Researchers analyzing new gorilla-like fossils from Ethiopia suggest that the human and gorilla lineages split around 10 million years ago. Based on genomic modeling and previous fossil finds, we thought that humans and great apes diverged relatively recently: The human-chimpanzee split was set at 4 million to 6 million years ago, while we split from gorillas less than 8 million years ago. The new findings are published in Nature this week.
Fossils from between 12 million and 7 million years ago are crucial for helping us understand the origins of ape and human ancestors, but assemblages from this period have rarely been reported in sub-Saharan Africa. One of the few key windows into Late Miocene mammalian evolution we do have is Ethiopia’s Chorora Formation, which extends for 100 kilometers (62 miles) at the transition zone between the southern Afar Rift and the northern Main Ethiopian Rift. Researchers in the 1970s originally dated the Chorora Formation at about 10.5 million years old, though work in the decades since have raised some doubts about this age.
In 2006, researchers working at Beticha near the Chorora Formation unearthed fossil teeth that belonged to an extinct ape they named Chororapithecus abyssinicus. This species shares a suite of molar features with gorillas, and they’re thought to represent an early phase of increased herbivory. If Chororapithecus is a primitive member of the gorilla lineage, knowing its exact age would be important for constraining the timing of the gorilla–human divergence.
Now, University of Tokyo’s Gen Suwa and colleagues have uncovered new fossils from the Chorora Formation, and they managed to date them using a combination of geochemical, magnetostratigraphic, and radioisotopic techniques. Chorora Formation sediments date to between 9 million and 7 million years, and the Chororapithecus fossils date back to 8 million years – which means that the human–gorilla split must have occurred closer to 10 million years ago.
These findings also suggest that the evolution of apes (and thus humans) took place in Africa – and not in Eurasia as recent work suggested.