A piece of skull found in a cave in Greece suggests our human ancestors left Africa far earlier than anthropologists thought.

A study published today in the journal Nature revealed that the skull, which was originally discovered in Greece in the 1970s, belonged to a member of an early population of Homo sapiens and is about 210,000 years old.

The earliest known fossilised remains of modern humans have all been found on the African continent, but this is the earliest evidence of modern humans outside Africa. It predates what researchers previously considered to be the earliest evidence of Homo sapiens in Europe by more than 160,000 years.

A tale of 2 skulls

In the late '70s, scientists discovered two partial skulls embedded in the rock of a coastal cave in southern Greece. They named them Apidima 1 and 2, after the cave in which they were found, which overhangs the Aegean Sea and is accessible only by boat.

Apidima 2 was identified as a Neanderthal, but Apidima 1 wasn't assigned a species.

Both skulls are incomplete (Apidima 1 consists of only the back of the skull, while Apidima 2 has more intact facial features), and it took anthropologists many years to extricate the fragmented fossils from the surrounding rock.

Assigning an age for the specimens also proved challenging, according to Katerina Harvati, the lead author of the new study.

Tools that measure age based on uranium decay suggested that even though the two skulls were found side by side, Apidima 1 was about 40,000 years older than the 170,000-year-old Apidima 2. However, there were no other identifiable animal remains or stone tools that Harvati's team could use to double-check those dating estimates.

After decades of work, the team was able to use CT scans of the bones to virtually reconstruct the damaged and missing parts of the skulls. By comparing the digital reconstructions with other Neanderthal and Homo sapiens fossils of similar ages, the researchers concluded that they had one of each type of human ancestor on their hands.

The smoking gun was the rounded shape of the back of Apidima 1 - a characteristic unique to modern humans.

Rauner Grün, another study author, said the skulls' isotopic signatures also revealed that the two individuals came from very different environments before they died.

"It's a wonder of nature that you find the two together," he said.

Jockeying with Neanderthals

Although the two skulls were deposited in the Greek cave about 40,000 years apart, the researchers said, it appears Homo sapiens and Neanderthals coexisted in this part of Southern Europe during the time that the owner of the Apidima 1 skull lived.

Havarti thinks this part of Eurasia might have served as a refuge for animals and human populations during that period, since the mild coastal climate was a preferable alternative to other parts of Europe. But these earliest Homo sapiens voyagers died out while local Neanderthal populations persisted, she said. Eric Delson, an anthropologist with Lehman College who was unaffiliated with the study, said in an accompanying Nature article that these groups "reached the Middle East and southeastern Europe, but did not persist in these regions." Neanderthals and early modern Homo sapiens likely jockeyed for dominance as the main hominin group in this part of the world, Delson said. So the individual that left behind the Apidima 1 skull fragment seems to have been part of a population that couldn't successfully compete. The same story likely applies to a roughly 194,000-year-old jawbone belonging to another modern human, which was found in Israel's Misliya cave.

However, our modern human ancestors did eventually migrate successfully out of Africa and replaced Neanderthals across Europe between 45,000 to 35,000 years ago. "All humans alive today outside Africa can trace their ancestry" to that final, successful dispersal, Harvati said, but early migrants like Apidima 1 didn't contribute any genetic material to humans living today.

Changing the timeline of human history

Ancient hominins - a term for any ancestors in the human lineage that are more closely related to each other than to chimpanzees - are thought to have left Africa in three waves. The first happened when members of Homo erectus migrated some 2 million years ago. Then the species that eventually gave rise to Neanderthals moved into Europe between 600,000 and 800,000 years ago.

The oldest evidence of our modern human ancestors (Homo sapiens) dates back 315,000 years and was found at a site called Jebel Irhoud in Morocco. Anthropologists have hypothesised that modern humans migrated out of Africa in a mass exodus roughly 70,000 years ago, gradually spreading into Europe, Asia, and the Pacific. Over time, these ancestors replaced Neanderthal populations in those areas.

Apidima 1 and other recent discoveries, however, have led researchers to think that some modern Homo sapiens left Africa long before that larger exodus. But those early migrants don't appear to have been successful.