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Discovery of 13 million-year-old ape skull shows what human ancestors may have looked like

Watch an animation of a 13 million-year-old skull (Video: Paul Tafforeau /ESRF)

To the untrained eye, the area west of Lake Turkana in northern Kenya appears to be barren of anything but rocky hills and volcanic ash.

But anthropologists know the Napudet region of the Turkana Basin as a promising new dig site for fossils from the Middle Miocene era, about 13 million years ago. And one professor's persistence there would pay off in a monumental discovery: a rare, complete skull of a baby ape that could give scientists a glimpse at what our common ancestors looked like.

The discovery almost didn't happen.

When Isaiah Nengo, an anthropology professor at De Anza College in California, sought to assemble a team for a three-week expedition there in 2014, no one wanted to go.

“There was nothing useful to be found,” others told Nengo, who also teaches at the Stony Brook University-affiliated Turkana Basin Institute.

Undeterred, Nengo, who had just spent two years at the University of Nairobi on a Fulbright scholarship, returned to Kenya and gathered a ragtag group of local fossil finders. There were six of them in total, including the camp cook.

For two weeks in August, the team dug and found … nothing. Though Nengo knew it wasn't unusual for the site (“You could go for days and days, weeks and weeks without finding anything"), he began hoping to come across some fossil scraps or bone fragments — anything to make the expedition worth it.

On Sept. 4, 2014, the team once again worked for hours at the dig site and came up empty-handed. Exhausted and disappointed, the crew packed up and began walking back to their land cruiser, parked about a mile away from where they had been working.

One team member, Kenyan fossil hunter John Ekusi, pulled out some tobacco and began rolling a cigarette.

“Man, you're gonna kill us with that smoke,” Nengo told him.

Ekusi ambled ahead until he was a couple hundred yards away from the group. After a short while, Nengo noticed Ekusi had stopped, and was inspecting something with a familiar fervor.

“If you're a fossil finder, you know that look,” he said. “It's like an atomic bomb can go off, and you don't care, you're so focused at what you're looking for.”

By the time the group caught up with Ekusi, he had brushed out the top of a fossil.

“Almost instantly we knew it was the skull of a primate,” Nengo said. “We just broke into a dance, we were so happy.”

What the team later excavated would end up being what is thought to be the most complete skull of an extinct ape species in the fossil record. After more than two years of sophisticated imaging work and additional geological research at the dig site, the discovery was published in the Aug. 10 issue of the journal Nature.

According to the article, “younger” fossil finds — those 6- to 7-million-years-old — have shed light on humans' common ancestors with chimpanzees. However, far less is known about the common ancestors of all living apes and humans from before 10 million years ago.

“Relevant fossils are scarce, consisting mostly of isolated teeth and partial jaw bones,” a statement accompanying the Nature article reads. “It has therefore been difficult to find answers to two fundamental questions: Did the common ancestor of living apes and humans originate in Africa, and what did these early ancestors look like?”

The discovery of the infant ape skull — nicknamed “Alesi” after the local Turkana word for “ancestor” — helps bridge some of those gaps, not only because of how intact the outside of the skull is but for what was preserved on the inside.

In September 2015, about a year after the fossil was excavated, Nengo obtained government clearance to hand-carry the skull from Kenya to the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. It was, he would later admit, one of the most nerve-racking air travel experiences he had ever had.

“I sat with that specimen in my lap all the way until we got to Grenoble,” Nengo said. “It did not leave my sight. If I was in the bathroom, it went with me.”

At the facility, which produces “the world's most intense X-rays,” scientists scanned the skull and arrived at startlingly clear 3-D images of the what it held.

“We were able to reveal the brain cavity, the inner ears and the unerupted adult teeth with their daily record of growth lines,” Paul Tafforeau, an ESRF scientist, said in a statement. “The quality of our images was so good that we could establish from the teeth that the infant was about 1 year and 4 months old when it died.”

At first, researchers suspected Alesi had been a baby gibbon because of the small snout. However, once scans revealed fully developed bony inner ear tubes and the unerupted adult teeth, it was clear Alesi had been an ape.

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

Alesi's teeth showed that the infant skull hadn't just belonged to just any ape, but one of a previously undiscovered species, now named Nyanzapithecus alesi. Up until then, scientists hadn't been certain if the Nyanzapithecus species were apes at all, or whether they had originated in Asia or Africa. Now, Nengo said, they could conclude that N. alesi had been part of a group of primates that lived more than 10 million years ago, and that they had originated in Africa.

“It's always very important to know when you're looking for ancestral lineages which continent they evolved. It helps you to explain the evolution of that particular group,” Nengo said. Alesi provides an important link between apes' and humans' common ancestors and the earliest humans. 

“To find this little baby that perished in volcanic ash 13 million years ago … it's a glimpse of what our prehuman stage looked like.”

Alesi is now back in Kenya. Nengo said he plans to continue fieldwork there and also to use Alesi as “kind of an anchor” for the study of babies and the role of babies in the evolution of apes and humans.

“The real work is coming now,” he said.

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