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Friday, May 23, 2008

Tracing Humanity's Path

This blog has been hitting the politics pretty heavily lately, which seems justifiable given that the future of humanity is at stake in the decisions we make now. But today in ScienceNOW, Science's online news service, I report on an interesting paper about humanity's past, which includes the surprising finding that the Americas might have been been colonized in multiple waves rather than one primary migration. The paper, in the journal PloS Genetics is available here for those who want all the technical details; my story for more general readers can be found here (it can be accessed for free for 30 days); and the text of the story is below.

Update: I just heard from team leader Daniel Falush that there is a YouTube version of the slideshow tracing humanity's journey, complete with music!

Tracing Humanity's Path

By Michael Balter
ScienceNOW Daily News
23 May 2008

Most researchers agree that modern humans got their start in Africa and then spread throughout the world beginning about 50,000 years ago. But scientists are still working out the details of how the planet was peopled, such as who went where, and when. A new study, employing sophisticated modeling techniques, confirms the prevailing Out of Africa model but also comes up with some surprises, including evidence that the Americas' first human inhabitants arrived in multiple waves.

Archaeologists and anthropologists worldwide have dug up plenty of skeletons over the years, but the bones seldom say much about where ancient peoples originally came from. Thus researchers have tried using variations in the genes of living individuals to trace their ancestries back to prehistoric times. In general, the closer two modern populations are genetically, the more likely that they share a common ancestry; yet this ancestral heritage is sometimes obscured by genetic changes that have taken place over thousands of years, as well as by interbreeding between populations. Happily, efforts to get around these complications have been boosted by an ever-growing mound of data about genetic differences between human populations.

A team led by geneticist Daniel Falush of University College Cork in Ireland developed a new mathematical model to compare not just individual genes or short DNA segments, as previous studies have done, but also very long stretches of DNA. Falush and his colleagues analyzed 32 DNA segments, each consisting of more than 300,000 base pairs, from 927 people representing 53 different populations from around the globe. Plugging this huge amount of data into computer simulations, the team worked out which migration scenarios were most likely to have created the genetic variation we see today. The results, reported today in PloS Genetics, suggest that modern humans peopled the world in nine phases, beginning in Africa, moving on to Europe and Asia, and finally colonizing the Americas and the Pacific islands. (The team illustrates humanity's journey in two movies accompanying the paper; see below.) The team did not try to date the migrations.

The study came up with two unexpected findings. One is that the people of the Orkney Islands, to the north of Scotland, share some ancestry with Siberians, possibly because some ancestors of modern Orcadians ventured to Asia via the Arctic Circle. The team also found that North and South America were colonized independently by at least two different waves of migration from different parts of Asia, although both waves appear to have arrived via the Bering Strait. This conclusion contradicts the conventional view, which postulates just one migratory wave out of Asia.

"I like the paper very much," says Jonathan Pritchard, a human geneticist at the University of Chicago in Illinois. "It's a very novel and creative way of thinking about the data" that "may provide a better representation of human history." Ripan Malhi, a molecular anthropologist at the University of Illinois, Urbana-Champaign, says that the team's approach "holds great potential to give us important and novel insights into the peopling of the Americas." Nevertheless, Malhi cautions that the multiple migrations Falush and his colleagues detect in the Americas might be an artifact of ancient population movements "more complex than the simple models created in this study can accommodate."

Related sites

Watch human populations march across the globe in two movies created by Falush and his colleagues:

  • http://www.plosgenetics.org/article/fetchFirstRepresentation.action?uri=info:doi/10.1371/journal.pgen.1000078.s008
  • http://www.plosgenetics.org/article/fetchFirstRepresentation.action?uri=info:doi/10.1371/journal.pgen.1000078.s009
  • 8 comments:

    Woody said...

    Interesting, but I have suspicions that the study misses a lot. It would seem almost impossible to reconstruct human migration correctly. Also, some skeletons found in the northwest U.S. and in South America don't seem to fit the conclusions as I understand them. You reported the research, but what's your gut feel about it?

    Michael Balter said...

    Well, Woody, the study has the advantage of comparing much bigger stretches of DNA in reconstructing ancient migrations, and the reactions from researchers were pretty positive on that score. There is nothing in the fossil record that would directly contradict the notion of multiple colonizations of the Americas that I am aware of, could you be more precise?

    Woody said...

    I don't have the references, but there were archeological finds of pre-historic non-Indians in the U.S. northwest, either Peru or Chile, and in the Great Lakes area--I think Minnesota. Those didn't seem to fit the immigration lines that I saw in the charts from the study you referenced. I'm guessing that they cover the main migrations but don't explain unique finds.

    Woody said...

    Balter, want to know how G.M. and I sound when we talk to each other?

    Computer 'Recreates' Neanderthal Speech

    via DADvocate

    Michael Balter said...

    Woody, I think what you are referring to is the evidence for so-called pre-Clovis migrations into the Americas prior to 13,000 years, for which there is evidence in both North and South America. The migration maps you saw were very rough and based on specific populations that exist today, so should not be taken too literally. But these pre-Clovis migrations should not be referred to as "non-Indian," in fact really calling the earliest Americans "Indians" is misleading. All early Americans, however, came from Asia, both pre-Clovis and Clovis groups, and thus all could be referred to as the founding Native American populations.

    Woody said...

    Thanks for the distinctions and clarifications. I'm too pressed for time to do a look-up, but the one earlier find in Minnesota(?) seemed to be Nordic. Perhaps the "native Americans" were not the original natives.

    Woody said...

    Just found this, which ties in.

    First Greenlanders left no descendents?

    "Here's another example of how genetic methods can shed light on archaeological questions, Paleo-Eskimo mtDNA Genome Reveals Matrilineal Discontinuity in Greenland:

    "...This suggests that the earliest migrants into the New World's northern extremes derived from populations in the Bering Sea area, and were neither directly related to Native Americans nor the later Neo-Eskimos that replaced them.

    "New Scientist has a popular press profile of the research & findings. Remember last year when it was confirmed that Polynesians had to have been visiting the coast of South America because of the phylogeny of chicken DNA extracted from subfossils? Though there have always been hints, I think this suggests greater complexities to our picture of the pre-Columbian world...."

    Dominique said...

    THE HUMAN EXPERIENCE – the story of a band of brothers who travel the world in search of the answers to the burning questions: Who am I? Who is Man? Why do we search for meaning? Their journey brings them into the middle of the lives of the homeless on the streets of New York City, the orphans and disabled children of Peru, and the abandoned lepers in the forests of Ghana, Africa. What the young men discover changes them forever. Through one on one interviews and real life encounters, the brothers are awakened to the beauty of the human person and the resilience of the human spirit.

    Check out the trailer on grassrootsfilms.com