It seems like new discoveries about the peopling of the Americas are a dime a dozen these days.  Without a doubt, the journey those first Americans took from the frozen wastelands of Asia down the Pacific coast into the Americas has been an active research subject for many decades.  Archaeologists, linguists, and now geneticists have all analyzed the data in their respective fields, and while we have seen progress in figuring out the overall timing and migration routes across the Bering Strait during the height of the Ice Age, many questions remain unanswered.  For example, there is still disagreement over whether there was a single wave of migrations into the New World around 18,000 years ago (a scenario generally favored by geneticists), or whether there were several separate migrations, each bringing across the Bering Strait its own distinct culture and languages (more popular among linguists).

Now, an international team of geneticists has added to the debate by trying things a bit differently.  While the majority of genetic studies have focused on the four most common mitochondrial DNA (mtDNA) types, or ‘haplogroups,’ among Native Americans, these authors switched things up a bit. In study published online Thursday by Current Biology, they focused instead on two of the most rare and localized mtDNA haplogroups in the New World: D4h3 and X2a.

Mitochondrial DNA is passed down from mother to child exclusively. So by comparing the mtDNA of different populations, geneticists can estimate where and when their female lines diverged from one another.

Haplogroup D4h3 is usually found along the Pacific coast of South America, while X2a has been found only in north-central North America.  The authors sampled a total of 55 individuals who fell into either one of these two groups.  They sequenced the entire mitochondrial genome for each, thereby adding to the considerable lack of knowledge on these haplogroups; this was the first time that anyone completely sequenced representatives of either D4h3 or X2a.

The analyses of haplogroups D4h3 and X2a revealed two distinct genetic histories. That difference suggests they may have come from separate regions of Asia and expanded in the New World in very different directions, even though they both may have arrived around the same time.

Specifically, the authors argue that that, even though it appears that both D4h3 and X2a individuals arrived in the New World at about the same time period – between 14,000 and 17,000 years ago – they took very different routes to get there. The authors argue that D4h3 individuals crossed from Asia to the Americas via a coastal route; the same path the ancestors of most people bearing the major haplogroups are believed to have taken. Then, they continued down the Pacific coast, settling in various places along the way.

However, X2a individuals seemed to have embarked on a different journey.  Avoiding the coast entirely, this group of people traveled through a small inland corridor between two major North American ice sheets, about 15,000 years ago.  Then, they continued into the heart of North America, settling in what is now central Canada, where their descendants still reside today.

The results presented by the authors provide – perhaps for the first time – clear evidence that at least two separate routes were used by the earliest Paleo-Indians as they left East Asia and entered the Americas.  But, and maybe even more importantly, it shows that the migrants who took these routes may have come from two different source populations.  We can only hope that future research on these two haplogroups can reveal where in East Asia they originated, and maybe even what made them cross the frozen landscape in the first place – by land and by sea.

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A study in last month’s American Journal of Human Genetics argued for an ancient and fast migration of prehistoric humans across the Bering Strait and down the Pacific Coast based on a genetic analysis of present-day Native Americans. That study was provocative because it significantly pushed back the arrival date of humans to the New World, to 18,000 years ago from the previous estimate of 13,000 years.

Now, results from a new study in this week’s Science further bolster that theory, this time using the prevalence of preserved seaweed as an indicator.

Anthropologists at Vanderbilt University have excavated a South American archaeological site known as Monte Verde II. Located in southern Chile, Monte Verde II is midway between the Pacific Coast and the Andes mountains. It dates to approximately 14,600 years ago, making it one of the oldest archaeological sites in the New World. The age of Monte Verde II, combined with its extreme southern location, lend additional support to the hypothesis of an early arrival of humans into South America.

And the sheer abundance of preserved seaweed in various huts at Monte Verde II lends additional support for the initial migration occurring down the Pacific Coast, rather than further inland. Seaweed is an incredibly fragile material, and does not preserve well. However, remains of seaweed are found all over Monte Verde II. The authors think the seaweed was probably used extensively both as food and for medicinal purposes, something that still goes on in present-day coastal communities.

The presence of some foods from inland areas indicates that the inhabitants of Monte Verde also traveled inland at some point during the year, or else mingled with other communities that did so.

So how do these findings help us to understand the earliest inhabitants of the Americas? Most importantly, this study supports the idea that the first Americans traveled down the length of the Americas via the Pacific Coast. But it also shows that there does seem to be some relationship between coastal communities, such as Monte Verde, and those groups that were more inland.

In fact, the authors of this study use this possible relationship to propose a slight variation on the theme of a coastal migration route, saying that the first inhabitants may have taken a slower journey down the coast, spreading out in the interior along the way. Although this hypothesis is in need of additional support, it is clear that the debate on the peopling of the Americas is far from over.

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Just about everything else is subject to debate: who the people were, where they originated, when they migrated, how numerous they were and what they did once they crossed into the Americas.

Finally, a solution to the decades-old dispute may be at hand. That’s because genetic analyses are beginning to converge on a scenario that fits in with all the archaeological, linguistic, climatological and ecological data relevant to the issue – and also seems to make sense.

In this month’s issue of the American Journal of Human Genetics, researchers from Brazil and California argue that the present-day pattern of genetic diversity among Native Americans suggests a rapid southward migration along the Pacific coast that began about 18,000 years ago and took only a few thousand years to reach the tip of South America.

It was only later, the researchers claim, that people made their way inland in pursuit of large game animals such as woolly mammoth. The traditional view is that these mammoth hunters, who appear in the archaeological record about 13,000 years ago, were the first Americans.

This is not the first time a migration down the Pacific coast has been proposed. But the researchers were able to show by comparing the mitochondrial DNA of 86 Native Americans that all five of the major genetic branches in the New World trace back to an ancestral population of a few thousand people that began expanding rapidly around 18,000 years ago.

At that time – not long after the peak of the Ice Age – glaciers thousands of feet thick blocked the land route from Alaska into the heart of North America. But the coast was clear – recent research indicates the Pacific rim of North America was relatively ice-free by about 19,000 years ago.

The DNA evidence also offers information about the ancestral Asian populations that gave rise to the first Americans. The researchers compared the Native American DNA samples to their closest known relatives in Asia, and found that the Asian and American lines appear to have split about 5,000 years before the rapid population expansion on the American side.

That suggests a long period of isolation, probably in the region that is now around (and underneath) the Bering Strait, before the first people flooded into the New World.

Two recent papers in the open-access journal Public Library of Science One suggest a similar scenario, albeit with slightly different dates. One study, published last month by University of Florida researchers, analyzed mitochondrial DNA donated by 77 Native Americans. That study found that the overall pattern of diversity was most consistent with a population that has experienced two episodes of rapid growth – one about 40,000 years ago and another about 15,000 years ago.

A second study, published last year by a global team of researchers, found that all of the major genetic branches of Native Americans appear to have started diversifying about 14,000 years ago, after being isolated for a long period of time.

The beauty of these genetic studies is their ability to explain the vexing archaeological evidence for the migration of people into the Americas. There are signs of humans living in Siberia, far from the major population centers of the time, as early as 30,000 years ago. And there is evidence of people at the Monte Verde archaeological site in southern Chile as early as 14,500 years ago.

Yet there are no archaeological sites intermediate between those two very distant locations in time and space. The genetic evidence suggest two reasons why: First, a small number of people migrating very quickly down the Pacific coast wouldn’t have left many artifacts behind. And second, if the migration happened shortly after the peak of the Ice Age, as the genetic evidence suggests, then the coastline it followed is now well offshore – sea level has risen 120 feet since then.

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