The Americas were the last great landmass to be occupied by humans, and it has been becoming increasingly evident that we have been oversimplifying the theory of the continent’s population.
It remains accepted that the prehistoric peoples crossed from the Old World to North America over Beringia, the land bridge that connected Siberia to Alaska when ocean levels were low because of the Ice Age. It also remains accepted that the earliest South Americans hailed from North America.
The surprise is that the early human remains found so far in North America and now in Central America too evince much more diversity in craniofacial morphology than had been expected.
The latest revelation stems from painstaking morphological analysis of four skulls found in a vast cave system on the coast of Quintana Roo, in the vicinity of the town of Tulum, Mexico. The skulls demonstrate unexpectedly high diversity, Prof. Mark Hubbe from Ohio State University, Alejandro Terrazas Mata from National Autonomous University of Mexico and colleagues reported in PLOS One on Wednesday.
Moreover, these paleo-Mexicans were much more diverse in their morphology than early human remains in South America – at least those found so far. It bears adding that fossil human remains in the Americas are really rare.
“The typical feature of early South Americans, who are more homogeneous, is to have [an] elongated and narrow skull, with a projecting face and relatively short orbits,” Hubbe tells Haaretz. “In South America, all early remains show strong affinity with groups from Australia and Africa, showing that they retained the ancestral morphological pattern of modern humans. Early Mexicans are more diverse.”
The authors theorize that the initial populations of North America were more morphologically diverse than had been assumed, that the diversity persisted to Central America – but then, the groups who moved onto South America encompassed only a fraction of that diversity.
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Meanwhile, in Quintana Roo
When people began crossing from Siberia to Alaska is not known. The general thinking is that the northern parts of the Old World, and Siberia, were a blasted, uninhabitable wasteland because of the Ice Age until around 35,000 years ago. It is also generally thought that people began to reach North America about 20,000 years ago, though some believe occupation of the new lands began quite a bit earlier.
We shall ignore the argument over whether humans could have been the ones who smashed mastodon bones in California 130,000 years ago, because it is as futile as it is unlikely. A more intriguing case is the Toca da Tira Peia rock shelter in Brazil, where redating suggests an age of 22,000 years for human occupation. (Asked for his thoughts on Peia, Hubbe explains that there are some sites in South America, and Mexico too, with postulated dates as old as 45,000 years, but that the record is too spotty for the scientific community to seriously address the possibility and rewrite history just yet.)
In any case, the simplistic assumption of a single original migration to the Americas has long been challenged, including through genetic, archaeological and linguistic research. Different studies defended just about all the models – a single basal migration from Siberia, or two, or three, or constant movement back and forth until the land bridge sank beneath the freezing waves. In short, we still don’t know.
Now a new factor argues against being overly simplistic: The skeletons of four people who lived approximately 14,000 to 8,000 years ago, found in caves along the coast of Quintana Roo. They are among the earliest actual human remains found in the Americas (as opposed to artifacts) and, the scientists were surprised to discover, they looked quite different from one another – and not much like latter-day Native Americans either.
Asked why their diversity is remarkable given that they lived so far apart in time, Hubbe explains: “This is surprising to us because, when you look at South America and also at genetic studies, the evidence points to low diversity of early Americans.”
The caves are largely underwater today, but weren’t then and the human remains are believed to have been deposited there before the caves’ submersion. Which means in the argot of archaeology that they are considered to be in situ – not moved since their demise, which is crucial to the dating process. The prehistoric Quintana Rooans lived near if not on the coast: the caves are, today, from a few hundred meters to 10 kilometers (6 miles) from the water.
Violence, virus and variety
Let us admit that craniofacial features may be affected by much more than genetics, and not only by getting clubbed in the face. Nutrition and disease may play a part; so may culture. An extreme example is skull-shaping – a strangely popular cultural artifact in prehistory that led the unwary to suspect the deformed crania were from aliens.
Moreover, when all we have are four specimens, there is always the question of outlying characteristics, and whether illness could have profoundly affected individual specimens. For instance, Homo floresiensis, the so-called hobbit discovered on the Indonesian island of Flores, was originally postulated to be a sick or dwarfed modern human, rather than a diminutivized descendent of Homo erectus, which is the present popular theory.
Anyway, generally, craniofacial analyses of ancient human remains in the Americas have suggested considerable and unexpected biological diversity over time.
Hubbe and the team checked the morphology of the Quintana Roo skulls against a worldwide dataset of people today.
The oldest belonged to a young woman who lived almost 14,000 years ago. She apparently shared some key features with modern arctic North Americans in Greenland and Alaska, who are quite distinct from Native Americans, the authors say. They do qualify that her skull was far from complete, with not that many anatomical landmarks.
The second-oldest skull was also badly fragmented, was probably a young male and demonstrated affinities with modern Europeans: a new finding for early American remains based on craniofacial comparisons. He showed no affinities with early or later American groups.
The third skull was a lady who seems to have achieved mid-adulthood – mazel tov! – though when is a little unclear (the margins of dating in her case were wide). She appeared to show associations with paleo-Brazilian and paleo-Japanese groups.
The fourth skull came from a middle-aged man who lived around 10,000 years ago. He was harder to classify by morphological associations, but did show some commonalities with arctic populations, as well as modern South American features.
Separate work on South American human remains consistently found morphological associations with modern Australo-Melanesian and African groups, and with Late Pleistocene specimens found in Europe and Asia.
Playing with crayons
So, it seems that early North American colonizers were highly diverse; the peoples migrating to Central America were also diverse; possibly there was constant or intermittent gene flow from the north to Mexico, which is en route to South America. But, based on the evidence so far, it seems that only some population groups dispersed into South America, resulting in diminished diversity.
It has been becoming clearer in the last couple of decades that contemporary Native Americans look very different from the earliest arrivers. The so-called Kennewick Man, a paleo-American who lived about 9,000 years ago and was found in 1996 on the banks of the Columbia River, seems to have been genetically closest to Native Americans – but had features more typical of Polynesians, aboriginal Japanese and Europeans. It seems that Native American craniofacial patterns took their present form later, during the Holocene.
Today’s Native American craniofacial patterns differ from the first arrivers to the Americas as much as Australian Aboriginals and Melanesians differ from East Asians, the authors write.
As for the earliest South American remains from the late Paleolithic, morphologically they have a strong, consistent association with Australo-Melanesians and Africans.
What does all this mean? It does not mean that Europeans or Polynesians were boating over to the Americas in prehistory. African and Melanesian features are explained as retention of ancestral human features, which was subsequently lost. The authors are not implying Pleistocene or even early Holocene migration from Europe to the Americas or ancestor-descendant relations between the reference groups and the paleo-Mexicans.
“The fact that these individuals resemble Europeans or Melanesians does not mean they are descendants from these populations,” Hubbe elaborates. “The genetic inheritance of skull morphology is very complex and we use other populations to create a scale of reference, to contextualize the differences. I stick so far to the genetic data available, which shows all Native American groups are descendants of Asian populations who crossed the Bering Strait.”
Ultimately, the new study does not really say much about the people who came into the Americas, Hubbe explains: It’s more about what happened to these people after they got here.
“Most of the times, researchers just assume that people got here and just moved south, following these long and linear arrows we draw on maps,” he tells Haaretz. “What we are arguing is that we should not be drawing those arrows... Talking about the settlement of the Americas is like building a 1,000-piece jigsaw puzzle with only about 20 pieces – which means we need to pick up crayons and make our own drawings to complete the gaps between pieces.”
Their new piece doesn’t change the pieces we know; it forces us to consider other ways we could draw the background to fill in the gaps, he says.
So, the history of the Americas’ occupation is apparently more complex than we had thought, the models so far are too simplistic, and we are likely to yet be surprised. Stay tuned.