| Orrorin | |
|---|---|
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| Cast ofO. tugenensis femur (BAR 1002'00),National Museum of Natural History | |
| Thedistal phalanx of the thumb ofO. tugenensis | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Primates |
| Suborder: | Haplorhini |
| Infraorder: | Simiiformes |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini |
| Genus: | †Orrorin Senut et al. 2001 |
| Type species | |
| †Orrorin tugenensis Senutet al., 2001 | |
| Other species | |
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| Synonyms | |
Orrorin is anextinctgenus ofprimate withinHomininae from theMioceneLukeino Formation andPlioceneMabaget Formation, both ofKenya.
The type species isO. tugenenesis, named in 2001,[1] and a second species,O. praegens,[2] assigned to the genus in 2022.[3]
The first part of theholotype, a lower molar, was discovered byMartin Pickford in 1974 and described by Pickford (1975).[4]
The team that found the rest of the holotype ofO. tugenensis was led byBrigitte Senut andMartin Pickford from the FrenchNational Museum of Natural History.[1] Starting from 17 October 2000, 20 fossils were found at four sites in theLukeino Formation, Kenya: of these, the fossils atCheboit andAragai are the oldest (6.1 Ma), while those inKapsomin andKapcheberek are found in the upper levels of the formation (5.7 Ma).[5]
Orrorin tugenensis was named and described by Senutet al. (2001).[1]
The second species,O. praegens, was first described by Ward (1985)[6] and Ward & Hill (1988),[7] and was initially described asHomoantiquus praegens by Ferguson (1989)[2] based on specimen KNM-TH 13150, a mandible discovered in thePlioceneMabaget Formation ofKenya during the early 1980s.[8] The mandible is known as the Tabarin mandible, which was previously classified withinArdipithecus ramidus (or cf.A. cf.ramidus),"Ardipithecus" praegens or"Praeanthropus" praegens.
Several referred remains ofO. praegens were collected between 2005 and 2011 by the Franco-Kenyan Kenya Palaeontology Expedition and they, alongside the Tabarin mandible, were classified byPickfordet al. (2022) as being separate fromHomo, so they were classified withinOrrorin asO. praegens.[3]
The name of genusOrrorin (pluralOrroriek) means "original man" inTugen,[1][9] and theepithet ofO. tugenensis derives fromTugen Hills inKenya, where the firstfossil was found in 2000.[9]
The epithet ofO. praegens means roughly “group of people who came before.”[3]
The 20 specimens belonging toO. tugenensis are believed to be from at least five individuals.[10] They include: the posterior part of amandible in two pieces; asymphysis and several isolated teeth; three fragments offemora; a partialhumerus; a proximalphalanx; and a distal thumb phalanx.[5]
Orrorin had small teeth relative to its body size. Its dentition differs from that found inAustralopithecus in that itscheek teeth are smaller and less elongatedmesiodistally and fromArdipithecus in that itsenamel is thicker. The dentition differs from both these species in the presence of a mesial groove on the upper canines. Thecanines are ape-like but reduced, like those found in Miocene apes and female chimpanzees.Orrorin had small post-canines and wasmicrodont, like modern humans, whereas australopithecines weremegadont.[5] However, some researchers have denied that this is compelling evidence thatOrrorin was more closely related to modern humans than australopithecines as early members of the genusHomo, who were almost certainly the direct ancestors of modern humans, were also megadonts.[11]
In the femur, thehead is spherical and rotated anteriorly; theneck is elongated and oval in section and thelesser trochanter protrudes medially. While these suggest thatOrrorin was bipedal, the rest of thepostcranium indicates it climbed trees. While the proximal phalanx is curved, the distalpollical phalanx is of human proportions and has thus been associated with toolmaking, but should probably be associated with grasping abilities useful for tree-climbing in this context.[5]
After the fossils were found in 2000, they were held at the Kipsaraman village community museum, but the museum was subsequently closed. Since then, according to the Community Museums of Kenya chairman Eustace Kitonga, the fossils are stored at a secret bank vault in Nairobi.[12]
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IfOrrorin proves to be a direct human ancestor, then according to some paleoanthropologists,australopithecines such asAustralopithecus afarensis ("Lucy") may be considered a side branch of the hominid family tree:Orrorin is both earlier, by almost 3 million years, and more similar to modern humans than isA. afarensis. The main similarity is that theOrrorin femur is morphologically closer to that ofHomo sapiens than isLucy's; there is, however, some debate over this point.[13] This debate is largely centered around the fact that Lucy was female and theOrrorin femur it has been compared to belonged to a male.[11]
Another point of view cites comparisons between Orrorin and otherMiocene apes, rather than extant great apes, which shows instead that the femur shows itself as an intermediate between that of Australopiths and said earlier apes.[14]
Other fossils (leaves and many mammals) found in the Lukeino Formation show thatOrrorin lived in a dry evergreen forest environment, not thesavanna assumed by many theories ofhuman evolution.[13]
The fossils ofOrrorin tugenensis share no derived features of hominoid great-ape relatives.[15] In contrast, "Orrorin shares several apomorphic features with modern humans, as well as some with australopithecines, including the presence of anobturator externus groove, elongated femoral neck, anteriorly twisted head (posterior twist inAustralopithecus),anteroposteriorly compressed femoral neck, asymmetric distribution of cortex in the femoral neck, shallow superior notch, and a well developed gluteal tuberosity which coalesces vertically with the crest that descends the femoral shaft posteriorly."[15] It does, however, also share many of such properties with several Miocene ape species, even showing some transitional elements between basal apes like theAegyptopithecus and Australopithecus.[14]
According to recent studiesOrrorin tugenensis is a basal hominid that adapted an early form of bipedalism.[16] Based on the structure of its femoral head it still exhibited some arboreal properties, likely to forage and build shelters.[16] The length of the femoral neck inOrrorin tugenensis fossils is elongated and is similar in shape and length to modern humans andAustralopithicines.[15] While it was originally claimed that its femoral head is larger in comparison toAustralopithicines and is much closer in shape and relative size toHomo sapiens,[15] this claim has been challenged by some researchers who have noted that the femoral heads of male australopithicines are more akin to those ofOrrorin, and by extension modern humans, than those of female australopithicines. Proponents of the notion thatOrrorin is more closely related to humans than Lucy is have addressed this by asserting that the male australopithicine femurs in question in fact belong to a different species than Lucy.[11]O. tugenensis appears to have developed bipedalism 6 million years ago.[16]
O. tugenensis shares an early hominin feature in which their iliac blade is flared to help counter the torque of their body weight; this shows that they adapted bipedalism around 6 MYA.[16] These features are shared with many species ofAustralopithecus.[16] It has been suggested by Pickford that the many featuresOrrorin shares with modern humans show that it is more closely related toHomosapiens than toAustralopithecus.[15] This would mean that Australopithecus would represent a side branch in the homin evolution that does not directly lead toHomo.[15] However the femora morphology ofO. tugenensis shares many similarities withAustralopithicine femora morphology, which weakens this claim.[16] Another study conducted by Almecija suggested thatOrrorin is more closely related to early hominins than toHomo.[14] An analysis of the BAR 10020' 00 femur showed thatOrrorin is an intermediate betweenPan andAustralopithecus afarensis.[14] The current prevailing theory is thatOrrorin tugenensis is a basal hominin and that bipedalism developed early in the hominin clade and successfully evolved down the human evolutionary tree.[16] While the phylogeny ofOrrorin is uncertain, the evidence of the evolution of bipedalism is an invaluable discovery from this early fossil hominin. A recent phylogenetic analysis also recoveredOrrorin as a hominin.[17]
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