13 million-year-old infant skull may unveil human-ape ancestor

 A remarkably complete fossil skull of an infant ape, nicknamed Alesi, who lived about 13 million years ago in Kenya may reveal how the common ancestor of all living apes and humans looked like, scientists say.

Among living primates, humans are most closely related to the apes, including chimpanzees, gorillas, orangutans and gibbons, but little is known about their last common ancestor.

"We've got many spectacular fossil finds showing how humans evolved in Africa over the last six to seven million years, since our lineage split from that of the chimpanzees," said Fred Spoor, professor at University College London in the UK.

"However, the evolution of the common ancestors of living apes and humans before 10 million years ago remains elusive," said Spoor.

"Until now, relevant fossils are scarce and incomplete which is why this new discovery is extremely important for furthering our knowledge," he said.

The ape fossil, nicknamed Alesi by its discoverers, and known by its museum number KNM-NP 59050, comes from a critical time period in the African past.

It is the most complete known in the fossil record and was spotted in 13 million year-old rock layers in the Napudet area, west of Lake Turkana in northern Kenya, by Kenyan fossil hunter John Ekusi in 2014.

Analysis of the skull indicates that the specimen belonged to a new species called Nyanzapithecus alesi, named after the Turkana word for ancestor "ales."

"Nyanzapithecus alesi was part of a group of primates that existed in Africa for over 10 million years. What the discovery of Alesi shows is that this group was close to the origin of living apes and humans and that this origin was African," said Isaiah Nengo, of the Turkana Basin Institute.

Many of the most informative parts of the skull are preserved inside the fossil, and to make these visible the team used an extremely sensitive form of 3D X-ray imaging at the synchrotron facility in France.

"We were able to reveal the brain cavity, the inner ears and the unerupted adult teeth with their daily record of growth lines," said Paul Tafforeau from the European Synchrotron Radiation Facility.

"The quality of our images was so good that we could establish from the teeth that the infant was about one year and four months old when it died," said Tafforeau.

"Until now, all Nyanzapithecus species were only known from teeth and it was an open question whether or not they were even apes," said John Fleagle from Stony Brook University in the US.

Alesi's skull is about the size of a lemon, and with its notably small snout it looks most like a baby gibbon.

"This gives the initial impression that it is an extinct gibbon; however, our analyses show that this appearance is not exclusively found in gibbons, and it evolved multiple times among extinct apes, monkeys, and their relatives," said Chris Gilbert from Hunter College in the US.

That the new species was certainly not gibbon-like in the way it behaved could be shown from the balance organ inside the inner ears.

"Gibbons are well known for their fast and acrobatic behaviour in trees, but the inner ears of Alesi show that it would have had a much more cautious way of moving around," said Spoor.