An ancestor's ancestor
In the Tugen Hills of central Kenya, paleontologists unearthed a fossil that changes the history of ape evolution. The partial skeleton, specimen KNM-TH 28860, belongs to Equatorius africanus, a hominoid that lived approximately 15.4 to 15.6 million years ago. This was the Middle Miocene, an era of dense forests, long before the open savannas that are linked to primates moving from the trees to the ground. The discovery of a semi-terrestrial ape from this period challenges the long-held idea that our ancestors' move to ground-level living was a direct result of shrinking forests.
The skeleton of Equatorius provides the earliest clear evidence for a shift away from a life spent entirely in the trees. Its anatomy is varied, combining features of earlier, tree-dwelling apes like Proconsul with new adaptations for life on the forest floor. Analysis of its hand bones, specifically the ratio between the phalanx and metacarpal lengths, shows proportions that are intermediate between fully arboreal monkeys and habitually terrestrial ones like baboons. The finger bones are less curved than those of climbing apes, and the vertebral column was more rigid, suggesting a body built for quadrupedal movement on solid ground.