Is This Baby Animal the Last Common Ancestor of Humans and Apes?

The 13-million-year-old skull found in Kenya combines early ape and gibbon-like features

Nyanzapithecus alesi
Nyanzapithecus alesi skull Fred Spoor

About 13 million years ago, when present-day Kenya was covered in forests, a baby ape died. Its tiny corpse was covered in ash from a nearby volcano, helping to perfectly preserve its fragile cranium. Now, reports Michael Greshko at National Geographic, that baseball-sized skull is giving researchers insight into a little understood period when the human and ape lineages split.

As Greshko reports, between 25 to 28 million years ago, apes diverged from Old World monkeys before splitting into many different lineages. While most of those evolutionary branches died off about 7 million years ago as the climate changed, one line remained, later branching into great apes, like chimps, gorillas and eventually humans. Reconstructing the history of that branch, however, has been difficult, mainly because the forests those common ancestors once lived  weren’t great at preserving fossils. Researchers have found bits of jaw, facial bones and foreheads, but a complete cranium is an almost miraculous find.

During an expedition three years ago, Kenyan fossil hunter John Ekusi discovered the infant skull in the Turkana Basin of northern Kenya, reports Michael Price at Science. Dating suggests that the skull was some 13 million years old and dental rings showed the creature was just one year, four months old when it perished. The shape of the teeth also showed it was a new species in the genus Nyanzapithecus, given a species designation of alesi. The research appears in the journal Nature.

As George Dvorsky at Gizmodo reports, the researchers believe N. alesi is the oldest common ancestor of human and apes discovered so far. If the animal was fully grown it would have weighed in at 25 pounds and looked like a gibbon. But a tiny semicircular canal in the skull suggests was markedly different from gibbons, reports Dvorsky. In tree-dwelling primates, like gibbons, the canal is larger and helps the animals keep their balance and orientation as they swing through the trees. So N. alesi was likely a slower-moving primate.

“Gibbons are well known for their fast and acrobatic behavior in trees,” co-author Fred Spoor of University College London and the Max Planck Institute of Evolutionary Anthropology says in a press release. “But the inner ears of N. alesi show that it would have had a much more cautious way of moving around.”

Brenda Benefit, an anthropologist at New Mexico State University, who was not involved in the study, tells Dvorsky that N. alesi combines some traits of the great apes with more primitive gibbon-like traits. But the inner ear helps researchers know that fossil comes from a period after monkeys and early apes diverged.

According to Price at Science, placing N. alesi in the ape-ancestor camp helps answer a big question in paleontology: whether the common ancestor of hominins and apes evolved in Africa or somewhere in Eurasia.

Determining this last common ancestor of apes and humans is also important in figuring out what pressures, such as climate, geography and ecology, led to the emergence of early human ancestors, reports Charles Q. Choi at LiveScience, 

“The living apes are found all across Africa and Asia—chimps and gorillas in Africa, orangutans and gibbons in Asia—and there are many fossil apes found on both continents, and Europe as well,” Christopher Gilbert, paleoanthropologist at Hunter College in New York and co-author of the paper, tells Choi. “So, as you can imagine, there are numerous possibilities for how that distribution came to be, and different researchers have suggested different hypotheses for where the common ancestor of the living apes and humans might be found.”

The discovery of N. alesi seems to squarely place that lineage in Kenya. But not everyone is convinced by the little skull. For one, David Begun, anthropologist at the University of Toronto in Canada argues that human and ape ancestors evolved in Europe before moving into Africa. He tells Dvorsky that he believes other ape specimens, including Proconsul and Ekembo are also good candidates for being the last common ancestor. In fact, he points out that paleontologists previously found a 17-million-year old specimen of Nyanzapithecus. “It therefore does not mean that the last common ancestor of all living apes lived 13 million years ago, the age of this fossil,” he says. “It was much older than that.”

There's just one solution: find more skulls. And, as Greshko reports, the researchers saw hints that more fossils may be trapped in the ash layer. They hope to soon return to look for more.

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