The Bottleneck and the Great Migration

on August 23, 2009

In the previous entry, I began exploring the idea that there may have been an evolutionary leap from almost-modern to fully-modern humans as recently as 80,000 years ago, when art and personal ornament first appear in the archaeological record.

A further piece of evidence for this theory is the extremely low genetic diversity of the human population, even today. It was estimated in 2003 that all modern humans are descended from no more than 2000 individuals who lived around 80,000 years ago.

Some scientists argue that this figure may be a bit on the low side, but it seems generally accepted that there was a significant population bottleneck. The reasons for the contraction, however, remain unclear. One possibility is that a natural disaster caused a significant die-back — but there is no geological evidence for such a disaster.

In addition, the bottleneck was immediately followed by the Great Migration, when modern humans spread from Africa throughout the world. That would seem like an odd adventure for a species which had just avoided extinction by the skin of its teeth.

A more satisfactory explanation might be that our genetic diversity is low because all of us are descended from a small, isolated population that underwent a relatively recent evolutionary leap followed by a rapid expansion.

That story would account for most of the known facts, leaving just a few loose ends. For example, it seems that some fully-modern humans must have intermarried with their almost-modern relatives as they spread out, reassimilating them into the general gene pool.

The same 2003 study that proposed the idea of a population bottleneck also indicated that the Khoisan (Bushmen) of southwestern Africa and the Pygmies of central Africa are closer to the human root stock than any other groups. A more recent survey confirms that the Khoisan display the greatest genetic diversity of any human population.

What this means is that even though the vast majority of the human species derives from a small group of common ancestors who lived about 80,000 years ago, the Khoisan can trace part of their ancestry back a great deal further. It appears that the Khoisan became genetically isolated from the rest of us around 150,000 years ago and only rejoined the party some 50,000 to 100,000 years later.

The present-day Khoisan are thus a hybrid population, presumably resulting from a combination of the people who had been there all along with the new arrivals who showed up with their art and shell beads 70-some thousand years ago.

As to the origin of those outsiders, the best current evidence suggests that fully-modern humans may have first appeared along the coast of the Red Sea, in present day Ethiopia or the Sudan (star), where the genetic lines are very deep and complex.

northafricaFrom there, they would have spread out to the north as well as to the south. Shell beads very similar to those in southern Africa have been found in Morocco, Algeria, and Israel (red dots), dating from either the same time or a few thousand years earlier.

This might seem to suggest a very simple scenario of expansion from a single point of origin. However, things are not quite that straightforward.

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 (arrow).

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.

That, in turn, raises the possibility that the leap from almost-human to fully-human could have taken place within that population as a whole. 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.

Geologists tell us that the colder and drier weather which closed off the Sinai route from Africa to Asia 75,000 years ago also led to lower sea levels, which narrowed the mouth of the Red Sea and made it possible for early humans to travel directly from Ethiopia to southern Arabia (arrow). As a result, accounts of the Great Migration often depict it as following a route along the coast of Yemen and Oman and then across the mouth of the Persian Gulf (arrow).

But looking at the map above, I’m struck by an alternative possibility. 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 (arrow).

They were, after all, fully-modern humans — and that is the sort of thing that us modern humans do all the time.

Related:

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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Read the Next Entry: The Dance of the Visions

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