Recent African origin of modern humans

From Wikipedia, the free encyclopedia

In paleoanthropology, the recent African origin of modern humans, or the "Out of Africa" theory, is the most widely accepted model of the geographic origin and early migration of anatomically modern humans. This model has incorporated the 2010 discovery of genetic evidence for some archaic human admixture with modern Homo sapiens.^[1] The theory is called the "(Recent) Out-of-Africa" model in the popular press, and academically the "recent single-origin hypothesis" (RSOH), "Replacement Hypothesis", and "Recent African Origin" (RAO) model. The concept was speculative until the 1980s, when it was corroborated by a study of present-day mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens.

Genetic studies and fossil evidence show that archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 60,000 years ago,^[2] that members of one branch of Homo sapiens left Africa by between 125,000 and 60,000 years ago, and that over time these humans replaced earlier human populations such as Neanderthals and Homo erectus.^[3] The date of the earliest successful "out of Africa" migration (earliest migrants with living descendants) has generally been placed at 60,000 years ago as suggested by genetics, although migration out of the continent may have taken place as early as 125,000 years ago according to Arabian archaeology finds of tools in the region.^[4] A 2013 paper reported that a previously unknown lineage had been found, which pushed the estimated date for the most recent common ancestor (Y-MRCA) back to 338,000 years ago.^[5]

The recent single origin of modern humans in East Africa is the predominant position held within the scientific community.^{[6][7][8][9][10]} There are differing theories on whether there was a single exodus or several. A multiple dispersal model involves the Southern Dispersal theory,^[11] which has gained support in recent years from genetic, linguistic and archaeological evidence. A growing number of researchers also suspect that "long-neglected North Africa" was the original home of the modern humans who first trekked out of the continent.^[12][13][14]

The major competing hypothesis is the multiregional origin of modern humans, which envisions a wave of Homo sapiens migrating earlier from Africa and interbreeding with local Homo erectus populations in multiple regions of the globe. Most multiregionalists still view Africa as a major wellspring of human genetic diversity, but allow a much greater role for hybridization.^[15][16]

Genetic testing in the last decade has revealed that several now extinct archaic human species may have interbred with modern humans. These species have been claimed to have left their genetic imprint in different regions across the world: Neanderthals in all humans except Sub-Saharan Africans, Denisova hominin in Australasia (for example, Melanesians, Aboriginal Australians and some Negritos) and there could also have been interbreeding between Sub-Saharan Africans and an as-yet-unknown hominin (possibly remnants of the ancient species Homo heidelbergensis). However, the rate of interbreeding was found to be relatively low (1-10%) and other studies have suggested that the presence of Neanderthal or other archaic human genetic markers in modern humans can be attributed to shared ancestral traits originating from a common ancestor 500,000 to 800,000 years ago.^{[17][18][19][20][21]}

History of the theory

With the development of anthropology in the early 19th century, scholars disagreed vigorously about different theories of human development. Those such as Johann Friedrich Blumenbach and James Cowles Prichard held that since the creation, the various human races had developed as different varieties sharing descent from one people (monogenism). Their opponents, such as Louis Agassiz and Josiah C. Nott, argued for polygenism, or the separate development of human races as separate species or had developed as separate species through transmutation of species from apes, with no common ancestor.

The frontispiece to Huxley's Evidence as to Man's Place in Nature (1863): the image compares the skeletons of apes to humans.

Charles Darwin was one of the first to propose common descent of living organisms, and among the first to suggest that all humans had in common ancestors who lived in Africa.^[22] Darwin first suggested the "Out of Africa" hypothesis after studying the behaviour of African apes, one of which was displayed at the London Zoo. The anatomist Thomas Huxley had also supported the hypothesis and suggested that African apes have a close evolutionary relationship with humans.^[23] These views were however opposed by Ernst Haeckel the German biologist who was a proponent of the Out of Asia theory. Haeckel argued that humans were more closely related to the primates of Southeast Asia and rejected Darwin’s hypothesis of Africa.^[24][25]

In the Descent of Man, Darwin speculated that humans had descended from apes which still had small brains but walked upright, freeing their hands for uses which favoured intelligence. Further, he thought such apes were African:^[26]

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

—Charles Darwin, Descent of Man^[27]

The prediction was insightful, because in 1871 there were hardly any human fossils of ancient hominids available. Almost fifty years later, Darwin's speculation was supported when anthropologists began finding numerous fossils of ancient small-brained hominids in several areas of Africa (list of hominina fossils).

The debate in anthropology had swung in favour of monogenism by the mid-20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who hypothesized as late as 1962 that Homo sapiens arose five times from Homo erectus in five places.^[28]
The "Recent African origin" of modern humans means "single origin" (monogenism) and has been used in various contexts as an antonym to polygenism.

In the 1980s Allan Wilson together with Rebecca L. Cann and Mark Stoneking worked on the so-called "Mitochondrial Eve" hypothesis. In his efforts to identify informative genetic markers for tracking human evolutionary history, he started to focus on mitochondrial DNA (mtDNA) – genes that sit in the cell, but not in the nucleus, and are passed from mother to child. This DNA material is important because it mutates quickly, thus making it easy to plot changes over relatively short time spans. By comparing differences in the mtDNA Wilson believed it was possible to estimate the time, and the place, modern humans first evolved. With his discovery that human mtDNA is genetically much less diverse than chimpanzee mtDNA, he concluded that modern human populations had diverged recently from a single population while older human species such as Neandertals and Homo erectus had become extinct. He and his team compared mtDNA in people of different ancestral backgrounds and concluded that all modern humans evolved from one 'lucky mother' in Africa about 150,000 years ago.^[29] With the advent of archaeogenetics in the 1990s, scientists were able to date the "out of Africa" migration with some confidence.

In 2000, the mitochondrial DNA (mtDNA) sequence of "Mungo Man 3" (LM3) of ancient Australia was published indicating that Mungo Man was an extinct subspecies that diverged before the most recent common ancestor of contemporary humans. The results, if correct, supports the multiregional origin of modern humans hypothesis.^[30][31] This work was later questioned^[32][33] and explained by W. James Peacock, leader of the team who sequenced Mungo man's aDNA.^[34] In addition, a large-scale genotyping analysis of aboriginal Australians, New Guineans, Southeast Asians and Indians in 2013 showed close genetic relationship between Australian, New Guinean, and the Mamanwa people, with divergence times for these groups estimated at 36,000 y ago. Further, substantial gene flow was detected between the Indian populations and aboriginal Australians, indicating an early "southern route" migration out of Africa, and arrival of other populations in the region by subsequent dispersal. This basically opposes the view that there was an isolated human evolution in Australia.^[35]

The question of whether there was inheritance of other typological (not de facto) Homo subspecies into the Homo sapiens genetic pool is debated.

Early Homo sapiens

 

Anatomical comparison of the skulls of a modern human (left) and Homo neanderthalensis (right).

Anatomically modern humans originated in Africa about 250,000 years ago. The trend in cranial expansion and the acheulean elaboration of stone tool technologies which occurred between 400,000 years ago and the second interglacial period in the Middle Pleistocene (around 250,000 years ago) provide evidence for a transition from H. erectus to H. sapiens.^[36] In the Recent African Origin (RAO) scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus.

Homo sapiens idaltu, found at site Middle Awash in Ethiopia, lived about 160,000 years ago.^[37] It is the oldest known anatomically modern human and classified as an extinct subspecies.^[38] Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated to 80,000 to 100,000 years ago. However these humans seem to have either become extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by south bound Neanderthals escaping the colder regions of ice age Europe.^[39] Hua Liu et al. analyzing autosomal microsatellite markers dates to c. 56,000±5,700 years ago mtDNA evidence. He interprets the paleontological fossil of early modern human from Qafzeh cave as an isolated early offshoot that retracted back to Africa.^[40]

All other fossils of fully modern humans outside Africa have been dated to more recent times. The oldest well dated fossils found outside Africa are from Lake Mungo, Australia, and have been dated to about 42,000 years ago.^[41][42] The Tianyuan cave remains in Liujiang region China have a probable date range between 38,000 and 42,000 years ago. They are most similar in morphology to Minatogawa Man, modern humans dated between 17,000 and 19,000 years ago and found on Okinawa Island, Japan.^[43][44] However, others have dated Liujang Man to 111,000 to 139,000 years before the present.^[45]

Beginning about 100,000 years ago evidence of more sophisticated technology and artwork begins to emerge and by 50,000 years ago fully modern behaviour becomes more prominent. Stone tools show regular patterns that are reproduced or duplicated with more precision while tools made of bone and antler appear for the first time.^[46][47]

Genetic reconstruction

Two pieces of the human genome are quite useful in deciphering human history: mitochondrial DNA and the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms that generate diversity with each generation: instead, these elements are passed down intact. According to the hypothesis, all people alive today have inherited the same mitochondria^[48] from a woman who lived in Africa about 160,000 years ago.^[49][50] She has been named Mitochondrial Eve. All men living today have inherited their Y chromosomes from a man who lived 140,000–500,000 years ago, probably in Africa. He has been named Y-chromosomal Adam. Based on comparisons of non-sex-specific chromosomes with sex-specific ones, it is now believed that more men than women participated in the out-of-Africa exodus of early humans.^[51] [Mendez et al.]

Mitochondrial DNA

 

Map of early diversification of modern humans according to mitochondrial population genetics (see: Haplogroup L).

The first lineage to branch off from Mitochondrial Eve is L0. This haplogroup is found in high proportions among the San of Southern Africa, the Sandawe of East Africa. It is also found among the Mbuti people.^[52][53]

These groups branched off early in human history and have remained relatively genetically isolated since then. Haplogroups L1, L2 and L3 are descendents of L1-6 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3. L3 is about 84,000 years old, and haplogroup M and N are almost identical in age at about 63,000 years old. ^[54]

Genomic analysis

Although mitochondrial DNA and Y-chromosomal DNA are particularly useful in deciphering human history, data on the genomes of dozens of population groups have also been studied. In June 2009, an analysis of genome-wide SNP data from the International HapMap Project (Phase II) and CEPH Human Genome Diversity Panel samples was published.^[55] Those samples were taken from 1138 unrelated individuals.^[55] Before this analysis, population geneticists expected to find dramatic differences among ethnic groups, with derived alleles shared among such groups but uncommon or nonexistent in other groups.^[56] Instead the study of 53 populations taken from the HapMap and CEPH data revealed that the population groups studied fell into just three genetic groups: Africans, Eurasians (which includes natives of Europe and the Middle East, and Southwest Asians east to present-day Pakistan), and East Asians, which includes natives of Asia, Japan, Southeast Asia, the Americas, and Oceania.^[56] The study determined that most ethnic group differences can be attributed to genetic drift, with modern African populations having greater genetic diversity than the other two genetic groups, and modern Eurasians somewhat more than modern East Asians.^[56] The study suggested that natural selection may shape the human genome much more slowly than previously thought, with factors such as migration within and among continents more heavily influencing the distribution of genetic variations.^[57] A May 2002 study examined three groups, African, European, and Asian. It found greater genetic diversity among Africans than among Eurasians, and that genetic diversity among Eurasians is largely a subset of that among Africans, supporting the 'out of Africa' model.^[58]

Movement out of Africa

Red Sea crossing

By some 70,000 years ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East. The date of this first wave of "out of Africa" migration was called into question in 2011, based on the discovery of stone tools in the United Arab Emirates, indicating the presence of modern humans between 100,000 and 125,000 years ago.^[4][59] New research showing slower than previously thought genetic mutations in human DNA published in 2012, indicating a revised dating for the migration of between 90,000 and 130,000 years ago.^[60]

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world,^[61] based in the fact that only descendents of L3 are found outside Africa. From that settlement, some others point to the possibility of several waves of expansion. For example, geneticist Spencer Wells says that the early travellers followed the southern coastline of Asia, crossed about 250 kilometres (155 mi) of sea, and colonized Australia by around 50,000 years ago. The Aborigines of Australia, Wells says, are the descendants of the first wave of migrations.^[62]
It has been estimated that from a population of 2,000 to 5,000 individuals in Africa,^[63] only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea.^[64] Of all the lineages present in Africa only the female descendants of one lineage, mtDNA haplogroup L3, are found outside Africa. Had there been several migrations one would expect descendants of more than one lineage to be found outside Africa. L3's female descendants, the M and N haplogroup lineages, are found in very low frequencies in Africa (although haplogroup M1 is very ancient and diversified in North and Northeast Africa) and appear to be recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and by the founder effect became the dominant haplogroups after the exodus from Africa. Alternatively, the mutations may have arisen shortly after the exodus from Africa.

Other scientists have proposed a Multiple Dispersal Model, in which there were two migrations out of Africa, one across the Red Sea travelling along the coastal regions to India (the Coastal Route), which would be represented by Haplogroup M. Another group of migrants with Haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai.
This group then branched in several directions, some moving into Europe and others heading east into Asia. This hypothesis is supported by relatively late dating of the arrival of modern humans into Europe as well as by both archaeological and DNA evidence. Results from mtDNA collected from aboriginal Malaysians called Orang Asli, and the creation of a phylogentic tree, indicates Hapologroups M and N share characteristics with original African groups dating approximately 85,000 years ago, and sharing characteristics with sub-haplogroups among coastal southeast Asian regions, such as Australasia, the Indian Subcontinent, and throughout continental Asia, which had dispersed and separated from its African origins approximately 65,000 years ago. This southern coastal dispersion would have occurred before the original theory of dispersion through the Levant approximately 45,000 years ago.^[65] This hypothesis attempts to explain why Haplogroup N is predominant in Europe and why Haplogroup M is absent in Europe. Evidence of the coastal migration is hypothesized to have been destroyed by the rise in sea levels during the Holocene epoch.^[11][66] Alternatively, a small European founder population that initially expressed both Haplogroup M and N could have lost Haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

Today at the Bab-el-Mandeb straits the Red Sea is about 20 kilometres (12 mi) wide, but 50,000 years ago sea levels were 70 m (230 ft) lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, they were narrow enough and there may have been islands in between to have enabled crossing using simple rafts.^[67][68] Shell middens 125,000 years old have been found in Eritrea,^[69] indicating the diet of early humans included seafood obtained by beachcombing.

Subsequent expansion

 

Map of early human migrations^[70]
1. Homo sapiens
2. Neanderthals
3. Early Hominids

From the Near East, these populations spread east to South Asia by 50,000 years ago, and on to Australia by 40,000 years ago, Homo sapiens for the first time colonizing territory never reached by Homo erectus. Europe was reached by Cro-Magnon some 40,000 years ago. East Asia (Korea, Japan) was reached by 30,000 years ago. It is disputed whether subsequent migration to North America took place around 30,000 years ago, or only considerably later, around 14,000 years ago.^[71]

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Persia until reaching India, which appears to be the first major settling point. Haplogroup M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred.^[72] Sixty percent of the Indian population belong to Haplogroup M.

The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from mainland Asia. They are evidence of the coastal route of early settlers that extends from India along the coasts of Thailand and Indonesia all the way to Papua New Guinea. Since M is found in high frequencies in highlanders from New Guinea as well, and both the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists believe they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration.

Notably, the findings of Harding et al. (2000, p. 1355) show that, at least with regard to dark skin color, the haplotype background of Papua New Guineans at MC1R (one of a number of genes involved in melanin production) is identical to that of Africans (barring a single silent mutation). Thus, although these groups are distinct from Africans at other loci (due to drift, bottlenecks, etc.), it is evident that selection for the dark skin color trait likely continued (at least at MC1R) following the exodus. This would support the hypothesis that suggests that the original migrants from Africa resembled pre-exodus Africans (at least in skin color), and that the present day remnants of this ancient phenotype can be seen among contemporary Africans, Andamanese and New Guineans.
Others suggest that their physical resemblance to Africans could be the result of convergent evolution.^[73][74]

From Arabia to India the proportion of haplogroup M increases eastwards: in eastern India, M outnumbers N by a ratio of 3:1. However, crossing over into East Asia, Haplogroup N reappears as the dominant lineage. M is predominant in South East Asia but amongst Indigenous Australians N reemerges as the more common lineage. This discontinuous distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.^[75]

Competing hypotheses

The multiregional hypothesis, initially proposed by Milford Wolpoff, holds that the evolution of humans from H. erectus at the beginning of the Pleistocene 1.8 million years BP has been within a single, continuous worldwide population. Proponents of multiregional origin reject the assumption of an infertility barrier between ancient Eurasian and African populations of Homo. Multiregional proponents point to the fossil record and genetic evidence in chromosomal DNA. One study suggested that at least 5% of the human modern gene pool can be attributed to ancient admixture, which in Europe would be from the Neanderthals.^[76] But the study also suggests that there may be other reasons why humans and Neanderthals share ancient genetic lineages.^[77][78]

Mammal

From Wikipedia, the free encyclopedia

 

Mammals Temporal range: 225–0 Ma (Kemp) or 167–0 Ma (Rowe) See discussion of dates in text PreЄ Є O S D C P T J K Pg N
Examples of various mammalian orders. Click to see originals. Row 1: common vampire bat, Virginia opossum, eastern grey kangaroo. Row 2: southern marsupial mole, humans, northern elephant seals. Row 3: fox squirrel, tree pangolin, African elephants. Row 4: platypus, colugo, reindeer. Row 5: humpback whale, star-nosed mole, giant panda. Row 6: giant armadillo, plains zebras, black and rufous elephant shrew.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Superclass:	Tetrapoda
Clade:	Mammaliaformes
Class:	Mammalia Linnaeus, 1758
Subgroups
Clade Australosphenida †Order Ausktribosphenida Order Monotremata †Order Eutriconodonta †Order Gobiconodonta †Order Multituberculata Clade Trechnotheria Subclass Theria Infraclass Marsupialia Infraclass Placentalia

Mammals (class Mammalia /məˈmeɪli.ə/) are a clade of endothermic amniotes distinguished from the reptiles and the birds by the possession of hair, three middle ear bones, mammary glands, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The mammals include the largest animals on the planet, the rorquals and some other whales, as well as some of the most intelligent, such as elephants, some primates and some cetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placentals, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,416 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders.^[1] In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.^[2] In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.

Except for the five species of monotremes (egg-laying mammals), all modern mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives).^[1]
While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.
The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of the dinosaurs 66 million years ago.

Varying definitions, varying dates

In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.^[3] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.^[4]

T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.^[5]

If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.^[6] The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.^[7][8] In any case, the temporal range of the group extends to the present day.

Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.

Many traits shared by all living mammals appeared among the earliest members of the group:

• Jaw joint - The dentary (the lower jaw bone which carries the teeth) and the squamosal (a small cranial bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists 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).
• Middle ear - In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus, and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.
• Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.^[9]
• Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.
• Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob.

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.
For palaeontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position.
Palaeontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.

Classification

 

The orders Rodentia (blue), Chiroptera (red), and Soricomorpha (yellow) together comprise over 70% of mammal species.

George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,^[10] is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though recent molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of many terms used for mammal groups above the species includes extinct mammals, as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.

Extinct groups are represented by a dagger (†).

Class Mammalia

• Subclass Prototheria: monotremes: echidnas and the platypus
• Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
  • Infraclass †Allotheria: multituberculates
  • Infraclass †Triconodonta: triconodonts
  • Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
    • Superlegion †Kuehneotheria
    • Supercohort Theria: live-bearing mammals
      • Cohort Marsupialia: marsupials
        • Magnorder Australidelphia: Australian marsupials and the monito del monte
        • Magnorder Ameridelphia: New World marsupials.
      • Cohort Placentalia: placentals
        • Magnorder Xenarthra: xenarthrans
        • Magnorder Epitheria: epitheres
          • Superorder Anagalida: lagomorphs, rodents, and elephant shrews
          • Superorder Ferae: carnivorans, pangolins, †creodonts, and relatives
          • Superorder Lipotyphla: insectivorans
          • Superorder Archonta: bats, primates, colugos, and treeshrews
          • Superorder Ungulata: ungulates
            • Order Tubulidentata incertae sedis: aardvark
            • Mirorder Eparctocyona: †condylarths, whales, and artiodactyls (even-toed ungulates)
            • Mirorder †Meridiungulata: South American ungulates
            • Mirorder Altungulata: perissodactyls (odd-toed ungulates), elephants, manatees, and hyraxes

Molecular classification of placentals

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon presence/absence data.^[11] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals- Afrotheria, Xenarthra, and Boreoeutheria- which diverged from early common ancestors in the Cretaceous. The relationships between these three lineages is contentious, and three different hypothesis have been proposed with respect to which group is basal with respect to other placental. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), and Exafroplacentalia (basal Afrotheria).^[12] Boreoeutheria in turn contains two major lineages- Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on type of DNA (e.g. nuclear or mitochondrial)^[13] and varying interpretations of paleogeographic data.^[12]

Group I: Afrotheria

• Clade Afroinsectiphilia
  • Order Macroscelidea: elephant shrews (Africa)
  • Order Afrosoricida: tenrecs and golden moles (Africa)
  • Order Tubulidentata: aardvark (Africa south of the Sahara)
• Clade Paenungulata
  • Order Hyracoidea: hyraxes or dassies (Africa, Arabia)
  • Order Proboscidea: elephants (Africa, Southeast Asia)
  • Order Sirenia: dugong and manatees (cosmopolitan tropical)

Group II: Xenarthra

• Order Pilosa: sloths and anteaters (neotropical)
• Order Cingulata: armadillos and extinct relatives (Americas)

Group III: Boreoeutheria

• Clade: Euarchontoglires (Supraprimates)
  • Superorder Euarchonta
    • Order Scandentia: treeshrews (Southeast Asia).
    • Order Dermoptera: flying lemurs or colugos (Southeast Asia)
    • Order Primates: lemurs, bushbabies, monkeys, apes, humans (cosmopolitan)
  • Superorder Glires
    • Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
    • Order Rodentia: rodents (cosmopolitan)
• Clade Laurasiatheria
  • Order Erinaceomorpha: hedgehogs
  • Order Soricomorpha: moles, shrews, solenodons
  • Clade Ferungulata
    • Clade Cetartiodactyla
      • Order Cetacea: whales, dolphins and porpoises
      • Order Artiodactyla: even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep, goats
    • Clade Pegasoferae
      • Order Chiroptera: bats (cosmopolitan)
      • Clade Zooamata
        • Order Perissodactyla: odd-toed ungulates, including horses, donkeys, zebras, tapirs, and rhinoceroses
        • Clade Ferae
          • Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
          • Order Carnivora: carnivores (cosmopolitan), including cats and 2dogs

Evolutionary history

Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from the lineage that led to reptiles and birds. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic.

Cladogram following,^[14] which takes Mammalia to be the crown group.

Mammaliaformes	Morganucodontidae Docodonta Haldanodon Mammalia Australosphenida (incl. Monotremata) Fruitafossor Haramiyavia Multituberculata Tinodon Eutriconodonta (incl. Gobiconodonta) Trechnotheria (incl. Theria)

A cladogram compiled by Mikko Haaramo and based on individual cladograms of After Rowe 1988; Luo, Crompton & Sun 2001; Luo, Cifelli & Kielan-Jaworowska 2001, Luo, Kielan-Jaworowska & Cifelli 2002, Kielan-Jaworowska, Cifelli & Luo 2004, and Luo & Wible 2005.^[15]

Mammaliaformes classification

†Adelobasilus cromptoni Lucas & Hunt 1990 †Sinoconodon rigneyi Patterson & Olson 1961 †Morganucodonta †Docodonta †Hadrocodium wui Luo, Crompton & Sun 2001 †Kuehneotheriida Mammalia Yinotheria †Shuotheriidae Australosphenida †Henosferidae †Ambondro mahabo †Ausktribosphenidae Monotremata †Fruitafossor windscheffeli Luo & Wible 2005 †Volaticotherium antiquum Meng et al. 2006 emend. Meng et al. 2007 Theriimorpha †Eutriconodonta †Allotheria Holotheria †Chronoperates paradoxus Fox, Youzwyshyn & Krause 1992 Trechnotheria †Amphiodontidae Gobiconodontidae Klameliidae Amphilestidae Spalacotheriida Symmetrodonta Triconodontidae Zhangheotherium quinquecuspidens Hu et al. 1997 Cladotheria Dryolestida Zatheria Mammaliaformes †Tricuspes von Huene 1933 [? non-mammaliaform chiniquodontoid] †Repenomamus Li et al. 2001 {Repenomamidae Li et al. 2001} [?gobiconodont] Mammalia Linnaeus 1758 sensu stricto Rowe 1988 (true mammals) †Reigitherium bunodontum Bonaparte 1990 {Reigitheriidae Bonaparte 1990} †Tikitherium copei Datta 2005 Trechnotheria †Argentoconodon fariasorum Rougier et al. 2007 †Austrotriconodon mckennai Bonaparte 1986 {Austrotriconodontidae Bonaparte 1990} †Dyskritodon Sigogneau-Russell 1995 †Hallautherium schalchi Clemens 1980 †Liaotherium gracile Zhou, Cheng & Wang 1991 †Jeholodentidae Luo et al. 2007 Paraphyletic Symmetrodonta Simpson 1925 †Thereuodon Sigogneau-Russell 1989 [Dryolestida, or Zatheria indet., milk tooth (Averianov, 2002)] †Atlasodon Sigogneau-Russell 1991 [Dryolestida, or Zatheria indet., milk tooth (Averianov, 2002)] †Eurylambda aequicrurius †Peralestes longirostris †Tribactonodon bonfieldi Sigogneau-Russell, Hooker & Ensom 2001

Evolution from amniotes in the Paleozoic

The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.

The first fully terrestrial vertebrates were amniotes. Like their amphibian predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Late Carboniferous. They descended from earlier reptiliomorph amphibians,^[16] which lived on land that was already inhabited by insects and other invertebrates as well as by ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which include mammals; and the sauropsids, which include turtles, lizards, snakes, crocodilians, dinosaurs and birds.^[17] Synapsids have a single hole (temporal fenestra) low on each side of the skull.

One synapsid group, the pelycosaurs, included the largest and fiercest animals of the early Permian.^[18]

Therapsids descended from pelycosaurs in the Middle Permian, about 265 million years ago, and took over their position as the dominant land vertebrates.^[19] They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisors which are equal in size.^[20] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:

• gradual development of a bony secondary palate.^[21]
• Progress was made towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
• The dentary gradually became the main bone of the lower jaw and, in the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

Nonmammalian synapsids are sometimes called "mammal-like reptiles".^[19][22]

The mammals appear

The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of the carnivores among the therapsids. In the early Triassic, all the medium to large land carnivore niches were taken over by archosaurs which, over an extended period of time (35 million years), came to include the crocodylomorphs, the pterosaurs, and the dinosaurs. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards; Castorocauda, for example, had adaptations for swimming, digging and catching fish.^[23]

The majority of the mammal species that existed in the Mesozoic Era were multituberculates, eutriconodonts and spalacotheriids.^[24]

The earliest known monotreme is Teinolophos, which lived about 123 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:

• They use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") – as lizards and birds also do.
• They lay eggs which 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 earliest known metatherian is Sinodelphys, found in 125 million-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.^[25]

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China," dated to 160 million years ago in the Late Jurassic.^[26] A later eutherian, Eomaia, dated to 125 million years ago in the Early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.^[27] In particular:

• Epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, and nontherian mammals like the multituberculates as well as in Ukhaatherium, an Early Cretaceous animal in the eutherian order Asioryctitheria.^[28] They are apparently an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.

• A narrow pelvic outlet 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.

Rise to dominance in the Cenozoic

Mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event emptied ecological space once filled by reptiles.^[29] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.^[29]
For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the dinosaurs.^[30]

Recent molecular phylogenetic studies suggest that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene.^[31] But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.^[32] The earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs.^[32] In particular, scientists have recently identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals.^[33][34] The earliest known ancestor of primates is Archicebus achilles^[35][36] from around 55 million years ago.^[35][36] This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.^[35][36]

During the Cenozoic, several groups of mammals appeared which were much larger than their nearest modern equivalents, but none was even close to the size of the largest dinosaurs with similar feeding habits.

Earliest appearances of features

Hadrocodium, whose fossils date from approximately 195 million years ago, in the Early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.^[37][38][39]

The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164 million years ago in the Middle Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur;^[40][41] it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina which are almost identical to those found in the nonmammalian cynodont Thrinaxodon.^[22][42] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon.^[43]

The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the Early Cretaceous or latest Jurassic; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated 125 million years ago.^[44]

When endothermy first appeared in the evolution of mammals is uncertain. Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,^[45] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.^[46] Some of the evidence found so far suggests that Triassic cynodonts had fairly high metabolic rates, but it is not conclusive. For small animals, an insulative covering like fur is necessary for the maintenance of a high and stable body temperature.

Anatomy and morphology

Skeletal system

The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales, and humans. The exceptions are the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.^[47]

Respiratory system

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.

Breathing is largely driven by the muscular diaphragm, which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli.
Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wall contracts, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.
Mammals take oxygen into their lungs, and discard carbon dioxide.

Nervous system

All mammalian brains possess a neocortex, a brain region unique to mammals. Placental mammals have a corpus callosum, unlike monotremes and marsupials. The size and number of cortical areas (Brodmann's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).

Integumentary system

The integumentary system is made up of three layers: the outermost epidermis, the dermis, and the hypodermis.

The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

Although other animals have features such as whiskers, feathers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class. Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies.

Some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies.^[48] The two-toed sloth and the polar bear sometimes appear to have green fur, but this color is caused by algal growths.

Reproductive system

 

Goat kids will stay with their mother until they are weaned.

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[49] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[50]

The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. The placentals give birth to complete and fully developed young, usually after long gestation periods.

Physiology

Endothermy

Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for nonavian reptiles and large insects.

Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.

A rare exception, the naked mole rat, produces little metabolic heat, so it is considered an operational poikilotherm. Birds and tuna are also endothermic, so endothermy is not peculiar to mammals.

Intelligence

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.^[51]

Social structure

Locomotion

Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, or fly. Some land mammals have toes that produce claws for climbing or hooves for running. Aquatic mammals like whales and dolphins have flippers which evolved from legs.

Terrestrial

Arboreal

Aquatic

Whales and dolphins propel themselves through the water by moving their tail flukes up and down, adjusting the angle of the flukes as needed. The more massive front of the body contributes stability.^[52][53]

Aerial[

Feeding

To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes subtypes such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores).
Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).^[51]

Specializations in herbivory include: Granivory "seed eating", folivory "leaf eating", frugivory "fruit eating", nectivory "nectar eating", gummivory "gum eating", and mycophagy "fungus eating".

Hybrid Mammals

The deliberate or accidental hybridising of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown in recent times for economic purposes. The number of successful interspecific mammalian hybrids is relatively small, although it has come to be known that there is a significant number of naturally occurring hybrids between forms or regional varieties of a single species. These may form zones of gradation known as clines. Indeed the distinction between some hitherto distinct species can become clouded once it can be shown that they may not only breed but produce fertile offspring. Some hybrid animals exhibit greater strength and resilience than either parent. This is known as hybrid vigor. The existence of the mule (donkey sire; horse dam) being used widely as a hardy draught animal throughout ancient and modern history is testament to this. Other well known examples are the lion/tiger hybrid, the liger, which is by far the largest big cat and sometimes used in circuses; and cattle hybrids such as between European and Indian domestic cattle or between domestic cattle and American bison, which are used in the meat industry and marketed as Beefalo. There is some speculation that the donkey itself may be the result of an ancient hybridisation between two wild ass species or sub-species. Hybrid animals are normally infertile partly because their parents usually have slightly different numbers of chromosomes, resulting in unpaired chromosomes in their cells, which prevents division of sex cells and the gonads from operating correctly, particularly in males. There are exceptions to this rule, especially if the speciation process was relatively recent or incomplete as is the case with many cattle and dog species. Normally behavior traits, natural hostility, natural ranges and breeding cycle differences maintain the separateness of closely related species and prevent natural hybridisation. However the widespread disturbances to natural animal behaviours and range caused by human activity, cities, dumping grounds with food, agriculture, fencing, roads and so on do force animals together which would not normally breed. Clear examples exist between the various sub-species of grey wolf, coyote and domestic dog in North America. As many birds and mammals imprint on their mother and immediate family from infancy, a practice used by animal hybridizers is to foster a planned parent in a hybridization program with the same species as the one with which they are planned to mate. A comprehensive web search will reveal further details.