Orrorin

Orrorin tugenensis is considered to be the second-oldest known hominin ancestor that is possibly related to modern humans and is the only species classified in genus Orrorin. The name was given by the discoverers who found Orrorin fossils in the Tugen Hills of Kenya. By using radiometric dating techniques, the volcanic tuffs and lavas, faunal correlation and magneto-stratigraphy, the strata in which the fossils were found were estimated to date between 6.1 and 5.8 million years ago, during the Miocene. This find is important because it is possibly an early bipedal hominin.

The fossils found so far come from at least five individuals. They include a proximal femur, which is insufficient evidence to prove that it was bipedal, though some scholars suggest that Orrorin walked upright; a right humerus shaft, suggestive of tree-climbing skills but not brachiation; and teeth that suggest a diet similar to Paranthropoids. The obturator externus groove on the posterior aspect of the neck of the fossil femur suggests that Orrorin tugenensis moved bipedally. The bunodont, microdont molars and small canines suggest that Orrorin ate mostly fruit and vegetables, with occasional meat. Orrorin was about the size of a modern chimpanzee.

The team that found these fossils in 2000 was led by Brigitte Senut and Martin Pickford from the Muséum national d'histoire naturelle. The discoverers conclude that Orrorin is a hominin on the basis of its bipedal locomotion and dental anatomy; based on this, they date the split between hominins and African great apes to at least 7 million years ago, in the Messinian. This date is markedly different from those derived using the molecular clock approach, but has found general acceptance among paleoanthropologists.

If Orrorin proves to be a direct human ancestor, then 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 H. sapiens than is Lucy's; there is, however, some debate over this point.

Other fossils (leaves and many mammals) found in the Lukeino Formation show that Orrorin lived in dry evergreen forest environment, not the savanna assumed by many theories of human evolution. Thus, the origins of bipedalism occurred in an arboreal precursor living in forest and not a quadrupedal ancestor living in open country. A recently published idea suggests that ancestral apes may have shared the technique used by modern orangutans of moving bipedally over small springy branches with the vertebral column oriented vertically (orthograde), using their arms for balance and keeping their legs straight. This kind of upright locomotion could have been used as a way of getting around on the ground when gaps opened in the forest canopy. Our closest extant relatives the gorillas and chimpanzees developed a flexed stance (with clinograde, (sloping) vertebral column) and are more adapted to tree climbing and to quadrupedal locomotion while on the ground. According to a minority of researchers, like humans, they have fused and strengthened wrist bones suggesting a shared period of knuckle walking.

After the fossils were found in 2000, they were held at the Kipsaraman village community museum, but the museum was closed later. Since then, according to the Community Museums of Kenya chairman Eustace Kitonga, the fossils are stored at a secret bank vault in Nairobi.