Around 210,000 years ago, an early human died in southern Greece — leaving scientists with the earliest evidence of human migration out of Africa and prompting them to reconsider the story of how our species spread throughout the planet.
A new analysis of that ancient person’s skull suggests Homo sapiens left their birthplace in Africa about 16,000 years earlier than previously thought, according to a report published Wednesday in the journal Nature.
“We are seeing evidence of human dispersals that are not just limited to one major exodus out of Africa, as perhaps we have thought in the past, but multiple dispersals,” said study leader Katerina Harvati, a paleoanthropologist at the University of Tübingen in Germany.
The skull was found about 40 years ago in Greece’s Apidima Cave. The specimen, dubbed Apidima 1, was situated nose to nose just 30 centimeters (about 12 inches) away from a second human-like skull known as Apidima 2.
The two partial skulls were not near anything that offered archaeologists useful clues about their origin: no stone tools, no animal remains, nothing.
In time, researchers determined that Apidima 2, the more complete of the two skulls, belonged to a Neanderthal.
But Apidima 1 did not get its due until the Museum of Anthropology at the University of Athens invited Harvati to use her expertise in imaging and 3-D virtual reconstruction to bring both of the skulls to life.
The results confirmed that Apidima 2 belonged to a Neanderthal who lived about 170,000 years ago.
Apidima 1, however, has features that distinguish it as a modern human. Its owner lived some 40,000 years before its Neanderthal neighbor, making it the oldest human skull found outside of Africa.
When the researchers removed the skulls from their surrounding rock using a chisel and hammer, tiny bone fragments became dislodged as well.
Instead of throwing these priceless fragments away, they kept them to determine the age of the skulls using a high-tech version of mass spectrometry.
This technique involves shooting a laser at the skull fragments to generate tiny particles that can be ionized. By examining the mass and charge of those ions, scientists can identify the elements their sample is made of.
When it comes to figuring out how old something is, archaeologists are interested in one element in particular: uranium.
Most of the uranium atoms found in nature are U-238, an isotope with 92 protons and 146 neutrons. Over time, U-238 atoms lose four of their neutrons and decay into another isotope known as U-234.
When bone is buried in the ground, it can absorb U-238 from the environment. So by determining the relative amounts of U-238 and U-234, scientists can determine the minimum age of bones.
The results indicated that although the two skulls were found so close to each other, they were from vastly different time periods.
The rock surrounding Apidima 1 was estimated to be about 210,000 years old, while the rock around Apidima 2 was only 170,000 years old.
The best explanation, said study co-author Rainer Grün, a geochemist at Griffith University in Australia, is that “Apidima 1 must come from quite a different environment originally, before it was deposited at the site.”
To determine the species of the skulls, the researchers took CT scans and used the images to generate a 3-D virtual reconstruction of each one. Then they compared the virtual skulls with a variety of other skulls of known species.
Apidima 2’s features were well within the range of Neanderthal specimens. The skull had a prominent brow ridge, a larger area below the eye sockets, and — most distinctively — a bulge at the back that resembles hair tied in a bun.
While only the back portion of the skull of Apidima 1 remained, it was enough for the researchers to make a positive identification. That’s because it was rounded in a way that’s unique to modern humans, Harvati said.
Apidima 1 also lacked any features that were typical of Neanderthals or other archaic human species, she said.
The incomplete nature of the Apidima 1 skull may leave some experts uncertain about its true origin, wrote Eric Delson, a paleoanthropologist from City University of New York, in a commentary that accompanied the study. If DNA or proteins could be extracted from the fossil, it could lead to a more convincing identification, he wrote.
But Mirjana Roksandic, a paleoanthropologist from the University of Winnipeg, said the skull will help scientists in their continuing quest to fit together the puzzle pieces of human evolution.
“Now we have fossil evidence that we might have had African-style modern humans in Europe much earlier than thought previously,” said Roksandic, who was not involved in the study. “I was just waiting for it to happen!”
Harvati said she’s already contemplating a brand new set of questions.
“What caused this early dispersal?” she said. “Were there any technological advances that enabled such a far geographic intrusion into Europe?”
The fact that these skulls were found in Greece could turn out to be an important clue, Roksandic said.
“The eastern Mediterranean is where the three continents actually meet,” she said. That means it’s where “three continental populations meet and interact.”