Sunday, December 18, 2005

Luzia, Kennewick and the Peopling of the Americas

National Geographic News has an interesting story on some research on a large collection of skeletons found in Brazil. The skulls themselves date to about 10,000 years ago. The story is incredibly interesting, since it impacts theories about the peopling of the americas.

The skeletal material comes from the Lagoa Santa region of Brazil. Caves containing skeleatal material were first discovered in 1842/1843. The first prefessional archaeological excavations were conducted in 1956 and further excavations were made in the 1970's (the skull above came from the 1970's excavations). During that time at least 250 skeletons were discovered:

The scientists examined 81 skulls unearthed over many decades in Brazil's Lagoa Santa region. They represent the largest collection of early American remains, many of which had to be tracked down in European museums.

The results of the study, which is actually one paper in an ongoing program, were recently published in the Proceedings of the National Academy of Science(Thanks for sending me the article Aydin!). I also managed to track down an earlier paper published in Genetics and Molecular Biology. I'll get to some of the results below, but would like to describe some of the things that are a part of the research program of Neves and Hubbe. At this point they are trying to date as many skeletons as possible via accelerator mass spectrometry, trying to get minimum dates for some of the skeletons by dating cristaline calcite layers sealing the deposits, excavating new sites to increase the number of skeletal material and to better understand the stratigraphy of the area and rexamining some of the original sites.

Absolute dates have been obtained on 22 of the skeletons and range from 7500 - 8500 years ago. An interesting aside, most of the sample was buried in a hypeflexed position in shallow graves covered with blocks of limestone or quartz. A small hearth was usually adjacent to the pit and burning coals werethrown into the pit before sealing.

A series of measurements (61 according to the data set - although not all of them were used de to missing value, etc.) was taken on the skulls and the measurments were compared to those Howells datbase (which can be found here). To assess morphological affinities three different principal components analysis (an explanation of principal component analysis can be found here, here and here) were conducted. The first, of a type I'm not terribly familiar with (and will research and post on later), compared principal coordinates on heritability corrected data (as I understand it this technique tries to get at how much of the variation in skull morphology is due to genetic contributions). The second principal component analysis compared individual measurments within the Lagoa Santo and Howells samples. The third compared group centroids.

The resilts of all three analysis indicated that the Lagoa Santo sample has a distinctly different cranial morphology compared to late and modern Northeastern Asians and Amerindians. The Lagoa Santo sample was more similar to present Australian/Melanesian and African samples than any other group. At this point you should be asking yourselves "Where have I heard afarensis talk about old bones being related to Melanesians and Australians before?"
The picture below is another hint.

I'll get back to that in a minute. First we have to talk about the National Geographic article:

Recent genetic studies of modern human populations have also suggested multiple early migrations across the Bering land bridge.

Neves and colleagues have not been able yet to extract ancient DNA from the Lagoa Santa remains—but excavations are yielding additional ancient remains.

"We have already found at least 20 new skeletons older than 8,000 years that are not part of our paper," he said.

Still, not all scientists are convinced that the variations found in the skulls are proof of multiple migrations to the Americas.

"There is a huge amount of variation among the first Americans, more than you see among any other population outside of the Pacific," said Joseph Powell, an anthropologist at the University of New Mexico in Albuquerque.

"Much of that is genetic, and it comes from the fact, I think, that these first Americans had very small colonizing populations, and they have a great degree of genetic variation due to genetic drift."

There two questions here. First, how many populations settled the America? We know of at least two, possibly three: those populations ancestral to modern day Native Americans and those ancestral to Athabascans and the Eskimoos. All of whom originated in Northeast Asia. Neves and Hubbe, and Neves et al, put a slightly different on this question. They argue that the Americca's were first populated by "...a generalized population of Homo sapiens very similar to the one that departed from East Asia to Australia around 50,000 BP..."(from Neves et al). Later in the paper, Neves et al go into a little more detail:
In the beginning of this paper we stated that the settlement of the Australian continent is currently seen as a direct expansion of early modern people from Africa...while the settlement of the America's is viewed as a much later colonization event, namely the expansion of the specialized Mongoloids from North Asia in relatively recent times...
If this view is accepted, it becomes clear that Native Americans had an ancestral population in mainland Asia which was not shared with Native Australians."

In essence, Neves et al, is saying that Paleoindians were part of the expansion of H. sapiens out of Africa, whereas Native Americans represent a later expansion of specialized populations. I find this suggestion intriguing. There are skeptics, as you can see from the above quote from The National Geographic article. Powell is arguing that because America was populated by small populations, genetic drift would kick in and you would see a lot of variation between populations and that the Lagoa Santo populations fit within the pattern af variation seen in Native American populations. If that is the case, I would expect one or two populations to display morphology similar to Australians and Melanesians. Instead, what we have is a series running from southern Chile to Florida - and now Kennewick in Washington (which was not included in either study)- all of which display morphology similar to Australians, Melanesians and Polynesians. As Neves and Hubbe put it:

One is a local microevolutionary process that transformed, in situ, the Paleoamerican morphology into that prevailing today among Native Americans. The other is that the Americas were successively occupied by two morphologically differentiated human stocks, with the Paleoamerican morphology entering first.

We believe the second hypothesis is more plausible for three reasons: first, it would be very unlikely that the same evolutionary event (directional morphological change) happened in the Americas and in East Asia in parallel at approximately the same time (the parsimony principle)...; second, because in South America, at least, the transition between the two morphological patterns was, as far as we know, abrupt...; and third, cranial morphology has recently been shown to respond adaptatively only to extreme environmental conditions, being therefore much less plastic than originally thought...

It would have been interesting to have included Kennewick in this study since it is withing the date range of some of the other skulls studied, not to mention some of the similar morphology...but considering the legal difficulties involved in previous attempts...