The Great Migration Revisited

on October 3, 2010

It’s been over a year since I turned aside from exploring the Old Stone Age to present what was intended to be a brief discussion of the sequence of historical visions. That side-trip unexpectedly took on a life of its own — but now it’s done and I’m ready to pick up the narrative where I left off.

My final entry in that earlier set concluded with the heretical suggestion that the Great Migration which brought modern humans out of Africa began not around 70,000 to 60,000 years ago, as generally assumed, but much earlier:

Over the last few decades, there has been increasing evidence that early modern humans reached the Middle East around 130,000 years ago, when the ice age prior to the most recent one was coming to an end and a moister climate in what is now the Sinai Desert made such a migration possible.

The climate remained mild for an extended period, until another ice age set in about 75,000 years ago. At that point, modern humans vanished from what is now Israel, being replaced in the fossil record by more cold-adapted Neanderthals. For that reason, those early Middle Easterners are generally written off as an evolutionary side note, irrelevant to the larger human story.

The recent finds of shell beads, however, present a very different picture. They suggest that during the entire interglacial period from 130 to 75 thousand years ago, the people of northern Africa and nearby parts of the Middle East may have formed a single population. …

And in that case, the people who lived in Israel then were not some isolated, backwoods anomaly but part of a vibrant and rapidly evolving human community. … Perhaps those ancient inhabitants of Israel, rather than quietly dying out when the cold weather came, were inspired instead to pull up stakes and set off to the east in search of greener pastures

To my great delight, several discoveries over the past year have both confirmed and extended what might just as easily have proved to be merely a wild flight of fancy. The first of these was presented in an article last spring, titled “British archaeologist: 125,000 years ago first human settlement began in Oman.”

If you look at the map to the right — which is the same one I used a year ago — Oman lies in the southeastern part of Arabia, at the base of the red arrow over at the right-hand edge. It lies squarely on what is generally assumed to have been the chief migration route out of Africa, which is the reason Dr. Jeffrey Rose chose that area to explore.

According to the article, “At long last, after a decade of searching, they found a site in Wadi Aybut with stone tools that represent the footprint of the human expansion. The artefacts demonstrate a specific method of making stone tools first discovered in Africa in the 1960’s called “Nubian Complex.” … Dates of the Nubian Complex range between 125,000 and 75,000 years ago. … The conclusion is that the first modern humans followed the rivers into Arabia 125,000 years ago in response to improved environmental conditions.”

The announcement of this discovery was followed just last month by another article — titled “Stone tools ‘change migration story'” — which describes related findings by Dr. Michael Petraglia in the Arabian peninsula and India.

“I believe that multiple populations came out of Africa in the period between 120,000 and 70,000 years ago,” he said. “Our evidence is stone tools that we can date.”

Most of the tools are from far inland — hundreds of kilometres from the coasts. This means it was more likely humans migrated by land than in boats, he said.

The tools are found in areas that are often very inhospitable now, but which at the time would have been much more conducive to migration.

“During the period we’re talking about, the environments were actually very hospitable,” he told BBC News. “So where there are deserts today, there used to be lakes and rivers, and there was an abundance of plants and animals.”

Put together, these two discoveries have the potential to completely rewrite our understanding of the migration out of Africa. Perhaps the most exciting implication is that for some 50,000 years there was a thriving human community in the Middle East whose existence has up to now been completely unsuspected.

In recent years, the prevailing scenario of human expansion has been the so-called beachcomber hypothesis, according to which a relatively small number of migrants crossed the Red Sea from Ethiopia into southern Arabia when the onset of the most recent Ice Age lowered sea levels. They are then assumed to have virtually raced along the shorelines of the Indian Ocean and around the tip of India, reaching what is now Indonesia within 10,000 years or even less.

There have always been problems with this hypothesis — not least that it would require an almost inconceivable rate of population growth for the original handful of wanderers to settle so much formerly uninhabited territory so quickly.

But now those problems are resolved, as we are offered the far more generous span of 50,000 years, during which modern humans could have moved eastwards in a natural series of stages — traveling inland rather than along the beaches and colonizing the river valleys and around the lakes.

First they would have established themselves in the fertile lands of Israel and Arabia. Then they would have crossed to the eastern shore of the Persian Gulf. And finally, no later than 80,000 years ago they would have reached Pakistan and India.

The details of those 50,000 years remain largely obscure — but there’s one more remarkable piece of research that may help to fill in a small corner of the picture.

Last spring, it was announced that 60% of the Neanderthal genome has now been deciphered and that, to the great surprise of the investigators, it turns out that “about 1 percent to 4 percent of DNA in modern people from Europe and Asia was inherited from Neandertals.”

“Scientists were surprised to find that people from China and Papua New Guinea (places where Neandertals never lived) have just as much Neandertal ancestry as people from France,” the summary in Science News reports. “The group did not find traces of Neandertal heritage in the two African people studied. The result probably means that interbreeding between Neandertals and humans took place about 50,000 to 80,000 years ago in the Middle East as humans began migrating out of Africa to colonize the rest of the world.”

On the basis of the discoveries mentioned above, those dates are almost certainly far too late. It seems more likely that any interbreeding would have occurred some 130,000 to 110,000 years ago — between when modern humans first crossed the Sinai peninsula into the limited area that was then Neanderthal territory (in red on the map) and when the ancestors of the Chinese and the Papuans began moving away to the east.

The prospect of human-Neanderthal interbreeding raises many questions, however. We already know from decades of archaeological fieldwork that the two species lived in the same locations, made exactly the same kinds of tools, and exploited the same local resources. And yet they never merged into a single population.

An article which appeared in Discover Magazine in 1995 attempted to address the seeming paradox that “for perhaps 50,000 years, two radically different types of human lived side by side in the same small land. And for all those millennia, the two apparently had nothing whatsoever to do with each other.”

“If these humans were isolated in neither space nor time but were truly contemporaneous, then how on earth did they fail to mate?” the author asked. “Only one solution to the mystery is left. Neanderthals and moderns did not interbreed in the Levant because they could not.”

But now we know that they apparently did mate — just not to the point of becoming a single population. And that is a puzzle.

I can see one possible factor which might give rise to such a situation. I read once that modern humans whose ancestors diverged 100,000 years ago — such as Europeans and Japanese — have a slightly higher than usual chance of reproductive failure, so that when they conceive children together there are more miscarriages and stillbirths.

In the same way, early modern humans and Neanderthals, with a 500,000 year divergence, might have been genetically far enough apart to make reproductive success possible but only on rare occasions.

And that, in turn, might have triggered some interesting cultural consequences.

I’ve suggested previously that the earliest human societies were dominated by what I’ve called the transformative vision — which placed particular emphasis on the cultivation of “female magic” to ensure fertility and successful childbirth — along with the kinship vision.

On both grounds, modern humans might have considered matings with Neanderthals to be “taboo” — not only because they fell outside the kinship rules for permissible marriages, but also because they were perceived as somehow “cursed.” And yet, paradoxically, the occasional child born of one of those matings, who would have inherited some of the superior strength of the Neanderthals, might have been honored as having a supernatural origin and powers.

The mythologies of the world are full of stories about human matings with non-humans and the superheroes — or monsters — that resulted. I’m hesitant to conclude that those stories are a direct reflection of events that occurred more than a hundred thousand years ago — and yet, once a trope gets embedded in story, it can endure long after the conditions which gave rise to it are forgotten.

In any case, it’s an interesting conceit to think that, at least in mythic terms, those among us who are of unalloyed African ancestry are the only “true” humans — while all the rest of us have at least a touch of goblin in our veins.

Related:

A follow-up on the subject of human-Neanderthal interbreeding can be found here.

A listing of all my posts on deep prehistory can be found here.

A general overview of the areas of interest covered at this blog can be found here.

A chronological listing of all entries at this blog, with brief descriptions, can be found here.

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