larger clade, the Mammaliamorpha.
Some writers have adopted this terminology noting, to avoid misunderstanding, that they have done so. Most paleontologists, however, still think that animals with the dentary-squamosal jaw joint and the sort of molars characteristic of modern mammals should formally be members of Mammalia.[7]
Where the ambiguity in the term 'mammal' may be confusing, this article uses 'mammaliaform' and 'crown mammal'.
Family tree – cynodonts to crown group mammals[edit]
(based on Cynodontia:Dendrogram – Palaeos)
Cynodontia |
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Morganucodontidae and other transitional forms had both types of jaw joint: dentary-squamosal (front) and articular-quadrate (rear).
Morganucodontidae[edit]
The Morganucodontidae first appeared in the late Triassic, about 205M years ago. They are an excellent example of transitional fossils, since they have both the dentary-squamosal and articular-quadrate jaw joints.[35] They were also one of the first discovered and most thoroughly studied of the mammaliaforms outside of the crown-group mammals, since an unusually large number of morganucodont fossils have been found.
Docodonts[edit]
Reconstruction of Castorocauda. Note the fur and the adaptations for swimming (broad, flat tail; webbed feet) and for digging (robust limbs and claws).
Docodonts, among the most common Jurassic mammaliaforms, are noted for the sophistication of their molars. They are thought to have had general semi-aquatic tendencies, with the fish-eating Castorocauda ('beaver tail'), which lived in the mid-Jurassic about 164M years ago and was first discovered in 2004 and described in 2006, being the most well-understood example. Castorocauda was not a crown group mammal, but it is extremely important in the study of the evolution of mammals because the first find was an almost complete skeleton (a real luxury in paleontology) and it breaks the 'small nocturnal insectivore' stereotype:[36]
- It was noticeably larger than most Mesozoic mammaliaform fossils — about 17 in (43 cm) from its nose to the tip of its 5-inch (130 mm) tail, and may have weighed 500–800 g (18–28 oz).
- It provides the earliest absolutely certain evidence of hair and fur. Previously the earliest was Eomaia, a crown group mammal from about 125M years ago.
- It had aquatic adaptations including flattened tail bones and remnants of soft tissue between the toes of the back feet, suggesting that they were webbed. Previously the earliest known semi-aquatic mammaliaforms were from the Eocene, about 110M years later.
- Castorocauda's powerful forelimbs look adapted for digging. This feature and the spurs on its ankles make it resemble the platypus, which also swims and digs.
- Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey.
Hadrocodium[edit]
The family tree above shows Hadrocodium as an 'aunt' of crown mammals. This mammaliaform, dated about 195M years ago in the very early Jurassic, exhibits some important features:[37]
- The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
- In therapsids and early mammaliaforms the eardrum may have stretched over a trough at the rear of the lower jaw. But Hadrocodium had no such trough, which suggests its ear was part of the cranium, as it is in crown-group mammals — and hence that the former articular and quadrate had migrated to the middle ear and become the malleus and incus. On the other hand, the dentary has a 'bay' at the rear that mammals lack. This suggests that Hadrocodium's dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that Hadrocodium or a very close ancestor may have been the first to have a fully mammalian middle ear.
- Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. Hadrocodium's braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain — in other words it now became possible for mammaliaforms both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.
Earliest crown mammals[edit]
The crown group mammals, sometimes called 'true mammals', are the extant mammals and their relatives back to their last common ancestor. Since this group has living members, DNA analysis can be applied in an attempt to explain the evolution of features that do not appear in fossils. This endeavor often involves molecular phylogenetics, a technique that has become popular since the mid-1980s.
Family tree of early crown mammals[edit]
Cladogram after Z.-X Luo.[32] († marks extinct groups)
Crown groupmammals |
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Color vision[edit]
Early amniotes had four opsins in the cones of their retinas to use for distinguishing colours: one sensitive to red, one to green, and two corresponding to different shades of blue.[38][39] The green opsin was not inherited by any crown mammals, but all normal individuals did inherit the red one. Early crown mammals thus had three cone opsins, the red one and both of the blues.[38] All their extant descendants have lost one of the blue-sensitive opsins but not always the same one: monotremes retain one blue-sensitive opsin, while marsupials and placentals retain the other (except cetaceans, which later lost the other blue opsin as well).[40] Some placentals and marsupials, including humans, subsequently evolved green-sensitive opsins; like early crown mammals, therefore, their vision is trichromatic.[41][42]
Australosphenida and Ausktribosphenidae[edit]
Ausktribosphenidae is a group name that has been given to some rather puzzling finds that:[43]
- appear to have tribosphenic molars, a type of tooth that is otherwise known only in placentals and marsupials.[44]
- come from mid-Cretaceous deposits in Australia — but Australia was connected only to Antarctica, and placentals originated in the Northern Hemisphere and were confined to it until continental drift formed land connections from North America to South America, from Asia to Africa and from Asia to India (the late Cretaceous map here shows how the southern continents are separated).
- are represented only by teeth and jaw fragments, which is not very helpful.
Australosphenida is a group that has been defined in order to include the Ausktribosphenidae and monotremes. Asfaltomylos (mid- to late Jurassic, from Patagonia) has been interpreted as a basal australosphenid (animal that has features shared with both Ausktribosphenidae and monotremes; lacks features that are peculiar to Ausktribosphenidae or monotremes; also lacks features that are absent in Ausktribosphenidae and monotremes) and as showing that australosphenids were widespread throughout Gondwanaland (the old Southern Hemisphere super-continent).[45]
Recent analysis of Teinolophos, which lived somewhere between 121 and 112.5 million years ago, suggests that it was a 'crown group' (advanced and relatively specialised) monotreme. This was taken as evidence that the basal (most primitive) monotremes must have appeared considerably earlier, but this has been disputed (see the following section). The study also indicated that some alleged Australosphenids were also 'crown group' monotremes (e.g. Steropodon) and that other alleged Australosphenids (e.g. Ausktribosphenos, Bishops, Ambondro, Asfaltomylos) are more closely related to and possibly members of the Therian mammals (group that includes marsupials and placentals, see below).[46]
Monotremes[edit]
Teinolophos, from Australia, is the earliest known monotreme. A 2007 study (published 2008) suggests that it was not a basal (primitive, ancestral) monotreme but a full-fledged platypus, and therefore that the platypus and echidna lineages diverged considerably earlier.[46] A more recent study (2009), however, has suggested that, while Teinolophos was a type of platypus, it was also a basal monotreme and predated the radiation of modern monotremes. The semi-aquatic lifestyle of platypuses prevented them from being outcompeted by the marsupials that migrated to Australia millions of years ago, since joeys need to remain attached to their mothers and would drown if their mothers ventured into water (though there are exceptions like the water opossum and the lutrine opossum; however, they both live in South America and thus don't come into contact with monotremes). Genetic evidence has determined that echidnas diverged from the platypus lineage as recently as 19-48M, when they made their transition from semi-aquatic to terrestrial lifestyle.[47]
Monotremes have some features that may be inherited from the cynodont ancestors:
- like lizards and birds, they use the same orifice to urinate, defecate and reproduce ('monotreme' means 'one hole').
- they lay eggs that are leathery and uncalcified, like those of lizards, turtles and crocodilians.
Unlike other mammals, female monotremes do not have nipples and feed their young by 'sweating' milk from patches on their bellies. The sims 4 dine out download.
These features are not visible in fossils, and the main characteristics from paleontologists' point of view are:[43]
- a slender dentary bone in which the coronoid process is small or non-existent.
- the external opening of the ear lies at the posterior base of the jaw.
- the jugal bone is small or non-existent.
- a primitive pectoral girdle with strong ventral elements: coracoids, clavicles and interclavicle. Note: therian mammals have no interclavicle.[48]
- sprawling or semi-sprawling forelimbs.
Multituberculates[edit]
Skull of the multituberculate Ptilodus
Multituberculates (named for the multiple tubercles on their 'molars') are often called the 'rodents of the Mesozoic', but this is an example of convergent evolution rather than meaning that they are closely related to the Rodentia. They existed for approximately 120 million years—the longest fossil history of any mammal lineage—but were eventually outcompeted by rodents, becoming extinct during the early Oligocene.
Some authors have challenged the phylogeny represented by the cladogram above. They exclude the multituberculates from the mammalian crown group, holding that multituberculates are more distantly related to extant mammals than even the Morganucodontidae.[49][50] Multituberculates are like undisputed crown mammals in that their jaw joints consist of only the dentary and squamosal bones-whereas the quadrate and articular bones are part of the middle ear; their teeth are differentiated, occlude, and have mammal-like cusps; they have a zygomatic arch; and the structure of the pelvis suggests that they gave birth to tiny helpless young, like modern marsupials.[51] On the other hand, they differ from modern mammals:
- Their 'molars' have two parallel rows of tubercles, unlike the tribosphenic (three-peaked) molars of uncontested early crown mammals.
- The chewing action differs in that undisputed crown mammals chew with a side-to-side grinding action, which means that the molars usually occlude on only one side at a time, while multituberculates' jaws were incapable of side-to-side movement—they chewed, rather, by dragging the lower teeth backwards against the upper ones as the jaw closed.
- The anterior (forward) part of the zygomatic arch mostly consists of the maxilla (upper jawbone) rather than the jugal, a small bone in a little slot in the maxillary process (extension).
- The squamosal does not form part of the braincase.
- The rostrum (snout) is unlike that of undisputed crown mammals; in fact it looks more like that of a pelycosaur, such as Dimetrodon. The multituberculate rostrum is box-like, with the large flat maxillae forming the sides, the nasal the top, and the tall premaxilla at the front.
Theria[edit]
Therian form of crurotarsal ankle. Adapted with permission from Palaeos
Theria ('beasts') is the clade originating with the last common ancestor of the Eutheria (including placentals) and Metatheria (including marsupials). Common features include:[52]
- no interclavicle.[48]
- coracoid bones non-existent or fused with the shoulder blades to form coracoid processes.
- a type of crurotarsal ankle joint in which: the main joint is between the tibia and astragalus; the calcaneum has no contact with the tibia but forms a heel to which muscles can attach. (The other well-known type of crurotarsal ankle is seen in crocodilians and works differently — most of the bending at the ankle is between the calcaneum and astragalus).
- tribosphenic molars.[44]
Metatheria[edit]
The living Metatheria are all marsupials (animals with pouches). A few fossil genera, such as the Mongolian late Cretaceous Asiatherium, may be marsupials or members of some other metatherian group(s).[53][54]
The oldest known metatherian is Sinodelphys, found in 125M-year-old early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.[55]
Lineage 2 Revolution Online
Didelphimorphia (common opossums of the Western Hemisphere) first appeared in the late Cretaceous and still have living representatives, probably because they are mostly semi-arboreal unspecialized omnivores.[56]
The best-known feature of marsupials is their method of reproduction:
- The mother develops a kind of yolk sack in her womb that delivers nutrients to the embryo. Embryos of bandicoots, koalas and wombats additionally form placenta-like organs that connect them to the uterine wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.[57]
- Pregnancy is very short, typically four to five weeks. The embryo is born at a very early stage of development, and is usually less than 2 in (5.1 cm) long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune system will attack the embryo.
- The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a nipple, which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her mammary glands, as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple was historically thought to have restricted metatherian evolution, as it was assumed that the forelimb couldn't become specialised intro structures like wings, hooves or flippers. However, several bandicoots, most notably the pig-footed bandicoot, have true hooves similar to those of placental ungulates, and several marsupial gliders have evolved.
Skull of thylacine, showing marsupial pattern of molars
Although some marsupials look very like some placentals (the thylacine, 'marsupial tiger' or 'marsupial wolf' is a good example), marsupial skeletons have some features that distinguish them from placentals:[58]
- Some, including the thylacine, have four molars; whereas no known placental has more than three.
- All have a pair of palatal fenestrae, window-like openings on the bottom of the skull (in addition to the smaller nostril openings).
Marsupials also have a pair of marsupial bones (sometimes called 'epipubic bones'), which support the pouch in females. But these are not unique to marsupials, since they have been found in fossils of multituberculates, monotremes, and even eutherians — so they are probably a common ancestral feature that disappeared at some point after the ancestry of living placental mammals diverged from that of marsupials.[59][60]Some researchers think the epipubic bones' original function was to assist locomotion by supporting some of the muscles that pull the thigh forwards.[61]
Eutheria[edit]
The time of appearance of the earliest eutherians has been a matter of controversy. On one hand, recently discovered fossils of Juramaia have been dated to 160 million years ago and classified as eutherian.[62] Fossils of Eomaia from 125 million years ago in the Early Cretaceous have also been classified as eutherian.[63] A recent analysis of phenomic characters, however, classified Eomaia as pre-eutherian and reported that the earliest clearly eutherian specimens came from Maelestes, dated to 91 million years ago.[64] That study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the Cretaceous–Paleogene boundary, about 66 million years ago.
Eomaia was found to have some features that are more like those of marsupials and earlier metatherians:
Fossil of Eomaia in the Hong Kong Science Museum.
- Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals — early mammaliaforms, non-placental eutherians, marsupials, and monotremes — as well as in the cynodonttherapsids that are closest to mammals. Their function is to stiffen the body during locomotion.[65] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[66]
- A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
- Five incisors in each side of the upper jaw. This number is typical of metatherians, and the maximum number in modern placentals is three, except for homodonts, such as the armadillo. But Eomaia's molar to premolar ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials.
Eomaia also has a Meckelian groove, a primitive feature of the lower jaw that is not found in modern placental mammals.
These intermediate features are consistent with molecular phylogenetics estimates that the placentals diversified about 110M years ago, 15M years after the date of the Eomaia fossil.
Eomaia also has many features that strongly suggest it was a climber, including several features of the feet and toes; well-developed attachment points for muscles that are used a lot in climbing; and a tail that is twice as long as the rest of the spine.
Placentals' best-known feature is their method of reproduction:
- The embryo attaches itself to the uterus via a large placenta via which the mother supplies food and oxygen and removes waste products.
- Pregnancy is relatively long and the young are fairly well-developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.
It has been suggested that the evolution of placental reproduction was made possible by retroviruses that:[67]
- make the interface between the placenta and uterus into a syncytium, i.e. a thin layer of cells with a shared external membrane. This allows the passage of oxygen, nutrients and waste products, but prevents the passage of blood and other cells that would cause the mother's immune system to attack the fetus.
- reduce the aggressiveness of the mother's immune system, which is good for the foetus but makes the mother more vulnerable to infections.
From a paleontologist's point of view, eutherians are mainly distinguished by various features of their teeth,[68] ankles and feet.[69]
Expansion of ecological niches in the Mesozoic[edit]
Restoration of Volaticotherium, a Middle Jurassic eutriconodont and the earliest known gliding mammal.
Skull cast of Late Cretaceous Didelphodon, showing its robust teeth adapted to a durophagous diet.
There is still some truth in the 'small, nocturnal insectivores' stereotype, but recent finds, mainly in China, show that some mammaliaforms and crown group mammals were larger and had a variety of lifestyles. For example:
- Castorocauda, a member of Docodonta which lived in the middle Jurassic about 164 million years, was about 42.5 cm (16.7 in) long, weighed 500–800 g (18–28 oz), had a beaver-like tail that was adapted for swimming, limbs adapted for swimming and digging, and teeth adapted for eating fish.[36] Another docodont, Haldanodon, also had semi-aquatic habits, and indeed aquatic tendencies were probably common among docodonts based on their prevalence in wetland environments.[70] The eutriconodontsLiaoconodon and Yanoconodon have more recently also have been suggested to be freshwater swimmers, lacking Castorocauda's powerful tail but possessing paddle-like limbs;[71] the eutriconodont Astroconodon has similarly been suggested as being semi-aquatic in the past, albeit to less convincing evidence.
- Multituberculates are allotherians that survived for over 125 million years (from mid-Jurassic, about 160M years ago, to late Eocene, about 35M years ago) are often called the 'rodents of the Mesozoic'. As noted above, they may have given birth to tiny live neonates rather than laying eggs.
- Fruitafossor, from the late Jurassic period about 150 million years ago, was about the size of a chipmunk and its teeth, forelimbs and back suggest that it broke open the nest of social insects to prey on them (probably termites, as ants had not yet appeared).[72]
- Similarly, the gobiconodontidSpinolestes possessed adaptations for fossoriality and convergent traits with placental xenarthrans like scutes and xenarthrous vertebrae, so it too might have had anteater like habits. It is also notable for the presence of quills akin to those of modern spiny mice.
- Volaticotherium, from the boundary the early Cretaceous about 125M years ago, is the earliest-known gliding mammal and had a gliding membrane that stretched out between its limbs, rather like that of a modern flying squirrel. This also suggests it was active mainly during the day.[73] The closely related Argentoconodon also shows similar adaptations that may also suggest aerial locomotion.[74]
- Repenomamus, a eutriconodont from the early Cretaceous 130 million years ago, was a stocky, badger-like predator that sometimes preyed on young dinosaurs. Two species have been recognized, one more than 1 m (39 in) long and weighing about 12–14 kg (26–31 lb), the other less than 0.5 m (20 in) long and weighing 4–6 kg (8.8–13.2 lb).[75][76]
- Schowalteria is a Late Cretaceous species almost as large if not larger than R. giganticus that shows speciations towards herbivory, comparable to those of modern ungulates.
- Zhelestidae is a lineage of Late Cretaceous herbivorous eutherians, to the point of being mistaken for stem-ungulates.[77]
- Similarly, mesungulatids are also fairly large sized herbivorous mammals from the Late Cretaceous
- Deltatheroidans were metatherians that were specialised towards carnivorous habits,[78][79] and possible forms like Oxlestes and Khudulestes might have been among the largest Mesozoic mammals, though their status as deltatheroidans is questionable.
- Ichthyoconodon, a eutriconodont from the Berriasian of Morocco, is currently known from molariforms found in marine deposits. These teeth are sharp-cusped and similar in shape to those of piscivorous mammals, and unlike the teeth of contemporary mammals they do not show degradation, so rather than being carried down by river deposits the animal died in situ or close. This has been taken to mean that it was a marine mammal, likely one of the few examples known from the Mesozoic.[80] Alternatively, its close relations to Volaticotherium and Argentoconodon might suggest that it was a flying mammal.[74]
- Didelphodon is a Late Cretaceous riverine species of stagodontidmarsupialiform with a durophagous dentition, robust jaws similar to a modern Tasmanian devil, and a postcranial skeleton very similar in size and shape to an otter. This animal has been lauded as the strongest bite of all Mesozoic mammals. It possibly specialized on eating freshwater crabs and molluscs.
- Tracks of a raccoon-sized mammaliaform representing the morphofamily Ameghinichnidae are described from the Early Cretaceous (late Aptian) Calonda Formation (Angola) by Mateuset al. (2017), who name a new ichnotaxon Catocapes angolanus.[81]
Evolution of major groups of living mammals[edit]
There are currently vigorous debates between traditional paleontologists and molecular phylogeneticists about how and when the modern groups of mammals diversified, especially the placentals. Generally, the traditional paleontologists date the appearance of a particular group by the earliest known fossil whose features make it likely to be a member of that group, while the molecular phylogeneticists suggest that each lineage diverged earlier (usually in the Cretaceous) and that the earliest members of each group were anatomically very similar to early members of other groups and differed only in their genetics. These debates extend to the definition of and relationships between the major groups of placentals — the controversy about Afrotheria is a good example.
Fossil-based family tree of placental mammals[edit]
Here is a very simplified version of a typical family tree based on fossils, based on Cladogram of Mammalia – Palaeos. It tries to show the nearest thing there is at present to a consensus view, but some paleontologists have very different views, for example:[82]
- The most common view is that placentals originated in the Southern Hemisphere, but some paleontologists argue that they first appeared in Laurasia (old supercontinent containing modern Asia, N. America and Europe).
- Paleontologists differ as to when the first placentals appeared, with estimates ranging from 20M years before the end of the Cretaceous to just after the end of the Cretaceous. Molecular biologists argue for a much earlier origin, even suggesting appearance in the Middle Jurassic.[83]
- Molecular data suggest that either Xenarthra, Afrotheria, or Atlantogenata (Xenarthra + Afrotheria), was the earliest-diverging group from the rest of the placental mammals.[84][85][86]
For the sake of brevity and simplicity, the diagram omits some extinct groups in order to focus on the ancestry of well-known modern groups of placentals — † marks extinct groups. The diagram also shows the following:
- the age of the oldest known fossils in many groups, since one of the major debates between traditional paleontologists and molecular phylogeneticists is about when various groups first became distinct.
- well-known modern members of most groups.
Eutheria |
|
This family tree contains some surprises and puzzles. For example:
- The closest living relatives of cetaceans (whales, dolphins, porpoises) are artiodactyls, hoofed animals, which are almost all pure herbivores.
- Bats are fairly close relatives of primates.
- The closest living relatives of elephants are the aquatic sirenians, while their next relatives are hyraxes, which look more like well-fed guinea pigs.
- There is little correspondence between the structure of the family (what was descended from what) and the dates of the earliest fossils of each group. For example, the earliest fossils of perissodactyls (the living members of which are horses, rhinos and tapirs) date from the late Paleocene, but the earliest fossils of their 'sister group', the Tubulidentata, date from the early Miocene, nearly 50M years later. Paleontologists are fairly confident about the family relationships, which are based on cladistic analyses, and believe that fossils of the ancestors of modern aardvarks have simply not been found yet.
Molecular phylogenetics based family tree of placental mammals[edit]
Molecular phylogenetics uses features of organisms' genes to work out family trees in much the same way as paleontologists do with features of fossils — if two organisms' genes are more similar to each other than to those of a third organism, the two organisms are more closely related to each other than to the third.
Molecular phylogeneticists have proposed a family tree that is very different from the one with which paleontologists are familiar. Like paleontologists, molecular phylogeneticists have different ideas about various details, but here is a typical family tree according to molecular phylogenetics:[87][88] Note that the diagram shown here omits extinct groups, as one cannot extract DNA from fossils.
Eutheria |
|
Here are the most significant of the many differences between this family tree and the one familiar to paleontologists:
- The top-level division is between Atlantogenata and Boreoeutheria, instead of between Xenarthra and the rest. However, analysis of transposable element insertions supports a three-way top-level split between Xenarthra, Afrotheria and Boreoeutheria [89][90] and the Atlantogenata clade does not receive significant support in recent distance-based molecular phylogenetics.[85]
- Afrotheria contains several groups that are only distantly related according to the paleontologists' version: Afroinsectiphilia ('African insectivores'), Tubulidentata (aardvarks, which paleontologists regard as much closer to odd-toed ungulates than to other members of Afrotheria), Macroscelidea (elephant shrews, usually regarded as close to rabbits and rodents). The only members of Afrotheria that paleontologists would regard as closely related are Hyracoidea (hyraxes), Proboscidea (elephants) and Sirenia (manatees, dugongs).
- Insectivores are split into three groups: one is part of Afrotheria and the other two are distinct sub-groups within Boreoeutheria.
- Bats are closer to Carnivora and odd-toed ungulates than to Primates and Dermoptera (colugos).
- Perissodactyla (odd-toed ungulates) are closer to Carnivora and bats than to Artiodactyla (even-toed ungulates).
The grouping together of the Afrotheria has some geological justification. All surviving members of the Afrotheria originate from South American or (mainly) African lineages — even the Indian elephant, which diverged from an African lineage about 7.6 million years ago.[91] As Pangaea broke up, Africa and South America separated from the other continents less than 150M years ago, and from each other between 100M and 80M years ago.[92][93] So it would not be surprising if the earliest eutherian immigrants into Africa and South America were isolated there and radiated into all the available ecological niches.
Nevertheless, these proposals have been controversial. Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly:[94]
- Mitochondrial DNA's mutation rate in mammals varies from region to region — some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species.[95][96]
- Mammalian mitochondrial DNA mutates so fast that it causes a problem called 'saturation', where random noise drowns out any information that may be present. If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged.[97]
Timing of placental evolution[edit]
Recent molecular phylogenetic studies suggest that most placental orders diverged late in the Cretaceous period, about 100 to 85 million years ago, but that modern families first appeared later, in the late Eocene and early Miocene epochs of the Cenozoic period.[98][99] Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic.[100] Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock runs fast during major evolutionary radiations.[101] On the other hand, there is fossil evidence from 85 million years ago of hoofed mammals that may be ancestors of modern ungulates.[102]
Fossils of the earliest members of most modern groups date from the Paleocene, a few date from later and very few from the Cretaceous, before the extinction of the dinosaurs. But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolatebackwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous.[103] However, statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene.[104][105]
Evolution of mammalian features[edit]
Jaws and middle ears[edit]
Hadrocodium, whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints and middle ears, in which the jaw joint is formed by the dentary and squamosal bones while the articular and quadrate move to the middle ear, where they are known as the incus and malleus.
One analysis of the monotreme Teinolophos suggested that this animal had a pre-mammalian jaw joint formed by the angular and quadrate bones and that the definitive mammalian middle ear evolved twice independently, in monotremes and in therian mammals, but this idea has been disputed.[106] In fact, two of the suggestion's authors co-authored a later paper that reinterpreted the same features as evidence that Teinolophos was a full-fledged platypus, which means it would have had a mammalian jaw joint and middle ear.[46]
Lactation[edit]
It has been suggested that lactation's original function was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):[107][108][109]
- While the amniote egg is usually described as able to evolve away from water, most reptile eggs actually need moisture if they are not to dry out.
- Monotremes do not have nipples, but secrete milk from a hairy patch on their bellies.
- During incubation, monotreme eggs are covered in a sticky substance whose origin is not known. Before the eggs are laid, their shells have only three layers. Afterwards, a fourth layer appears with a composition different from that of the original three. The sticky substance and the fourth layer may be produced by the mammary glands.
- If so, that may explain why the patches from which monotremes secrete milk are hairy. It is easier to spread moisture and other substances over the egg from a broad, hairy area than from a small, bare nipple.
Later research demonstrated that caseins already appeared in the common mammalian ancestor approximately 200–310 million years ago.[110] The question of whether secretion of a substance to keep egg moist translated into actual lactation in therapsids is open. A small mammaliomorph called Sinocodon, generally assumed to be the sister group of all later mammals, had front teeth in even the smallest individuals. Combined with a poorly ossified jaw, they very probably did not suckle.[111] Thus suckling may have evolved right at the pre-mammal/mammal transition. However, tritylodontids, generally assumed to be more basal, show evidence of suckling.[112]Morganucodontans, also assumed to be basal Mammaliaformes, also show evidence of lactation.[113]
Hair and fur[edit]
The first clear evidence of hair or fur is in fossils of Castorocauda and Megaconus, from 164M years ago in the mid-Jurassic.[36] As both mammals Megaconus and Castorocauda have a double coat of hair, with both guard hairs and an undercoat, it may be assumed that their last common ancestor did as well. This animal must have been Triassic as it was an ancestor of the Triassic Tikitherium.[32] More recently, the discovery of hair remnants in Permian coprolites pushes back the origin of mammalian hair much further back in the synapsid line to Paleozoic therapsids.[114]
In the mid-1950s, some scientists interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels that supplied blood vessels and nerves to vibrissae (whiskers) and suggested that this was evidence of hair or fur.[115][116] It was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae; the modern lizard Tupinambis has foramina that are almost identical to those found in the non-mammalian cynodont Thrinaxodon.[12][117] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.[118] A trace fossil from the Lower Triassic had been erroneously regarded as a cynodont footprint showing hair,[119] but this interpretation has been refuted.[120] A study of cranial openings for facial nerves connected whiskers in extant mammals indicate the Prozostrodontia, small immediate ancestors of mammals, presented whiskers similar to mammals, but that less advanced therapsids would either have immobile whiskers or no whisker at all.[121] Fur may have evolved from whiskers.[122] Whiskers themselves may have evolved as a response to nocturnal and/or burrowing lifestyle.
Ruben & Jones (2000) note that the Harderian glands, which secrete lipids for coating the fur, were present in the earliest mammals like Morganucodon, but were absent in near-mammalian therapsids like Thrinaxodon.[123] The Msx2 gene associated with hair follicle maintenance is also linked to the closure of the parietal eye in mammals, indicating that fur and lack of pineal eye is linked. The pineal eye is present in Thrinaxodon, but absent in more advanced cynognaths (the Probainognathia).[121]
Insulation is the 'cheapest' way to maintain a fairly constant body temperature, without consuming energy to produce more body heat. Therefore, the possession of hair or fur would be good evidence of homeothermy, but would not be such strong evidence of a high metabolic rate.[124][125]
Erect limbs[edit]
Understanding of the evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. Some scientists think that the parasagittal (non-sprawling) limb posture is limited to the Boreosphenida, a group that contains the therians but not, for example, the multituberculates. In particular, they attribute a parasagittal stance to the therians Sinodelphys and Eomaia, which means that the stance had arisen by 125 million years ago, in the Early Cretaceous. However, they also discuss that earlier mammals had more erect forelimbs as opposed to the more sprawling hindlimbs, a trend still continued to some extent in modern placentals and marsupials.[126]
Warm-bloodedness[edit]
'Warm-bloodedness' is a complex and rather ambiguous term, because it includes some or all of the following:
- Endothermy, the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
- Homeothermy, maintaining a fairly constant body temperature. Most enzymes have an optimum operating temperature; efficiency drops rapidly outside the preferred range. A homeothermic organism needs only to possess enzymes that function well in a small range of temperatures.
- Tachymetabolism, maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature because of the Q10 effect: biochemical processes run about half as fast if an animal's temperature drops by 10 °C.
Since scientists cannot know much about the internal mechanisms of extinct creatures, most discussion focuses on homeothermy and tachymetabolism. However, it is generally agreed that endothermy first evolved in non-mammalian synapsids such as dicynodonts, which possess body proportions associated with heat retention,[127] high vascularised bones with Haversian canals,[128] and possibly hair.[129] More recently, it has been suggested that endothermy evolved as far back as Ophiacodon.[130]
Modern monotremes have a low body temperature compared to marsupials and placental mammals, around 32 °C (90 °F).[131]Phylogenetic bracketing suggests that the body temperatures of early crown-group mammals were not less than that of extant monotremes. There is cytological evidence that the low metabolism of monotremes is a secondarily evolved trait.[132]
Respiratory turbinates[edit]
Modern mammals have respiratory turbinates, convoluted structures of thin bone in the nasal cavity. These are lined with mucous membranes that warm and moisten inhaled air and extract heat and moisture from exhaled air. An animal with respiratory turbinates can maintain a high rate of breathing without the danger of drying its lungs out, and therefore may have a fast metabolism. Unfortunately these bones are very delicate and therefore have not yet been found in fossils. But rudimentary ridges like those that support respiratory turbinates have been found in advanced Triassic cynodonts, such as Thrinaxodon and Diademodon, which suggests that they may have had fairly high metabolic rates.[115][133][134]
Bony secondary palate[edit]
Mammals have a secondary bony palate, which separates the respiratory passage from the mouth, allowing them to eat and breathe at the same time. Secondary bony palates have been found in the more advanced cynodonts and have been used as evidence of high metabolic rates.[115][116][135] But some cold-blooded vertebrates have secondary bony palates (crocodilians and some lizards), while birds, which are warm-blooded, do not.[12]
Diaphragm[edit]
A muscular diaphragm helps mammals to breathe, especially during strenuous activity. For a diaphragm to work, the ribs must not restrict the abdomen, so that expansion of the chest can be compensated for by reduction in the volume of the abdomen and vice versa. Diaphragms are known in caseid pelycosaurs, indicating an early origin within synapsids, though they were still fairly inefficient and likely required support from other muscle groups and limb motion.[136]
The advanced cynodonts have very mammal-like rib cages, with greatly reduced lumbar ribs. This suggests that these animals had more developed diaphragms, were capable of strenuous activity for fairly long periods and therefore had high metabolic rates.[115][116] On the other hand, these mammal-like rib cages may have evolved to increase agility.[12] However, the movement of even advanced therapsids was 'like a wheelbarrow', with the hindlimbs providing all the thrust while the forelimbs only steered the animal, in other words advanced therapsids were not as agile as either modern mammals or the early dinosaurs.[137] So the idea that the main function of these mammal-like rib cages was to increase agility is doubtful.
Limb posture[edit]
The therapsids had sprawling forelimbs and semi-erect hindlimbs.[116][138] This suggests that Carrier's constraint would have made it rather difficult for them to move and breathe at the same time, but not as difficult as it is for animals such as lizards, which have completely sprawling limbs.[139] Advanced therapsids may therefore have been significantly less active than modern mammals of similar size and so may have had slower metabolisms overall or else been bradymetabolic (lower metabolism when at rest).
Brain[edit]
Mammals are noted for their large brain size relative to body size, compared to other animal groups. Recent findings suggest that the first brain area to expand was that involved in smell.[140] Scientists scanned the skulls of early mammal species dating back to 190–200 million years ago and compared the brain case shapes to earlier pre-mammal species; they found that the brain area involved in the sense of smell was the first to enlarge.[140] This change may have allowed these early mammals to hunt insects at night when dinosaurs were not active.[140]
See also[edit]
- Evolution of ungulates
Notes[edit]
- ^Rougier, G. W.; Martinelli, A. G.; Forasiepi, A. M.; Novacek, M. J. (2007). 'New Jurassic mammals from Patagonia, Argentina: A reappraisal of australosphenidan morphology and interrelationships'(PDF). American Museum Novitates. 3566 (1): 1–54. doi:10.1206/0003-0082(2007)507[1:NJMFPA]2.0.CO;2. ISSN0003-0082.
- ^Ben Waggoner (February 2, 1997). 'Introduction to the Synapsida'. University of California Museum of Paleontology. Retrieved April 28, 2012.
- ^ abcWhite, A. T. (May 18, 2005). 'Amniota – Palaeos'. Archived from the original on December 20, 2010. Retrieved January 23, 2012.
- ^ abMammalia: Overview – PalaeosArchived June 15, 2008, at the Wayback Machine
- ^Cowen, R. (2000). History of Life. Oxford: Blackwell Science. p. 432. ISBN978-0-7266-0287-0.
- ^K. A. Kermack; Frances Mussett; H. W. RIgney (January 1981). 'The skull of Morganucodon'. Zoological Journal of the Linnean Society. 71 (1): 148. doi:10.1111/j.1096-3642.1981.tb01127.x.
- ^ abKemp, T. S. (2005). The Origin and Evolution of Mammals. Oxford University Press. p. 3. ISBN978-0-19-850760-4.
- ^Carroll R.L. (1991): The origin of reptiles. In: Schultze H.-P., Trueb L., (ed) Origins of the higher groups of tetrapods — controversy and consensus. Ithaca: Cornell University Press, pp 331-353.
- ^'Synapsida: Varanopseidae – Palaeos'. Retrieved 15 October 2013.
- ^ ab'Therapsida – Palaeos'. Archived from the original on 2007-04-15.
- ^Kermack, D.M.; Kermack, K.A. (1984). The evolution of mammalian characters. Croom Helm. ISBN978-0709915348.
- ^ abcdBennett, A.F.; Ruben, J.A. (1986). 'The metabolic and thermoregulatory status of therapsids'. In Hotton III, N; MacLean, P.D.; Roth, J.J.; et al. (eds.). The ecology and biology of mammal-like reptiles. Washington: Smithsonian Institution Press, Washington. pp. 207–218.
- ^'Therapsida: Biarmosuchia – Palaeos'. Retrieved 16 October 2013.
- ^'Dinocephalia – Palaeos'.
- ^'Ammodontia – Palaeos'. Retrieved 16 October 2013.
- ^'Theriodontia – Paleos'. Retrieved 2013-10-15.
- ^'Cynodontia Overview – Palaeos'.
- ^Groenewald, G.H.; Welman, J.; MacEachern, J.A. (April 2001). 'Vertebrate Burrow Complexes from the Early Triassic Cynognathus Zone (Driekoppen Formation, Beaufort Group) of the Karoo Basin, South Africa'. PALAIOS. 16 (2): 148–160. doi:10.1669/0883-1351(2001)016<0148:VBCFTE>2.0.CO;2. ISSN0883-1351. Retrieved 2008-07-07.
- ^'Olenekian Age of the Triassic – Palaeos'.
- ^ abBenton, M.J. (2004). Vertebrate Palaeontology (3rd ed.). Oxford: Blackwell Science. ISBN978-0-632-05637-8
- ^Campbell, J.W. (1979). C.L. Prosser (ed.). Comparative Animal Physiology (3rd ed.). W. B. Sauders. pp. 279–316.
- ^Darren Naish, Episode 38: A Not Too Shabby Podcarts
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References[edit]
- Robert L. Carroll, Vertebrate Paleontology and Evolution, W. H. Freeman and Company, New York, 1988 ISBN0-7167-1822-7. Chapters XVII through XXI
- Nicholas Hotton III, Paul D. MacLean, Jan J. Roth, and E. Carol Roth, editors, The Ecology and Biology of Mammal-like Reptiles, Smithsonian Institution Press, Washington and London, 1986 ISBN0-87474-524-1
- T. S. Kemp, The Origin and Evolution of Mammals, Oxford University Press, New York, 2005 ISBN0-19-850760-7
- Zofia Kielan-Jaworowska, Richard L. Cifelli, and Zhe-Xi Luo, Mammals from the Age of Dinosaurs: Origins, Evolution, and Structure, Columbia University Press, New York, 2004 ISBN0-231-11918-6. Comprehensive coverage from the first mammals up to the time of the Cretaceous–Paleogene extinction event.
- Luo, Zhe-Xi (13 December 2007). 'Transformation and diversification in early mammal evolution'(PDF). Nature. 450 (7172): 1011–1019. Bibcode:2007Natur.450.1011L. doi:10.1038/nature06277. PMID18075580. Archived from the original(PDF) on 2012-11-24. A survey article with 98 references to the scientific literature.
External links[edit]
- The Cynodontia covers several aspects of the evolution of cynodonts into mammals, with plenty of references.
- Mammals, BBC Radio 4 discussion with Richard Corfield, Steve Jones & Jane Francis (In Our Time, Oct. 13, 2005)
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Evolution_of_mammals&oldid=899133058'
The 'Paleontological Tree of the Vertebrates,' from the 5th edition of The Evolution of Man (London, 1910) by Ernst Haeckel. The evolutionary history of species has been described as a tree, with many branches arising from a single trunk.
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Evolution is the process of change in all forms of life over generations, and evolutionary biology is the study of how evolution occurs. Biological populations evolve through genetic changes that correspond to changes in the organisms' observable traits. Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA. As the genetic variation of a population drifts randomly over generations, natural selection gradually leads traits to become more or less common based on the relative reproductive success of organisms with those traits.
The age of the Earth is about 4.54 billion years.[1][2][3] The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago.[4][5][6] Evolution does not attempt to explain the origin of life (covered instead by abiogenesis), but it does explain how early lifeforms evolved into the complex ecosystem that we see today.[7] Based on the similarities between all present-day organisms, all life on Earth is assumed to have originated through common descent from a last universal ancestor from which all known species have diverged through the process of evolution.[8]
All individuals have hereditary material in the form of genes received from their parents, which they pass on to any offspring. Among offspring there are variations of genes due to the introduction of new genes via random changes called mutations or via reshuffling of existing genes during sexual reproduction.[9][10] The offspring differs from the parent in minor random ways. If those differences are helpful, the offspring is more likely to survive and reproduce. This means that more offspring in the next generation will have that helpful difference and individuals will not have equal chances of reproductive success. In this way, traits that result in organisms being better adapted to their living conditions become more common in descendant populations.[9][10] These differences accumulate resulting in changes within the population. This process is responsible for the many diverse life forms in the world.
The modern understanding of evolution began with the 1859 publication of Charles Darwin's On the Origin of Species. In addition, Gregor Mendel's work with plants helped to explain the hereditary patterns of genetics.[11] Fossil discoveries in paleontology, advances in population genetics and a global network of scientific research have provided further details into the mechanisms of evolution. Scientists now have a good understanding of the origin of new species (speciation) and have observed the speciation process in the laboratory and in the wild. Evolution is the principal scientific theory that biologists use to understand life and is used in many disciplines, including medicine, psychology, conservation biology, anthropology, forensics, agriculture and other social-cultural applications.
- 4Genetic drift
- 6Evidence for evolution
- 6.2Comparative anatomy
- 9Mechanism
Simple overview[edit]
The main ideas of evolution may be summarized as follows:
- Life forms reproduce and therefore have a tendency to become more numerous.
- Factors such as predation and competition work against the survival of individuals.
- Each offspring differs from their parent(s) in minor, random ways.
- If these differences are beneficial, the offspring is more likely to survive and reproduce.
- This makes it likely that more offspring in the next generation will have beneficial differences and fewer will have detrimental differences.
- These differences accumulate over generations, resulting in changes within the population.
- Over time, populations can split or branch off into new species.
- These processes, collectively known as evolution, are responsible for the many diverse life forms seen in the world.
Natural selection[edit]
Charles Darwin proposed the theory of evolution by natural selection.
In the 19th century, natural history collections and museums were popular. The European expansion and naval expeditions employed naturalists, while curators of grand museums showcased preserved and live specimens of the varieties of life. Charles Darwin was an English graduate educated and trained in the disciplines of natural history. Such natural historians would collect, catalogue, describe and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board HMS Beagle, assigned to a five-year research expedition around the world. During his voyage, he observed and collected an abundance of organisms, being very interested in the diverse forms of life along the coasts of South America and the neighboring Galápagos Islands.[12][13]
Darwin noted that orchids have complex adaptations to ensure pollination, all derived from basic floral parts.
Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places. Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In 1838, he described how a process he called natural selection would make this happen.[14]
The size of a population depends on how much and how many resources are able to support it. For the population to remain the same size year after year, there must be an equilibrium, or balance between the population size and available resources. Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation. There must be a competitive struggle for resources that aid in survival. As a result, Darwin realised that it was not chance alone that determined survival. Instead, survival of an organism depends on the differences of each individual organism, or 'traits,' that aid or hinder survival and reproduction. Well-adapted individuals are likely to leave more offspring than their less well-adapted competitors. Traits that hinder survival and reproduction would disappear over generations. Traits that help an organism survive and reproduce would accumulate over generations. Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term natural selection to describe this process.[15][16]
Observations of variations in animals and plants formed the basis of the theory of natural selection. For example, Darwin observed that orchids and insects have a close relationship that allows the pollination of the plants. He noted that orchids have a variety of structures that attract insects, so that pollen from the flowers gets stuck to the insects' bodies. In this way, insects transport the pollen from a male to a female orchid. In spite of the elaborate appearance of orchids, these specialised parts are made from the same basic structures that make up other flowers. In his book, Fertilisation of Orchids (1862), Darwin proposed that the orchid flowers were adapted from pre-existing parts, through natural selection.[17]
Darwin was still researching and experimenting with his ideas on natural selection when he received a letter from Alfred Russel Wallace describing a theory very similar to his own. This led to an immediate joint publication of both theories. Both Wallace and Darwin saw the history of life like a family tree, with each fork in the tree’s limbs being a common ancestor. The tips of the limbs represented modern species and the branches represented the common ancestors that are shared amongst many different species. To explain these relationships, Darwin said that all living things were related, and this meant that all life must be descended from a few forms, or even from a single common ancestor. He called this process descent with modification.[16]
Darwin published his theory of evolution by natural selection in On the Origin of Species in 1859.[18] His theory means that all life, including humanity, is a product of continuing natural processes. The implication that all life on Earth has a common ancestor has met with objections from some religious groups. Their objections are in contrast to the level of support for the theory by more than 99 percent of those within the scientific community today.[19]
![Game Game](/uploads/1/2/6/6/126634653/266147935.jpg)
How to download from rhapsody. Natural selection is commonly equated with survival of the fittest, but this expression originated in Herbert Spencer's Principles of Biology in 1864, five years after Charles Darwin published his original works. Survival of the fittest describes the process of natural selection incorrectly, because natural selection is not only about survival and it is not always the fittest that survives.[20]
Source of variation[edit]
Darwin's theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that the organisms in populations varied from each other, that some of these variations were inherited, and that these differences could be acted on by natural selection. However, he could not explain the source of these variations. Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that features acquired during an organism's lifetime could be passed on to its offspring. He looked for examples, such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all.[21] This misunderstanding was called the inheritance of acquired characters and was part of the theory of transmutation of species put forward in 1809 by Jean-Baptiste Lamarck. In the late 19th century this theory became known as Lamarckism. Darwin produced an unsuccessful theory he called pangenesis to try to explain how acquired characteristics could be inherited. In the 1880s August Weismann's experiments indicated that changes from use and disuse could not be inherited, and Lamarckism gradually fell from favor.[22]
The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of Gregor Mendel. Mendel's experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilisation. This is like mixing different hands of playing cards, with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information factors; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the information that directs the physical development and behavior of organisms.
Genes are made of DNA. DNA is a long molecule made up of individual molecules called nucleotides. Genetic information is encoded in the sequence of nucleotides, that make up the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the 'letters' of the DNA alphabet. Put together, the entire set of these genes gives enough information to serve as an 'instruction manual' of how to build and run an organism. The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the cell, the genes are carried in chromosomes, which are packages for carrying the DNA. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring. Since genes interact with one another during the development of an organism, novel combinations of genes produced by sexual reproduction can increase the genetic variability of the population even without new mutations.[23] The genetic variability of a population can also increase when members of that population interbreed with individuals from a different population causing gene flow between the populations. This can introduce genes into a population that were not present before.[24]
Evolution is not a random process. Although mutations in DNA are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment. The outcome of evolution is not a perfectly designed organism. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve progress towards an ultimate goal. Evolution does not strive for more advanced, more intelligent, or more sophisticated life forms.[25] For example, fleas (wingless parasites) are descended from a winged, ancestral scorpionfly, and snakes are lizards that no longer require limbs—although pythons still grow tiny structures that are the remains of their ancestor's hind legs.[26][27] Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time.
Rapid environmental changes typically cause extinctions.[28] Of all species that have existed on Earth, 99.9 percent are now extinct.[29] Since life began on Earth, five major mass extinctions have led to large and sudden drops in the variety of species. The most recent, the Cretaceous–Paleogene extinction event, occurred 66 million years ago.[30]
Genetic drift[edit]
Genetic drift is a cause of allelic frequency change within populations of a species. Alleles are different variations of specific genes. They determine things like hair color, skin tone, eye color and blood type; in other words, all the genetic traits that vary between individuals. Genetic drift does not introduce new alleles to a population, but it can reduce variation within a population by removing an allele from the gene pool. Genetic drift is caused by random sampling of alleles. A truly random sample is a sample in which no outside forces affect what is selected. It is like pulling marbles of the same size and weight but of different colors from a brown paper bag. In any offspring, the alleles present are samples of the previous generations alleles, and chance plays a role in whether an individual survives to reproduce and to pass a sample of their generation onward to the next. The allelic frequency of a population is the ratio of the copies of one specific allele that share the same form compared to the number of all forms of the allele present in the population.[31]
Genetic drift affects smaller populations more than it affects larger populations.[32]
Hardy–Weinberg principle[edit]
The Hardy–Weinberg principle states that under certain idealized conditions, including the absence of selection pressures, a large population will have no change in the frequency of alleles as generations pass.[33] A population that satisfies these conditions is said to be in Hardy–Weinberg equilibrium. In particular, Hardy and Weinberg showed that dominant and recessive alleles do not automatically tend to become more and less frequent respectively, as had been thought previously.
The conditions for Hardy-Weinberg equilibrium include that there must be no mutations, immigration, or emigration, all of which can directly change allelic frequencies. Additionally, mating must be totally random, with all males (or females in some cases) being equally desirable mates. This ensures a true random mixing of alleles.[34] A population that is in Hardy–Weinberg equilibrium is analogous to a deck of cards; no matter how many times the deck is shuffled, no new cards are added and no old ones are taken away. Cards in the deck represent alleles in a population’s gene pool.
In practice, no population can be in perfect Hardy-Weinberg equilibrium. The population's finite size, combined with natural selection and many other effects, cause the allelic frequencies to change over time.
Population bottleneck[edit]
Model of population bottleneck illustrates how alleles can be lost
A population bottleneck occurs when the population of a species is reduced drastically over a short period of time due to external forces.[35] In a true population bottleneck, the reduction does not favor any combination of alleles; it is totally random chance which individuals survive. A bottleneck can reduce or eliminate genetic variation from a population. Further drift events after the bottleneck event can also reduce the population's genetic diversity. The lack of diversity created can make the population at risk to other selective pressures.[36]
A common example of a population bottleneck is the Northern elephant seal. Due to excessive hunting throughout the 19th century, the population of the northern elephant seal was reduced to 30 individuals or less. They have made a full recovery, with the total number of individuals at around 100,000 and growing. The effects of the bottleneck are visible, however. The seals are more likely to have serious problems with disease or genetic disorders, because there is almost no diversity in the population.[37]
Founder effect[edit]
In the founder effect, small new populations contain different allele frequencies from the parent population.
The founder effect occurs when a small group from one population splits off and forms a new population, often through geographic isolation. This new population's allelic frequency is probably different from the original population's, and will change how common certain alleles are in the populations. The founders of the population will determine the genetic makeup, and potentially the survival, of the new population for generations.[34]
One example of the founder effect is found in the Amish migration to Pennsylvania in 1744. Two of the founders of the colony in Pennsylvania carried the recessive allele for Ellis–van Creveld syndrome. Because the Amish tend to be religious isolates, they interbreed, and through generations of this practice the frequency of Ellis–van Creveld syndrome in the Amish people is much higher than the frequency in the general population.[38]
Modern synthesis[edit]
The modern evolutionary synthesis is based on the concept that populations of organisms have significant genetic variation caused by mutation and by the recombination of genes during sexual reproduction. It defines evolution as the change in allelic frequencies within a population caused by genetic drift, gene flow between sub populations, and natural selection. Natural selection is emphasised as the most important mechanism of evolution; large changes are the result of the gradual accumulation of small changes over long periods of time.[39][40]
The modern evolutionary synthesis is the outcome of a merger of several different scientific fields to produce a more cohesive understanding of evolutionary theory. In the 1920s, Ronald Fisher, J.B.S. Haldane and Sewall Wright combined Darwin's theory of natural selection with statistical models of Mendelian genetics, founding the discipline of population genetics. In the 1930s and 1940s, efforts were made to merge population genetics, the observations of field naturalists on the distribution of species and sub species, and analysis of the fossil record into a unified explanatory model.[41] The application of the principles of genetics to naturally occurring populations, by scientists such as Theodosius Dobzhansky and Ernst Mayr, advanced the understanding of the processes of evolution. Dobzhansky's 1937 work Genetics and the Origin of Species helped bridge the gap between genetics and field biology by presenting the mathematical work of the population geneticists in a form more useful to field biologists, and by showing that wild populations had much more genetic variability with geographically isolated subspecies and reservoirs of genetic diversity in recessive genes than the models of the early population geneticists had allowed for. Mayr, on the basis of an understanding of genes and direct observations of evolutionary processes from field research, introduced the biological species concept, which defined a species as a group of interbreeding or potentially interbreeding populations that are reproductively isolated from all other populations. Both Dobzhansky and Mayr emphasised the importance of subspecies reproductively isolated by geographical barriers in the emergence of new species. The paleontologist George Gaylord Simpson helped to incorporate paleontology with a statistical analysis of the fossil record that showed a pattern consistent with the branching and non-directional pathway of evolution of organisms predicted by the modern synthesis.[39]
Evidence for evolution[edit]
During the second voyage of HMS Beagle, naturalist Charles Darwin collected fossils in South America, and found fragments of armor which he thought were like giant versions of the scales on the modern armadillos living nearby. On his return, the anatomist Richard Owen showed him that the fragments were from gigantic extinct glyptodons, related to the armadillos. This was one of the patterns of distribution that helped Darwin to develop his theory.[14]
Scientific evidence for evolution comes from many aspects of biology and includes fossils, homologous structures, and molecular similarities between species' DNA.
Fossil record[edit]
Research in the field of paleontology, the study of fossils, supports the idea that all living organisms are related. Fossils provide evidence that accumulated changes in organisms over long periods of time have led to the diverse forms of life we see today. A fossil itself reveals the organism's structure and the relationships between present and extinct species, allowing paleontologists to construct a family tree for all of the life forms on Earth.[42]
Modern paleontology began with the work of Georges Cuvier. Cuvier noted that, in sedimentary rock, each layer contained a specific group of fossils. The deeper layers, which he proposed to be older, contained simpler life forms. He noted that many forms of life from the past are no longer present today. One of Cuvier’s successful contributions to the understanding of the fossil record was establishing extinction as a fact. In an attempt to explain extinction, Cuvier proposed the idea of 'revolutions' or catastrophism in which he speculated that geological catastrophes had occurred throughout the Earth’s history, wiping out large numbers of species.[43] Cuvier's theory of revolutions was later replaced by uniformitarian theories, notably those of James Hutton and Charles Lyell who proposed that the Earth’s geological changes were gradual and consistent.[44] However, current evidence in the fossil record supports the concept of mass extinctions. As a result, the general idea of catastrophism has re-emerged as a valid hypothesis for at least some of the rapid changes in life forms that appear in the fossil records.
A very large number of fossils have now been discovered and identified. These fossils serve as a chronological record of evolution. The fossil record provides examples of transitional species that demonstrate ancestral links between past and present life forms.[45] One such transitional fossil is Archaeopteryx, an ancient organism that had the distinct characteristics of a reptile (such as a long, bony tail and conical teeth) yet also had characteristics of birds (such as feathers and a wishbone). The implication from such a find is that modern reptiles and birds arose from a common ancestor.[46]
Comparative anatomy[edit]
The comparison of similarities between organisms of their form or appearance of parts, called their morphology, has long been a way to classify life into closely related groups. This can be done by comparing the structure of adult organisms in different species or by comparing the patterns of how cells grow, divide and even migrate during an organism's development.
Taxonomy[edit]
Taxonomy is the branch of biology that names and classifies all living things. Scientists use morphological and genetic similarities to assist them in categorising life forms based on ancestral relationships. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping referred to as a family—in this case the family called Hominidae. These animals are grouped together because of similarities in morphology that come from common ancestry (called homology).[47]
A bat is a mammal and its forearm bones have been adapted for flight.
Strong evidence for evolution comes from the analysis of homologous structures: structures in different species that no longer perform the same task but which share a similar structure.[48] Such is the case of the forelimbs of mammals. The forelimbs of a human, cat, whale, and bat all have strikingly similar bone structures. However, each of these four species' forelimbs performs a different task. The same bones that construct a bat's wings, which are used for flight, also construct a whale's flippers, which are used for swimming. Such a 'design' makes little sense if they are unrelated and uniquely constructed for their particular tasks. The theory of evolution explains these homologous structures: all four animals shared a common ancestor, and each has undergone change over many generations. These changes in structure have produced forelimbs adapted for different tasks.[49]
The bird and the bat wing are examples of convergent evolution.
However, anatomical comparisons can be misleading, as not all anatomical similarities indicate a close relationship. Organisms that share similar environments will often develop similar physical features, a process known as convergent evolution. Both sharks and dolphins have similar body forms, yet are only distantly related—sharks are fish and dolphins are mammals. Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming have been favored. Thus, over time, they developed similar appearances (morphology), even though they are not closely related.[50]
Embryology[edit]
In some cases, anatomical comparison of structures in the embryos of two or more species provides evidence for a shared ancestor that may not be obvious in the adult forms. As the embryo develops, these homologies can be lost to view, and the structures can take on different functions. Part of the basis of classifying the vertebrate group (which includes humans), is the presence of a tail (extending beyond the anus) and pharyngeal slits. Both structures appear during some stage of embryonic development but are not always obvious in the adult form.[51]
Because of the morphological similarities present in embryos of different species during development, it was once assumed that organisms re-enact their evolutionary history as an embryo. It was thought that human embryos passed through an amphibian then a reptilian stage before completing their development as mammals. Such a reenactment, often called recapitulation theory, is not supported by scientific evidence. What does occur, however, is that the first stages of development are similar in broad groups of organisms.[52] At very early stages, for instance, all vertebrates appear extremely similar, but do not exactly resemble any ancestral species. As development continues, specific features emerge from this basic pattern.
Vestigial structures[edit]
Homology includes a unique group of shared structures referred to as vestigial structures. Vestigial refers to anatomical parts that are of minimal, if any, value to the organism that possesses them. These apparently illogical structures are remnants of organs that played an important role in ancestral forms. Such is the case in whales, which have small vestigial bones that appear to be remnants of the leg bones of their ancestors which walked on land.[53] Humans also have vestigial structures, including the ear muscles, the wisdom teeth, the appendix, the tail bone, body hair (including goose bumps), and the semilunar fold in the corner of the eye.[54]
Biogeography[edit]
Four of the Galápagos finch species, produced by an adaptive radiation that diversified their beaks for different food sources
![Revolution Revolution](/uploads/1/2/6/6/126634653/340328468.jpg)
Biogeography is the study of the geographical distribution of species. Evidence from biogeography, especially from the biogeography of oceanic islands, played a key role in convincing both Darwin and Alfred Russel Wallace that species evolved with a branching pattern of common descent.[55] Islands often contain endemic species, species not found anywhere else, but those species are often related to species found on the nearest continent. Furthermore, islands often contain clusters of closely related species that have very different ecological niches, that is have different ways of making a living in the environment. Such clusters form through a process of adaptive radiation where a single ancestral species colonises an island that has a variety of open ecological niches and then diversifies by evolving into different species adapted to fill those empty niches. Well-studied examples include Darwin's finches, a group of 13 finch species endemic to the Galápagos Islands, and the Hawaiian honeycreepers, a group of birds that once, before extinctions caused by humans, numbered 60 species filling diverse ecological roles, all descended from a single finch like ancestor that arrived on the Hawaiian Islands some 4 million years ago.[56] Another example is the Silversword alliance, a group of perennial plant species, also endemic to the Hawaiian Islands, that inhabit a variety of habitats and come in a variety of shapes and sizes that include trees, shrubs, and ground hugging mats, but which can be hybridised with one another and with certain tarweed species found on the west coast of North America; it appears that one of those tarweeds colonised Hawaii in the past, and gave rise to the entire Silversword alliance.[57]
Molecular biology[edit]
A section of DNA
Every living organism (with the possible exception of RNAviruses) contains molecules of DNA, which carries genetic information. Genes are the pieces of DNA that carry this information, and they influence the properties of an organism. Genes determine an individual's general appearance and to some extent their behavior. If two organisms are closely related, their DNA will be very similar.[58] On the other hand, the more distantly related two organisms are, the more differences they will have. For example, brothers are closely related and have very similar DNA, while cousins share a more distant relationship and have far more differences in their DNA. Similarities in DNA are used to determine the relationships between species in much the same manner as they are used to show relationships between individuals. For example, comparing chimpanzees with gorillas and humans shows that there is as much as a 96 percent similarity between the DNA of humans and chimps. Comparisons of DNA indicate that humans and chimpanzees are more closely related to each other than either species is to gorillas.[59][60][61]
The field of molecular systematics focuses on measuring the similarities in these molecules and using this information to work out how different types of organisms are related through evolution. These comparisons have allowed biologists to build a relationship tree of the evolution of life on Earth.[62] They have even allowed scientists to unravel the relationships between organisms whose common ancestors lived such a long time ago that no real similarities remain in the appearance of the organisms.
Artificial selection[edit]
The results of artificial selection: a Chihuahuamix and a Great Dane
Artificial selection is the controlled breeding of domestic plants and animals. Humans determine which animal or plant will reproduce and which of the offspring will survive; thus, they determine which genes will be passed on to future generations. The process of artificial selection has had a significant impact on the evolution of domestic animals. For example, people have produced different types of dogs by controlled breeding. The differences in size between the Chihuahua and the Great Dane are the result of artificial selection. Despite their dramatically different physical appearance, they and all other dogs evolved from a few wolves domesticated by humans in what is now China less than 15,000 years ago.[63]
Artificial selection has produced a wide variety of plants. In the case of maize (corn), recent genetic evidence suggests that domestication occurred 10,000 years ago in central Mexico.[64][unreliable source?] Prior to domestication, the edible portion of the wild form was small and difficult to collect. Today The Maize Genetics Cooperation • Stock Center maintains a collection of more than 10,000 genetic variations of maize that have arisen by random mutations and chromosomal variations from the original wild type.[65]
In artificial selection the new breed or variety that emerges is the one with random mutations attractive to humans, while in natural selection the surviving species is the one with random mutations useful to it in its non-human environment. In both natural and artificial selection the variations are a result of random mutations, and the underlying genetic processes are essentially the same.[66] Darwin carefully observed the outcomes of artificial selection in animals and plants to form many of his arguments in support of natural selection.[67] Much of his book On the Origin of Species was based on these observations of the many varieties of domestic pigeons arising from artificial selection. Darwin proposed that if humans could achieve dramatic changes in domestic animals in short periods, then natural selection, given millions of years, could produce the differences seen in living things today.
Coevolution[edit]
Coevolution is a process in which two or more species influence the evolution of each other. All organisms are influenced by life around them; however, in coevolution there is evidence that genetically determined traits in each species directly resulted from the interaction between the two organisms.[58]
An extensively documented case of coevolution is the relationship between Pseudomyrmex, a type of ant, and the acacia, a plant that the ant uses for food and shelter. The relationship between the two is so intimate that it has led to the evolution of special structures and behaviors in both organisms. The ant defends the acacia against herbivores and clears the forest floor of the seeds from competing plants. In response, the plant has evolved swollen thorns that the ants use as shelter and special flower parts that the ants eat.[68]Such coevolution does not imply that the ants and the tree choose to behave in an altruistic manner. Rather, across a population small genetic changes in both ant and tree benefited each. The benefit gave a slightly higher chance of the characteristic being passed on to the next generation. Over time, successive mutations created the relationship we observe today.
Speciation[edit]
There are numerous species of cichlids that demonstrate dramatic variations in morphology.
Given the right circumstances, and enough time, evolution leads to the emergence of new species. Scientists have struggled to find a precise and all-inclusive definition of species. Ernst Mayr defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. (The members of a species cannot produce viable, fertile offspring with members of other species).[69] Mayr's definition has gained wide acceptance among biologists, but does not apply to organisms such as bacteria, which reproduce asexually.
Speciation is the lineage-splitting event that results in two separate species forming from a single common ancestral population.[15] A widely accepted method of speciation is called allopatric speciation. Allopatric speciation begins when a population becomes geographically separated.[48] Geological processes, such as the emergence of mountain ranges, the formation of canyons, or the flooding of land bridges by changes in sea level may result in separate populations. For speciation to occur, separation must be substantial, so that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations that can no longer interbreed if they are reunited.[15] Barriers that prevent interbreeding are either prezygotic (prevent mating or fertilisation) or postzygotic (barriers that occur after fertilisation). If interbreeding is no longer possible, then they will be considered different species.[70] The result of four billion years of evolution is the diversity of life around us, with an estimated 1.75 million different species in existence today.[71][72]
Usually the process of speciation is slow, occurring over very long time spans; thus direct observations within human life-spans are rare. However speciation has been observed in present-day organisms, and past speciation events are recorded in fossils.[73][74][75] Scientists have documented the formation of five new species of cichlid fishes from a single common ancestor that was isolated fewer than 5,000 years ago from the parent stock in Lake Nagubago.[76] The evidence for speciation in this case was morphology (physical appearance) and lack of natural interbreeding. These fish have complex mating rituals and a variety of colorations; the slight modifications introduced in the new species have changed the mate selection process and the five forms that arose could not be convinced to interbreed.[77]
Mechanism[edit]
The theory of evolution is widely accepted among the scientific community, serving to link the diverse specialty areas of biology.[19] Evolution provides the field of biology with a solid scientific base. The significance of evolutionary theory is summarised by Theodosius Dobzhansky as 'nothing in biology makes sense except in the light of evolution.'[78][79] Nevertheless, the theory of evolution is not static. There is much discussion within the scientific community concerning the mechanisms behind the evolutionary process. For example, the rate at which evolution occurs is still under discussion. In addition, there are conflicting opinions as to which is the primary unit of evolutionary change—the organism or the gene.
Rate of change[edit]
Darwin and his contemporaries viewed evolution as a slow and gradual process. Evolutionary trees are based on the idea that profound differences in species are the result of many small changes that accumulate over long periods.
Gradualism had its basis in the works of the geologists James Hutton and Charles Lyell. Hutton's view suggests that profound geological change was the cumulative product of a relatively slow continuing operation of processes which can still be seen in operation today, as opposed to catastrophism which promoted the idea that sudden changes had causes which can no longer be seen at work. A uniformitarian perspective was adopted for biological changes. Such a view can seem to contradict the fossil record, which often shows evidence of new species appearing suddenly, then persisting in that form for long periods. In the 1970s paleontologists Niles Eldredge and Stephen Jay Gould developed a theoretical model that suggests that evolution, although a slow process in human terms, undergoes periods of relatively rapid change (ranging between 50,000 and 100,000 years)[80] alternating with long periods of relative stability. Their theory is called punctuated equilibrium and explains the fossil record without contradicting Darwin's ideas.[81]
Unit of change[edit]
A common unit of selection in evolution is the organism. Natural selection occurs when the reproductive success of an individual is improved or reduced by an inherited characteristic, and reproductive success is measured by the number of an individual's surviving offspring. The organism view has been challenged by a variety of biologists as well as philosophers. Richard Dawkins proposes that much insight can be gained if we look at evolution from the gene's point of view; that is, that natural selection operates as an evolutionary mechanism on genes as well as organisms.[82] In his 1976 book, The Selfish Gene, he explains:
Lineage 2 Revolution
“ | Individuals are not stable things, they are fleeting. Chromosomes too are shuffled to oblivion, like hands of cards soon after they are dealt. But the cards themselves survive the shuffling. The cards are the genes. The genes are not destroyed by crossing-over, they merely change partners and march on. Of course they march on. That is their business. They are the replicators and we are their survival machines. When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.[83] | ” |
Others view selection working on many levels, not just at a single level of organism or gene; for example, Stephen Jay Gould called for a hierarchical perspective on selection.[84]
See also[edit]
References[edit]
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Bibliography[edit]
- Bowler, Peter J. (2003). Evolution: The History of an Idea (3rd completely rev. and expanded ed.). Berkeley, CA: University of California Press. ISBN978-0-520-23693-6. LCCN2002007569. OCLC49824702.
- Campbell, Neil A.; Reece, Jane B. (2002). 'The Evolution of Populations'. Biology. 6th. San Francisco, CA: Benjamin Cummings. ISBN978-0-8053-6624-2. LCCN2001047033. OCLC47521441.
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Further reading[edit]
- Charlesworth, Brian; Charlesworth, Deborah (2003). Evolution: A Very Short Introduction. Very Short Introductions. Oxford; New York: Oxford University Press. ISBN978-0-19-280251-4. LCCN2003272247. OCLC51668497.
- Ellis, R. John (2010). How Science Works: Evolution: A Student Primer. Dordrecht; New York: Springer. doi:10.1007/978-90-481-3183-9. ISBN978-90-481-3182-2. LCCN2009941981. OCLC465370643.
- Horvitz, Leslie Alan (2002). The Complete Idiot's Guide to Evolution. Indianapolis, IN: Alpha Books. ISBN978-0-02-864226-0. LCCN2001094735. OCLC48402612.
- Krukonis, Greg; Barr, Tracy L. (2008). Evolution For Dummies. Hoboken, NJ: John Wiley & Sons. ISBN978-0-470-11773-6. LCCN2008922285. OCLC183916075.
- Pallen, Mark J. (2009). The Rough Guide to Evolution. Rough Guides Reference Guides. London; New York: Rough Guides. ISBN978-1-85828-946-5. LCCN2009288090. OCLC233547316.
- Sís, Peter (2003). The Tree of Life: A Book Depicting the Life of Charles Darwin, Naturalist, Geologist & Thinker (1st ed.). New York: Farrar, Straus and Giroux. ISBN978-0-374-45628-3. LCCN2002040706. OCLC50960680.
- Thomson, Keith (2005). Fossils: A Very Short Introduction. Very Short Introductions. Oxford; New York: Oxford University Press. ISBN978-0-19-280504-1. LCCN2005022027. OCLC61129133.
- Villarreal, Luis (2005). Viruses and the Evolution of Life. ASM Press.
- Zimmer, Carl (2010). The Tangled Bank: An Introduction to Evolution. Greenwood Village, CO: Roberts & Company Publishers. ISBN978-0-9815194-7-0. LCCN2009021802. OCLC403851918.
External links[edit]
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This audio file was created from a revision of the article 'Introduction to evolution' dated 2012-07-10 , and does not reflect subsequent edits to the article. (Audio help)
- 'Introduction to evolution and natural selection'. Khan Academy. Retrieved 2015-01-06.
- Brain, Marshall (July 25, 2001). 'How Evolution Works'. HowStuffWorks. Retrieved 2015-01-06.
- 'The Talk.Origins Archive: Evolution FAQs'. TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-12.
- Melton, Lisa (2007); Reza, Julie (2007); Jones, Ian (2014); Staves, Jennifer Trent (2014) (January 2007). 'Evolution'. Big Picture. London: Wellcome Trust. Updated for the Web in 2014.
- 'Understanding Evolution: your one-stop resource for information on evolution'. University of California, Berkeley. Retrieved 2015-01-08.
- 'An Introduction To Evolution'. Vectors (Web resource). Greg Goebel. Retrieved 2010-06-01.
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