Tuesday, October 16, 2012

Evidence Mounts For Out Of Africa 100-130 KYA

The Nubian Complex Culture Source In Africa For The First Wave Of Modern Humans To Leave Africa For Inland Arabia Has Been Identified And Accurately Dated Based Upon Archaelogical Evidence From Many Sites In Both Places.

A new published academic paper adds to the increasingly overwhelming evidence which has been published only in the last several years, that modern humans who were part of a Sudanese archaeological culture, called the Nubian Complex, migrated out of Africa to Arabia (for example, into what is now Southern Oman, into Central and Eastern Yemen, and into Central Arabia) sometime in the time frame of 100,000 to 130,000 years ago. 

The Nubian Complex people became a long standing Arabian culture that continued to have a presence in inland Arabia at least until the Middle Paleolithic to Upper Paleolithic transition around 40,000 to 60,000 years ago.  It is quite possible that people regularly moved between Arabia and African during the Middle Stone Age more or less continuously from the time that they first arrived until the Upper Paleolithic transition.

It appears likely that members of the Nubian Complex culture engaged in culturally influential back migrations to Africa during the Middle Stone Age and that members of the Nubian Complex culture made culturally influential migrations to the Levant from interior Arabia as well. 

Arabian Nubian Complex People Are The Likely Source Of The Lithic Tool Innovations That Mark The Middle Stone Age To Upper Paleolithic Transition.

It appears likely, indeed, that the Arabian branch of the Nubian Complex culture may have been the innovators who were the source of the lithic tool transition that appears in modern humans at the dawn of the Upper Paleolithic era.  Their tools in the late Middle Stone Age and early Upper Paleolithic look like clear antecedents to Upper Paleolithic lithic tools in the Levant and Egypt at the dawn of the Upper Paleolithic era, while the Nubian Complex lithic tools of Sudan at that point do not appear to be nearly so closely related.

The Numbian Complex People Were Not The Only Population Of Modern Humans In Arabia During The Middle Stone Age.

By the time that the Upper Paleolithic transition took place, around 40,000 to 60,000 years ago, there were other communities of modern humans in Arabia who had different lithic cultures from this culture. 

The Nubian Complex people of Arabia were probably not very closely related to them (although at the time depths involved, there would have been plenty of time for different branches of the original migrant population to grow quite distinct from each other over time). The origin of the other cultures is not known and may have origins in one or more Africa to Arabia migrations fairly close in time to the Africa to Arabia migration of people belonging to the Nubian Complex culture.

There Are Multiple Scenarios By Which The Eurasian Baseline Of Neanderthal Admixture Could Have Arisen And Become Somewhat Differentiated Between East Eurasians and West Eurasians.

Not later than the Upper Paleolithic transition, and possibly much earlier (perhaps 100,000 to 75,000 years ago), all or almost all Eurasian modern human populations had probably acquired some baseline level in Neanderthal admixture, either as (1) a single common founding population that split into Western Eurasian and Eastern Eurasian founding populations (and perhaps other founding populations that later when extinct) before the Neanderthal genes that had introgressed into the founding population had time to fully homogenize, or (2) a single common founding population that experienced additional and comparable amounts of admixture separately after splitting into a West Eurasian and East Eurasian founding population, or (3) in completely separate but similar parallel admixture events after splitting into West Eurasian and East Eurasian populations with reasonably closely related populations of Neanderthals.

Still, all of these scenarios fit the genetic data best if the split of the Middle Stone Age modern human population of Southwest Asia into proto-West Eurasians and proto-East Eurasians happened prior to or not long after a period of Neanderthal admixture in Southwest Asia or West Asia.

We Have Some Data To Provide A Framwork For Our Understanding Of The Out of Arabia To Asia Migration(s), But Many Details Are Still Not Known.

It increasingly appears that the migration of modern humans from Arabia to the rest of Eurasia came many thousands of years later, although there is fairly clear evidence that some population of modern humans (not necessary a group with ancestors among the Nubian Complex people) had migrated to Southern India no later than 75,000 years ago, and probably considerably earlier although just how much earlier is hard to tell.  Thus, there may have been a delay of as much as 55,000 years between the Out of Africa and the Out of Arabia migration, although the new more accurate dating of older Levantine and Arabian traces of modern humans and new and older finds of traces of modern humans from India could instead narrow the separation between the Out of Africa migration and the Out of Arabia migration to as little as a few thousand years.

It isn't at all clear that the first Out of Arabia to Asia wave that was present in Southern India at the time of the Toba erruption about 75,000 years ago are descendants of the Nubian Complex people. 

We have only relics that are clearly modern human in origin, not human remains, to rely upon this far back in time in Southern India.  The Arabian archaelogical record is also far richer in relics than in human remains although there are some modern human remains at some Arabian Nubian Complex archaeological sites.  The lithic tools in South India are not quite so strikingly similar to the Arabian ones as the Arabian ones are to lithic tools at the African Nubian Complex sites, or at the Levantine and Egyptian sites derived from Arabian Nubian Complex innovations arond the time of the Upper Paleolithic transition. 

Since it is likely that there were multiple modern human populations in Arabia at the approximately the same time that the Nubian Complex people were there during the Middle Stone Age, and since the modern human lithic tools at the earliest Southern Indian sites don't show a particularly clear continuity with Nubian Complex lithic tools, another of the modern human groups in Arabia at the time could have been ancestral to the first wave of modern humans in Southern India.

We know that sometime around 45,000 to 50,000 years ago, presumably via a route that made the connection from Africa to Southeast Asia that passed through India in some way or another, that modern humans arrived in Papua New Guinea and Australia (which were a single land mass at the time) having by then acquired the Neanderthal admixture shared by all Eurasians (and in particular, the East Eurasian subset of that admixture) and the Denisovan admixture which are largely restricted to Aboriginal Australians, Papuans and other populations genetically related to those populations. 

There are traces of modern humans in the Southeast Asian highlands that are no older than 64,000 years old and no younger than 50,000 years ago, probably close to the older date than the younger one.  Modern humans appear to have arrived in Southeast Asia en route to Australia although, of course, they didn't know where they where going at the time) sometime in a twenty-five thousand year time frame bounded by the Toba erruption on one side and the Upper Paleolithic revolution on the other.

The case that hominin remains from East Asia, which have claimed dates of around 100,000 years ago, are really modern humans rather than archaic hominins who may have been experiencing some convergent evolution towards some of the same traits which modern humans evolved, and are really accurately dated, is tenuous at best.  New evidence, however, could strengthen that claim and greatly complicate the emerging narrative of modern human expansion into Eurasia.

Out of West Asia To Europe: Were Cro-Magnons Descendants Of Nubian Complex People?

We also know that sometime around 45,000 to 43,000 years ago, modern human Cro-Magnons who already had West Eurasian genetic affinities distinct from East Eurasian populations arrived in a Europe that was populated by Neanderthals.  It isn't clear what population genetic affinity these Cro-Magnons or later waves of modern human migrants to Europe had to the Nubian Complex people.  But, given that the Nubian Complex people are likely to have been a source of Upper Paleolithic lithic tool technologies, at the very least, the Nubian Complex people were an important cultural inflence on the Cro-Magnons, and it wouldn't be at all surprising given the trend of recent ancient DNA and archaeological finds to reflect "Pots and People" rather than "Pots Not People" migrations, if the Nubian Complex people were key contributors to the gene pool of the Cro-Magnons.

While hardly definitive, what we know so far seems to identify the descendants of the Arabian Nubian Complex people a bit more closely with modern West Eurasians than with modern East Eurasians, who could be more closely related to one or more of the other Middle Stone Age modern human populations of Arabia that were in Arabia at the same time that the Nubian Complex people were there.

The Cro-Magnons very likely became more Neanderthal admixed than the Eurasian baseline level of Neanderthal admixture than they had been when they arrived over the course of the many thousands of years  that they co-existed with Neanderthals between their arrival in Europe and the extinction of the last Neanderthals arond 29,000 years ago. 

But, this pre-Last Glacial Maximum population of European hunter-gatherers that co-existed with Neanderthal was so diluted in subsequent waves of migration to Europe by modern humans.  The later waves of migrants to Europe lacked Neanderthal admixture in excess of the Eurasian baseline level.  So the Neanderthal admiture excess attributable to admixture in Europe's early Cro-Magnons is almost impossible to discern statistically in contemporary European populations.

How can we be confident that the Cro-Magnons did have elevated Neanderthal admixture then? 

Because people with significant amounts of ancestry from European hunter-gatherers were living in Europe as late as Otzi the Iceman who died 5,300 years ago and was preserved as a mummy in the ice on mountains between what are now Austria and Italy did have elevated levels of Neanderthal admixture.

It Isn't Entirely Clear What Relationship The Arabian Nubian Complex People Have To Linguistically Afro-Asiatic Africans Today.  They May Have Been A First Wave Of Back Migrating Eurasian Genetic Contributors To This Population.

These data points would be consistent with a hypothesis that many of the older non-replicating Y-DNA and mitochondrial DNA haplogroups that now coincide rather strongly with modern Afro-Asiatic populations could derive from a back migration of Nubian Complex people from Arabia into Africa in which the technological edge that allowed them to expand more effectively into territory previously inhabited by other peoples may have been the tool kit of the lithic tools that would become signatures of the Upper Paleolithic transition.  Of course, none of these data points can rule out a much later Neolithic or Epipaleolithic back migration as the main source of Asian to African back migrated haplogroups either.

Without ancient DNA we can't know for sure that this Nubian Complex population was ancestral to modern Eurasians, or to modern North and East Africans, at all.  There have been multiple extremely arid periods since the Nubian Complex people left Africa and came to Arabia when Arabia has been entirely or almost entirely depopulated.

No known language family today has a time depth of much more than 8,000 to 10,000 years from a most recent common proto-language, and some of the well defined subfamilies of the Afro-Asiatic language family are considerably young.  So, I am not willing to conclude that the direct ancestor of the Afro-Asiatic language family itself was spoken 40,000 or more years ago.  But, a migration into the modern Afro-Asiatic language region at that time may have had an important and formative impact on the population genetics of that region.  The people of that region may even have had enduring culture ties with each other relative to other contemporaneous modern humans for most of time period since then. 

Populations with powerful new technologies and cultural innovations (such as Neolithic era or even Copper Age peoples) could easily have brought about a language shift of this entire, far flung population to their language, however.  And, a Neolithic or Copper Age population could have brought about language shift with only a modest population genetic impact at some later date.


There are still many questions to be resolved about human pre-history in the Paleolithic era.  But, our understanding of modern human range expansion outside Africa during the Paleolithic era is much greater than it was even a decade ago.

Our ability to link the first Out of Africa wave of modern human migration to a particular archaeological culture has many practical benefits for further research.  For example, it provides an additional reasonably certain calibration point for mutation rate based dating of the point in time at which the defining mutations of genetic haplogroups arose.  It also provides an independent and direct means of estimating critical demographic parameters like the effective population size of the proto-Eurasian population that together with the calibration point and our knowledge of modern human mutation rates from other sources can be used to more accurately determine the parameters to use when modeling the population genetics of Eurasian prehistory in variety of contexts.  These details can reduce the size of the error bars associated with all efforts to assign absolute dates to ancestry informative genetic mutations.

1 comment:

terryt said...

A good summary. In other words, I mostly agree with it.