Theevolutionary history of theprimates can be traced back 57-90 million years.[1] One of the oldest known primate-like mammal species,Plesiadapis, came from North America;[2] another,Archicebus, came from China.[3] Other such early primates includeAltiatlasius andAlgeripithecus, which were found in Northern Africa.[4][5] Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of thePaleocene andEocene.Purgatorius is the genus of the four extinct species believed to be among the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago.
The surviving tropical population of primates, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of theFaiyum depression southwest of Cairo, gave rise to all living species—lemurs of Madagascar,lorises ofSoutheast Asia,galagos or "bush babies" of Africa, and theanthropoids:platyrrhine or New World monkeys,catarrhines or Old World monkeys, and theapes, includingHomo sapiens.
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The origins and early evolution of primates is shrouded in mystery due to lack of fossil evidence. They are believed to have split fromplesiadapiforms in Eurasia around the early Eocene or earlier. The first true primates so far found in the fossil record are fragmentary and already demonstrate the major split betweenstrepsirrhines andhaplorhines.
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The earliest strepsirrhines are known asadapiforms, a diverse group that ranged throughout Eurasia and North America. An early branch of thisclade gave rise tolemuriform primates, which includeslemurs and their kin.
David Begun has theorised that early primates flourished in Eurasia and that a lineage leading to the African apes and humans, includingDryopithecus, migrated south fromEurope or Western Asia into Africa.[6] However, earlier potential ancestors of primates such asAltiatlasius are known to have lived on the African continent much earlier thanDryopithecus.[4] And besidesDryopithecus, the early European fauna is also exemplified byDarwinius, abasalstrepsirrhine dated to 47 million years (earlyEocene)[7]
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The earliest haplorrhine primates from the fossil record are theomomyids, which resembled modern daytarsiers. Like the strepsirrhine adapiforms, omomyids were diverse and ranged throughout Eurasia and North America. Thephylogeny of omomyids, tarsiers, andsimians is currently unknown.
For many years, it was assumed that primates had first evolved in Africa, and this assumption and the excavations that resulted from it yielded many early simian fossils that chronicled their evolution. Due to the lack of fossils linking simians to the earliest haplorrhines, a more recently discoveredstem group calledeosimiids found in Asia are thought to have dispersed to Africa and evolved into simians. Eosimiids were very small and similar to tarsiers, though their dentition more closely resembles that of simians.
Some of the primates' vertebrate ancestors weretetrachromats, but their nocturnal mammalian ancestors lost two of their four cones during the mesozoic. Most modern primates, however, have evolved to betrichromats. All old world monkeys and apes are trichromats, but new world monkeys are polymorphic trichromats, meaning that males and homozygous females aredichromats while heterozygous females are trichromats (with the exceptions ofhowler monkeys andnight monkeys, who have more and less robust color vision respectively).
There are four prevailing theories as to what the evolutionary pressure was for primates to develop trichromatic vision. The Fruit Theory suggests that it was easier for trichromatic primates to find ripe fruit against a green background. While there is data supporting the Fruit Theory, there is some dispute about whether or nottrichromacy was more advantageous for determining how ripe fruit was up close or spotting fruit from afar. The Young Leaf hypothesis suggests that primates with more advanced color vision could better spot younger and more nutritious leaves during fruit shortages, while there are also theories that suggest more advanced color vision was better for recognizing changes in skin tone, allowing primates to better determine the bloodoxygen saturation of others. Still other theories suppose that primates' color vision evolved alongside their sense of smell, though research has shown no direct correlation between concentration of olfactory receptors and acquisition of color vision.
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Following the emergence of Simiformes in Africa,Platyrrhini split fromCatarrhini during the Eocene when New World monkeys dispersed to South America, likely byrafting on mats of vegetation across the Atlantic Ocean. The Atlantic is estimated to have been possibly 1,000 km (600 mi) narrower, based on estimates from the expansion of the Atlanticmid-ocean ridge formation processes (2.5 cm/1 in per year.) It is also possible that during this rafting process, there were a number of islands between Africa and South America which have since been submerged.
Bayesian estimates of divergence time using "conservative but realistic fossil constraints" have indicated themost recent common ancestor of new world monkeys to have existed between 27-31 million years ago.[8]
The earliest knowncatarrhine isKamoyapithecus from uppermost Oligocene at Eragaleit in thenorthern Kenya Rift Valley, dated to 24 million years ago.[9] Its ancestry is thought to be species related toAegyptopithecus,Propliopithecus, andParapithecus from the Faiyum depression, at around 35 million years ago.[10] In 2010,Saadanius was described as a close relative of the last common ancestor of thecrown catarrhines, and tentatively dated to 29–28 million years ago, helping to fill an 11-million-year gap in the fossil record.[11] Notable species also includeNsungwepithecusgunnelli andRukwapithecus fleaglei of the Oligocene.[12]
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In the earlyMiocene, about 22 million years ago, the many kinds of arboreally adapted primitive catarrhines from East Africa suggest a long history of prior diversification. Fossils dated to be 20 million years old include fragments attributed toVictoriapithecus, believed to be the earliest Old World monkey.[citation needed] Among the genera thought to be in the ape lineage leading up to 13 million years ago areProconsul,Rangwapithecus,Dendropithecus,Limnopithecus,Nacholapithecus,Equatorius,Nyanzapithecus,Afropithecus,Heliopithecus, andKenyapithecus, all from East Africa.
The presence of other generalized non-cercopithecids of the middle Miocene age from sites far distant—Otavipithecus from cave deposits in Namibia, andPierolapithecus andDryopithecus from France, Spain and Austria—is evidence of a wide diversity of forms across Africa and the Mediterranean basin during the relatively warm and equable climatic regimes of the early and middle Miocene. The youngest of the Miocene hominoids,Oreopithecus, is from coal beds in Italy that have been dated to 9 million years ago.
Molecular evidence indicates that the lineage ofgibbons (familyHylobatidae) diverged from Great Apes some 18–12 million years ago, and that oforangutans (subfamily Ponginae) diverged from the other Great Apes at about 12 million years; there are no fossils that clearly document the ancestry of gibbons, which may have originated in a so-far-unknown South East Asian hominoid population, but fossil proto-orangutans may be represented bySivapithecus from India andGriphopithecus from Turkey, dated to around 10 million years ago.[13]
Human evolution is theevolutionary process that led to the emergence ofanatomically modern humans, beginning with theevolutionary history ofprimates – in particulargenusHomo – and leading to the emergence ofHomo sapiens as a distinct species of thehominid family, the great apes. This process involved the gradual development of traits such ashuman bipedalism andlanguage.[14]
The study ofhuman evolution involves many scientific disciplines, includingphysical anthropology,primatology,archaeology,paleontology,neurobiology,ethology,linguistics,evolutionary psychology,embryology andgenetics.[15] Early genetic studies suggested that primates diverged from othermammals about85 million years ago, but newer research questions this suggesting a date possibly in thePaleocene,[16] consistent with the earliestfossils being found, around55 million years ago.[17]
Within the superfamilyHominoidea (apes), thefamilyHominidae diverged from the familyHylobatidae (gibbons) some 15–20 million years ago; Africangreat apes (subfamilyHomininae) diverged fromorangutans (Ponginae) about14 million years ago; the tribeHominini (humans,Australopithecines and other extinct biped genera, andchimpanzee) parted from the tribeGorillini (gorillas) between9 million years ago and8 million years ago; and, in turn, the subtribesHominina (humans and biped ancestors) andPanina (chimpanzees) separated about7.5 million years ago to5.6 million years ago.[18]
In primates, thepelvis consists of four parts—the left and the righthip bones which meet in the mid-line ventrally and are fixed to the sacrum dorsally and thecoccyx. Each hip bone consists of three components, theilium, theischium, and thepubis, and at the time of sexual maturity these bones become fused together, though there is never any movement between them. In humans, the ventral joint of the pubic bones is closed.
The most striking feature of evolution of the pelvis in primates is the widening and the shortening of the blade called theilium. Because of the stresses involved in bipedal locomotion, the muscles of the thigh move the thigh forward and backward, providing the power for bi-pedal and quadrupedal locomotion.[19]
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