More than 13 million years ago in what’s now northern Kenya, an infant ape ended up dead in a lush forest, its body blanketed in ashfall from a nearby volcanic eruption.
Millions of years later, scientists uncovered the baby ape’s skull, the best-preserved of its kind ever found, and got an extraordinarily glimpse into the early stages of ape evolution.
“We’ve been looking for ape fossils for years—this is the first time we’re getting a skull that’s complete,” says Isaiah Nengo, the De Anza College anthropologist who led the discovery, supported by a National Geographic Society grant and the Stony Brook University-affiliated Turkana Basin Institute.
Roughly the size of a lemon, the skull belongs to a newly identified species of early ape named Nyanzapithecus alesi. Some of its features resemble those of today’s living Old World monkeys and apes, and the face bears a striking resemblance to today’s infant gibbons.
What’s more, N. alesi offers insight into early apes’ brains, the team reports in their study, published today in Nature. With a volume of about seven tablespoons, N. alesi’s brain cavity was more than double that of other Old World monkeys from the time.
And the skull’s intact braincase, which has preserved impressions of the brain’s exterior surface, also contains the infant ape’s unerupted adult teeth.
After diverging from Old World monkeys’ ancestors between 25 and 28 million years ago, apes diversified near the middle of the Miocene epoch. Many of those lineages, however, died out roughly 7 million years ago amid a bout of natural climate change. Modern great apes and humans are the descendants of one of the surviving Miocene ape lineages.
The details of this evolutionary tale have remained murky, however, in part because early apes lived in rainforests, which rarely offer conditions favorable to fossilization. Until N. alesi, only one other Miocene ape skull had been found with the brain case, or neurocranium, intact.
“For those species where we have any of the cranium at all, often they’ll have jaws, the face, and sometimes the very beginning of the forehead bone,” says Brenda Benefit, a New Mexico State University anthropologist who reviewed the study before publication. “You do not get a complete neurocranium—that’s unheard of.”
Finding N. alesi took determination and a fantastic stroke of luck. The Leakeys, a family of preeminent paleoanthropologists, had previously excavated northern Kenya’s Napudet dig site. When Nengo took over excavations in 2013, few had high hopes that he would find much of note.
But one day in early 2014, one of the expedition’s assistants, John Ekusi, walked away from the crew to smoke a hand-rolled cigarette. The rest of the team grew puzzled as they watched Ekusi from afar: After a few minutes, he began circling something on the ground that had captured his attention.
Ekusi told the rest of the crew that he may have spotted the head of an elephant femur, gesturing to a rounded bony surface peeking out of the rock. Closer inspection revealed a far rarer find: a small ape skull, only gently squashed from its true-to-life proportions. The crew broke out into a dance in excitement.
With night fast approaching, however, the crew was forced to rebury the skull and wait until the next morning to excavate it. “I tell you, nobody was sleeping that night,” Nengo says.
Dating the sediment layer around the fossil told the team that the ape skull is about 13 million years old. But even with the skull’s fantastic preservation, initial inspection of the prepared fossil couldn’t confirm where the skull belonged on the primate family tree.
To pin that down, Nengo and his colleagues needed glimpses of its adult teeth, which hadn’t yet erupted. So the team took their find to the European Synchrotron Radiation Facility in Grenoble, France, where technicians subjected it to high-powered x-rays that allowed them to peer into the skull without damaging it.
The scans gave Nengo’s team 3-D reconstructions of the teeth, and their distinctive shapes firmly placed the skull among the nyanzapithecenes, an extinct sister group to gibbons, great apes, and humans.
“If they had not done [the synchrotron scans], they never would have been able to identify this,” says Benefit. “To me, that’s a miracle of modern technology.”
Now that N. alesi has been unveiled, Nengo is brimming with ideas for what aspect to study next. He and his colleagues are gearing up to examine the brain impressions on the skull’s interior. They are also doubling back to the ape’s exquisitely preserved ear and are working to reconstruct how N. alesi would have looked in life.
Nengo also plans to go back to Napudet, to track down other fossils he saw hints of in the ancient rock.
“That’s the plan,” he says. “There’re a few interesting things to do.”