Given the fragmentary nature of the skulls, they were hard to remove and clean, though that eventually happened in the 1990s. For example, the earlier human skull came from a time when the surrounding environment was warmer and wetter than the cold and arid conditions the later Neanderthal specimen would have lived in.
A reconstruction of Apidima 1. Image credits: Katerina Harvati, Eberhard Karls University of Tübingen.
An global team - including Manchester University experts - created virtual reconstructions of each using state-of-the-art technology. The other was that of a Neanderthal who lived 170,000 years ago.
The older of the two Apidima skulls does not match the features of the Neanderthals, another extinct human species that once roamed Eurasia, coexisting and possibly interbreeding with modern humans.
The researchers speculate that there could have been contact, or perhaps even conflict, between the Neanderthals and the Apidima people.
But the recent study from a multinational team led by Katerina Harvati reconstructed the specimens digitally and dated them by measuring their radioactive decay.
"I couldn't believe it at first", Harvati tells Yong, "but all the analyses we conducted gave the same result". Southeast Europe is considered to be a major migration corridor out of Africa.
A Griffith University researcher has played a key role in dating the fossil of an early human found in Greece as 210,000 years old.
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In response, Harvati said the back of the skull is very useful for differentiating Homo sapiens from Neanderthals and other related species and that several lines of evidence support the identification.
A skull unearthed in Greece has been dated to 210,000 years ago, at a time when Europe was occupied by the Neanderthals.
Finding both in the same cave illustrates that multiple early migrations out of Africa, rather than a single event, helped early humans spread, according to the researchers. Given that no similarly old human fossils have been found in Europe, it is possible that Apidima 1 belonged to a population that could not compete with the continent's resident Neanderthals, paleoanthropologist Eric Delson writes in a Nature article about the new paper. Bone remains recovered from a small breccia sample fell into the same distinct isotopic groups as the skulls. Previously known Homo sapiens remains in Europe are more than 150,000 years younger.
The human skull was one of two cranial fossils found in Apidima Cave, one of a series of cave sites along the southwestern coast of the Peloponnese in Greece. They want to know the underlying cause for the early migrations, if there were technological advancements that allowed for those migrations and why some of the modern human populations didn't persist in the areas where they migrated.
But not all experts are convinced. They found that one of them, dating from 170,000 years ago is indeed Neanderthal.
"Homo sapiens... is extremely clever, extremely resourceful, opportunistic", he said.
The latest discovery reported in Nature show these began earlier - with the adventurers reaching further than previously thought.
"Proteins are composed of a sequence of amino acids, and this sequence is coded for in the genome", Frido Welker, the author of that study explained at the time. It was not one exodus out of Africa but lots of small ones.