Ancient infant skull yields insights into human-ape lineage

Ancient infant skull yields insights into human-ape lineage

Researchers believe a 13 million-year-old skull recovered in Kenya belongs to the earliest common ancestor of humans and all living apes. It likely belonged to a fruit-eating, slow-climbing primate that resembled a baby gibbon, the researchers said. 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. An astounding dearth of paleontological evidence exists for the Miocene period, an important era that ran from 23 to five million years ago. In a new study published today in Nature, a Stony Brook University research team led by Isaiah Nengo describe the almost complete skull, showing a number of adaptations that would go on to influence ape and human evolutionary histories.

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. "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".

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.

"We have a attractive ape cranium (skull) from a period that we knew virtually nothing about, and this is one of those wonderful cases where discovery leads to all sorts of new and interesting perspectives", says Craig Feibel, a professor of geology and anthropology at Rutgers University. Reconstructing the history of that branch, however, has been hard, mainly because the forests those common ancestors once lived weren't great at preserving fossils. "It also provided us with the critical volcanic minerals by which we were able to date the fossil".

Our common ancestor with chimpanzees lived in Africa six to seven million years ago, and many spectacular fossil finds have revealed how humans evolved since then. Ellen Miller, an anthropology professor at Wake Forest University, contributed to the study. "We never had information on that before - it was always a mystery".

The lemon-size skull still had the roots of its baby teeth, and none of the adult teeth had erupted from the jaw yet. 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. They also said that several aspects of the new species link it to living apes.

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Alesi's teeth showed researchers that it's a new species. 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.

"The majority of that group, and the oldest members of that group, are African but we would not have been able to resolve all of that without Alesi", said Nengo.

"There was some discussion for a while about whether the modern apes actually originated in Africa or in Eurasia, because gibbons today live in Southeast Asia, and this pretty squarely confirms that the origin of apes was in Africa".

The researchers can not tell if Alesi was male or female, as the infant was too young for the features of the skull that distinguish the sexes to have emerged, the researchers said. If the animal was fully grown it would have weighed in at 25 pounds and looked like a gibbon. "Because they are probably close to the ancestor of all living apes, the specimen may help give us some sort of idea of what the common ancestor of all living apes and modern humans might have looked like, and because our specimen looks most similar to gibbons among living apes, it would potentially support the idea that the common ancestor of living apes and humans looked like a gibbon", Gilbert said.

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. After more than two years of sophisticated imaging work and additional geological research at the dig site, the discovery was published in the August 10 issue of the journal Nature.