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Orrorin tugenensis is considered as the second oldest possible hominin ancestor related to modern humans (the oldest being Sahelanthropus tchadensis) 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 where the fossils were found date to between 6.1 and 5.8 million years ago, during the Miocene. This find is very significant because it could represent one of the earliest fossils with evidence of bipedal locomotion in human ancestors.

The fossils found so far come from at least five individuals. They include a femur, suggesting that Orrorin walked upright; a thick right humerus, suggestive of tree-climbing skills but not brachiation; and teeth that suggest a diet much like that of modern humans. The fact that the fossil Orrorin tugenensis possesses the obturator externus groove on the posterior neck of the femur suggests that it moved bipedally. The full 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 Martin Pickford. Pickford claims that Orrorin is clearly a hominin; based on this, he dates the split between hominins and other African great apes to at least 7 million years ago. This date is markedly different from those derived using the molecular clock approach.

If Orrorin proves to be a direct human ancestor, the australopithecines such as Australopithecus afarensis ("Lucy") may be considered a side branch of the hominid family tree: Orrorin is both earlier, by over 1.5 million years, and more similar to us than A. afarensis. The main similarity is that the Orrorin humerus seems closer to H. sapiens in comparison to Lucy's; there is, however, significant controversy over this point, and other researchers assert that Pickford et al. gloss over a number of uncertainties.

Other fossils found in these rocks show that Orrorin lived in a wooded environment, not the savanna assumed by many theories of human evolution and, in particular, the origins of bipedalism. A recent theory suggests that ancestral apes shared the technique used by modern orangutans of moving bipedally over small springy branches, using their arms for balance and keeping their legs straight. This upright walking could have been used as a way of getting around on the ground when gaps opened in the forest canopy, so that bipedalism had evolved by the time of Orrorin. Our closest relatives the gorillas and chimpanzees developed a flexed stance and are more adapted to tree climbing, though like humans they have fused and strengthened wrist bones suggesting a shared period of knuckle walking.^[1][2][3]

• B. Senut, M. Pickford, D. Gommery, P. Mein, K. Cheboi, and Y. Coppens, "First hominid from the Miocene (Lukeino Formation, Kenya)". Comptes Rendus de l'Académie de Sciences, vol. 332, pp. 137-144, 2001.
• Orrorin Tugenensis: Pushing back the hominin line

• Martin Pickford answers a few questions about this month's fast breaking paper in field of Geosciences
• BBC News: First chimpanzee fossils found