25 March 2011

Pre-Clovis Strata In New World Definitively Dated

A Texas archaeological dig has cleanly established the existence of a human settlement from around 15,500 to 13,200 years ago. This is about two thousand years earlier than the distinctive Clovis culture strata that is found across North America, and about a thousand years older than the oldest archaeological remains found in South America. This age is consistent with an entry of the earliest hominins to the Americas via the Beringian land bridge (where Alaska and its Aleutian Islands are today) and an expansion beyond Beringia when glacial melt would have opened a passage to the continental interior.

This site is notable because its dating is particularly "clean", because it clearly pre-dates the Clovis sites, and because it is in the interior of North America. It had seemed possible before this find, that earlier pre-Clovis sites fit a pattern that could have arguably been confined to the Pacific Coast of the Americas and their near vicinity, rather than also including the interior of the Americas and Eastern Coasts.

Open Question: Why do the earliest skeletons in the Americas seem to look different from later skeletons in the Americas?

The new find does not have bones, so ancient DNA and physical anthropology describing the people who inhabited the earliest strata of settlements in the Americas is not possible from this site. Some of the oldest skeletons in the Americas are from people who would have looked physically very different from modern Native Americans in all eras starting a few thousand years later, and somewhat similar to the very oldest East Asian skeletons. It isn't clear if the differences between the oldest skeletons and the later ones in physical type are a case of population replacement, human evolution as a result of new conditions, or factors like disease that could produce a few fluke skeletons that have been misinterpreted as seemingly belonging to a different racial type.

It is clear from genetic evidence that the vast majority of Native American genes found today have their origin in a pretty small population that expanded almost everywhere at about the same time when humans first emerged onto the continent from Beringia. The biggest divide in Native American genetics seems to suggest, however, that the genetic mix in the Native American groups that settled the North American interior is somewhat different than the genetic mix in the Native American groups that settled the Pacific Coast. It isn't clear if this was a product of random founder effects, or if there was some degree of population structure in the Beringian source population for the Americas.

It also isn't clear if there were one or more subsequent waves of people who arrived from Asia, perhaps by boat or perhaps in separate waves of migration that were closely spaced in time (the Na-Dene are often seen as a candidate population that could have given rise to the genetic difference between North America and South America), outside of the circumpolar region, that had a minor but detectable impact of Native American population genetics.

Finally, we can't rule out the possibility that the wave of humans arriving in the New World via the Beringian land bridge initially included modern humans from Asia of both an "archaic race" (paleoindians) and a more "modern race" and that the archaic modern humans vanished due to some combination of failure to adapt to the New World, conflict between the two populations that they lost, and admixture to the point where they were diluted into invisibility in modern Native American gene pools.

The plausiblity of a scenario with a "lost race" that inhabited the Americas as the same time as the ancestors of modern Native Americans is enhanced by the fact that we know that at least two other human populations, the Vikings, and a paleo-Eskimo population that arrived after the closing of the Beringian land bridge and then were replaced by the Eskimos, arrived in the Americans only the disappear without leaving a notable genetic trace in contemporary Native American populations.

Open Question: When did human first arrive in America?

The new find does, however, help us to bette calibrate the amount of genetic mutation that is observed in the Americas in indigeneous populations to a time of divergence.

That task is complicated by the fact that we don't know how much time the proto-Native Americans spent as a genetically distinct population in Beringia and Northest Asia before entering into the interior of the Americas. In particular, it isn't entirely clear if the proto-Native American population was drawn from pre-Last Glacial Maximum populations of Northeast Asia, post-Last Glacial Maximum populations of Northeast Asia, or a mix of the two. In one scenario, there was a population that entered Beringia, was trapped their by ice in both directions for many thousands of years, and then expanded out afterwards (Beringia's weather would have been mild for its lattitude due to ocean currents and prevailing winds). In another scenario, the stayover of the proto-Native Americans in Beringia en route to the Americas would have been relatively brief.

But, in all of the scenarios not ruled out by contrary genetic evidence or other evidence, the proto-Native American population would have been in a bottleneck situation in Beringia for some period of time before rapidly expanding out into virgin territory across all parts of North America and South America. So, timing of the "breakout" into the Americas can still be inferred by the amount of genetic diversity in different regions of the Americas, without knowing too much about how long that population was in the Americas. And, in theory, one can look at the amount of genetic differences between all American populations and plausible source populations in Asia, to estimate the age of the most recent common ancestor of the two populations. By subtracting the apparent expansion date of the Native American popluations of about 15,000 years ago, from the apparent age of the most recent common ancestor of the Asia souce populations and the American populations, it ought to be possible to estimate how long a distinct proto-Native American population was genetically isolated either in Berginia itself or in some territory in Northeast Asia.

In practice, this isn't so easy, because the margin of error in dating the oldest human habitation sites discovered (roughly plus or minus 900 years) and the margin of error in estimating divergence times based on genetic evidence (roughly plus or minus a few thousand years) in large in relationship to the probable length of the time that this population was isolated pre-expansion, which is a few hundred years at the low end of theoretical expectation (i.e. about 14,500 years ago) and about seven thousand years at the high end of theoretical expectation (i.e. about 22,000 years ago).

Open Question: Where there pre-land bridge or post-land bridge waves of human migration to the Americas?

When glaciers melted, the land bridge between Asia and the Americas was submerged, effectively isolating the Americas from the rest of the world until the arrival of Columbus in 1492, with a handful of very limited exceptions:
* some circumpolar Eskimo and paleo-Eskimo interchange between North America and Siberia,
* a brief and failed interlude of Viking efforts to colonize a few places on the North Atlantic coast of North America about a thousand years ago, and
* a probable brief episode of contact (but not population exchange) between the Austronesians and the South Americans on the Pacific Coast that brough the kumara (a kind of South American origin yam) and some associated names for it that are used in South American languages, to the Polynesians sometime in the last two thousand years or so.

It isn't clear what lasting cultural impact, if any, this brief episode of Polynesian-South American contact had on the South Americans. There are musical similarites between South American music and the musics of some cultures in Southeast Asia and Melanesia, and this is one time that this musical culture could have arrived, but the music of Polynesian cultures does not share the traits found in both Southeast Asia and Melanesia on one hand, and South America on the other, so music as a cultural impact seems unlikely.

The circumpolar contacts are the only known contracts between the Americas and the rest of the world between the closing of the Beringian land bridge and 1492 CE, that are known to have had a genetic impact on modern Native American populations.

There are a couple of New World sites that claim much older ages (ca. 30,000 years ago), but there is considerable dispute over the methodology used to establish those dates from cinders that may or may not have had human origins and whose dating may or may not be reliable. If there were populations in the Americas that old, one also has to explain why modern humans expanding into the virgin territories of the Americas did not show the signs found in all other cases where that happened: megafauna extinction, widespread population expansion leaving archaeological traces, etc., yet did leave isolated traces at very far flung sites. And, one also has to presume a level of sea faring expertise greater than was known to be available in any candidate populations around that time. And, one has to explain the lack of the distinct population genetic signatures you would expect from that population.

Obviously, new discoveries can only push earliest Native American populations further back in time, but the knowledge we have of ancient climate and the timing of megafauna extinctions widely believed to be related to the arrival of humans in the Americas strongly disfavors a scenario in which humans arrived very much earlier. Even the sites with archaic modern human physical anthropology are quite close in time (within a few thousand years) of the sites with modern human physical anthropology quite simmilar to modern populations.

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