Anthropologists have waited decades to find the complete cranium of a Miocene ape from Africa—one that lived in the hazy period before the human lineage split off from the common ancestors we share with chimpanzees some 7 million years ago. Now, scientists in Kenya have found their prize at last: an almost perfectly preserved skull roughly the size of a baseball. The catch? It’s from an infant. That means that although it can give scientists a rough idea of what the common ancestor to all living apes and humans would have looked like, drawing other meaningful conclusions could be challenging.
“This is the sort of thing that the fossil record loves to do to us,” says James Rossie, a biological anthropologist at the State University of New York in Stony Brook who wasn’t involved with the study. “The problem is that we learn from fossils by comparing them to others. When there are no other infant Miocene ape skulls to which to make those comparisons, your hands are tied.”
The remarkably complete skull was discovered in the Turkana Basin of northern Kenya 3 years ago. As the sun sank behind the Napudet Hills west of Lake Turkana, primate paleontologist Isaiah Nengo of De Anza College in Cupertino, California, and his team started walking back to their jeep. Kenyan fossil hunter John Ekusi raced ahead to smoke a cigarette. Suddenly he began circling in place. When Nengo caught up, he saw a dirt-clogged eye socket staring up at him. “There was this skull just sticking out of the ground,” Nengo recalls. “It was incredible because we had been going up and down that path for weeks and never noticed it.”
The team carefully extracted the fossil from the rocky ground using small dental picks and brushes. Nengo knew immediately it was a primate skull, but that he wouldn’t learn much more until he and colleagues performed a more sophisticated analysis.
At the Noble Gas Laboratory at Rutgers University in New Brunswick, New Jersey, researchers measured argon isotopes—which decay at a fixed, predictable rate—within the fossil’s rock layer, revealing that it was about 13 million years old. Back then, the dry, rocky landscape of today’s Turkana Basin was a lush rainforest.
Although the fossil looks a bit like a gibbon skull on first blush, Nengo says, its dental pattern and teeth shape suggest its closest relatives are other Miocene fossil primates from the genus Nyanzapithecus, also found in Kenya. Yet its molars are much larger than those of the known nyanzapithecines, indicating a new species. The researchers named it N. alesi, or Alesi for short, after the Turkana word for “ancestor.”
Extremely sensitive x-ray imaging performed at the European Synchrotron Radiation Facility in Grenoble, France, allowed the team to count growth lines in the fossil’s unerupted adult teeth like tree rings, telling them Alesi was about 485 days (or 1 year and 4 months) old when it died. The x-rays also revealed the presence of bony ear tubes in the skull, which act as a balance organ. Primatologists have long debated whether the Nyanzapithecus genus belonged to the ape or monkey line, but the presence of these tubes, combined with the size and shape of the teeth, solidly mark Alesi—and by extension the other nyanzapithecines—as apes, the researchers report today in Nature. What’s more, they claim, the ear tubes present strong evidence that it’s an evolutionary cousin to the ancestral line of apes from which humans and living apes derive.
That could help answer a long-standing question in primate evolution: Did the common ancestor to living apes and humans evolve in Africa or Eurasia? Nengo says the new finding supports an African origin. “Africa has been acting like a petri dish for millions of years, conducting experiments in evolution,” he says. “Humans and our close ape relatives are just the latest evolutionary experiments to come out of that petri dish.”
David Begun, an anthropologist at the University of Toronto in Canada, isn’t convinced. He points to the fact that fossil hominines—a group whose descendants include African apes and humans—have been found in Europe dating to 12.5 million years ago, but they don’t conclusively show up in the African fossil record until 7 million years ago. To him, that suggests the common ancestor evolved in Europe before heading back into Africa. The discovery of N. alesi does nothing to change that. “Nyanzapithecus is an early ape,” Begun says. “Whether it’s the closest thing we know to the last common ancestor … is questionable.”