A huge study of fossil apes clarifies which extinct species are most closely related to humans. But it can’t resolve one of the most controversial questions in human evolution: whether the last common ancestor we shared with living African apes like chimpanzees lived in Africa or Eurasia.
Primatologist at the American Museum of Natural History in New York looked at apes that lived during the Miocene epoch, between 23 and 5.3 million years ago. She focused on those from the middle and late Miocene that have been proposed to be closely related to humans and living apes.
“Miocene apes are amazingly diverse in their [physical forms], with combinations of features we don’t find in any living primate,” says at the University of Kent in Canterbury, UK, who wasn’t involved in the study.
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For each of 30 Miocene apes, Pugh measured 274 characteristics, from the sizes of their braincases to the shapes of their teeth. Pugh also looked at the genus Homo – which includes our species Homo sapiens – and Australopithecus, a genus of human-like apes that lived in Africa between about 4 and 2 million years ago. She also included other living apes including gibbons, gorillas, chimpanzees, bonobos and orangutans. She used this data to estimate which species were most closely related to which. The result is the first family tree of the Miocene apes to be published in two decades.
“It’s superb,” says at Arizona State University in Tempe. “It’s incredibly thorough, extremely well documented.”
The family tree groups the apes according to how closely they are related to us. The most distant relatives were the oldest Miocene apes, all of which lived in Africa, like the 18-million-year-old Ekembo from what is now Kenya. But by 16 million years ago, some apes were living outside Africa, in both Europe and Asia. The first group to break away includes the ancestors of modern gibbons, which today live in Asia.
The remaining apes are all classed as hominids. They include humans and other hominins, plus all the living great apes. Of these, humans are assumed to be more closely related to the African great apes – chimpanzees, bonobos and gorillas – than orangutans.
Some European hominids, including Dryopithecus from western Europe and Rudapithecus from what is now Hungary, had previously been proposed to be more closely related to humans and the living African great apes than to orangutans, implying that the common ancestor we shared with chimps and gorillas lived in Europe rather than Africa. However, Pugh found that these two apes were all much more distant cousins, on branches of the family tree that broke away before the orangutans and the African great apes split apart. She expects pushback on that, but says “I’m quite confident in it”.
The group containing the orangutans – known formally as the pongines – branched away next. The pongines included the 2-million-year-old Gigantopithecus from southern Asia, which stood at least 2.5 metres tall. Today the only surviving pongines are the three orangutan species in south-east Asia.
The Miocene apes that are most closely related to humans and the living African great apes in Pugh’s family tree are a mixed bunch. The closest is probably , which lived in what is now Kenya 9.8 million years ago and is only known from one jawbone and a handful of teeth. However, the next closest are two species of Ouranopithecus, which lived in what is now Greece and Turkey between 9.6 and 7.4 million years ago, and Graecopithecus in what is now Greece 7.2 million years ago.
This means the last common ancestor of humans, chimps and gorillas may have lived in Africa or in Eurasia. It’s “an open question”, says Pugh.
A key problem is the sparsity of ape fossils from the very end of the Miocene, especially in Africa, combined with the total lack of fossils of the African apes alive today. “We are really desperate to find fossil chimpanzees or fossil gorillas,” says Pugh.
This also means we still don’t know what the last common ancestor of humans and chimpanzees, our closest living relatives, looked like. It may have swung under branches of trees like a chimp, or it may not. . “It is very possible, from what we see in the Miocene, that living African apes and orangutans are not representative of what was found in the fossil record,” says Pugh.
Journal of Human Evolution
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