|The illustration of Orrorin tugenensis fossils|
|The distal phalanx of the thumb|
Senut et al. 2001
Orrorin tugenensis is a postulated early species of Homininae, estimated at and discovered in 2000. It is not confirmed how Orrorin is related to modern humans. Its discovery was used to argue against the hypothesis that australopithecines are human ancestors, as much as it still remains the most prevalent hypothesis of human evolution as of 2012.
The name of genus Orrorin (plural Orroriek) means "original man" in Tugen, and the name of the only classified species, O. tugenensis, derives from Tugen Hills in Kenya, where the first fossil was found in 2000. As of 2007, 20 fossils of the species have been found.
The 20 specimens found as of 2007 include: the posterior part of a mandible in two pieces; a symphysis and several isolated teeth; three fragments of femora; a partial humerus; a proximal phalanx; and a distal thumb phalanx.
Orrorin had small teeth relative to its body size. Its dentition differs from that found in Australopithecus in that its cheek teeth are smaller and less elongated mesiodistally and from Ardipithecus in that its enamel is thicker. The dentition differs from both these species in the presence of a mesial groove on the upper canines. The canines are ape-like but reduced, like those found in Miocene apes and female chimpanzees. Orrorin had small post-canines and was microdont, like modern humans, whereas robust australopithecines were megadont.
In the femur, the head is spherical and rotated anteriorly; the neck is elongated and oval in section and the lesser trochanter protrudes medially. While these suggest that Orrorin was bipedal, the rest of the postcranium indicates it climbed trees. While the proximal phalanx is curved, the distal pollical 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.
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.
If Orrorin proves to be a direct human ancestor, then according to some paleoanthropologists, australopithecines such as Australopithecus 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 is A. afarensis. The main similarity is that the Orrorin femur is morphologically closer to that of Homo sapiens than is Lucy's; there is, however, some debate over this point.
However, another point of view cites comparisons between Orrorin and other Miocene 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.
Evolution of bipedalism
The fossils of Orrorin tugenensis shares no derived features of hominoid great-ape relatives. In contrast, "Orrorin shares several apomorphic features with modern humans, as well as some with australopithecines, including the presence of an obturator externus groove, elongated femoral neck, anteriorly twisted head (posterior twist in Australopithecus), anteroposteriorly compressed femoral neck, asymmetric distribution of cortexin the femoral neck, shallow superior notch, and a well developed gluteal tuberosity which coalesces vertically with the crest that descends the femoral shaft poste-riorly." It does, however, also share many of such properties with several Miocene ape species, even showing some transitional elements between basal apes like the Aegypropithecus and Australopithecus. According to recent studies Orrorin tugenensis is a basal hominid that adapted an early form of bipedalism. Based on the structure of its femoral head it still exhibited some arboreal properties, likely to forage and build shelters. The length of the femoral neck in Orrorin tugenensis fossils is elongated and is similar in shape and length to Australopithicines and modern humans. Additionally, its femoral head is larger in comparison to Australopithicines and is much closer in shape and relative size to Homo sapiens. This archaic morphology suggests that O. tugenensis developed bipedalism 6 million years ago.
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. These features are shared with many species of Australopithecus. It has been suggested by Pickford that the many features Orrorin shares with modern humans show that it is more closely related to Homo sapiens than to Australopithecus. This would mean that Australopithecus would represent a side branch in the homin evolution that does not directly lead to Homo. However the femora morphology of O. tugenensis shares many similarities with Australopithicine femora morphology, which weakens this claim. Another study conducted by Almecija suggested that Orrorin is more closely related to early hominins than to Homo. An analysis of the BAR 10020' 00 femur showed that Orrorin is an intermediate between Pan and Australopithecus afarensis. The current prevailing theory is that Orrorin tugenensis is a basal hominin and that bipedalism developed early in the hominin clade and successfully evolved down the human evolutionary tree. It is clear that the phylogeny of Orrorin is uncertain, however the evidence of the evolution of bipedalism is an invaluable discovery from this early fossil hominin.
The team that found these fossils in 2000 was led by Brigitte Senut and Martin Pickford from the Muséum national d'histoire naturelle. The 20 fossils have been found at four sites in the Lukeino Formation, located in Kenya: of these, the fossils at Cheboit and Aragai are the oldest ( ), while those in Kapsomin and Kapcheberek are found in the upper levels of the formation ( ).
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