Nicknamed "millennium ancestor", fossil remains for Orrorin tugenensis that have been found at Tugen Hills, Kenya, exhibits a combination of primitive ape-like upper limb morphology and derived lower limb morphology. The proximal femur of O. tugenensis exhibits morphology consistent with bipedalism (e.g. a larger femoral head and longer femoral neck length), which, if true, may offer the oldest definitive evidence for hominin bipedalism. O. tugenensis has reduced canines, cheek teeth smaller than Australopithecus, and appears to lack a honing complex.

O. tugenensis lived at the same time as the last common ancestor to apes and humans. Some anthropologists consider O. tugenensis a basal hominin that is ancestral to modern humans and unrelated to australopithecines. Others believe O. tugenensis is an extinct off-branch of the human lineage that left behind no descendants.

The type specimen for O. tugenensis, BAR 1000’00, was found at Tugen Hills and dates to approximately 6 million years ago.