On a belated “foundation” event: the peopling of the New World

[Jacques Cinq-Mars]

All,

For your information:

EurekAlert -- Public release date: 23-May-2005

Contact: Joseph Blumberg
blumberg@ur.rutgers.edu
732-932-7084 x652
Rutgers, the State University of New Jersey

Founding fathers & mothers: How many crossed the land bridge?

NEW BRUNSWICK/PISCATAWAY, N.J. - Programs on the Discovery Channel and PBS have sparked fresh interest in the prehistoric peopling of the New World. Now, for the first time, we have a realistic estimate of how many ancients made that ice age trek across the long-lost land bridge from Asia to become the first Native Americans.

Jody Hey, a professor of genetics at Rutgers, The State University of New Jersey, has developed a computational method that uses genetic information to create models of population divergence - where a group has split off from its ancestral population to pursue its own destiny.

In a paper appearing in the June 2005 issue of PLoS (Public Library of Science) Biology, Hey disclosed his findings. "The estimated effective size of the founding population for the New World is about 70 individuals," Hey said. "Calculations also showed that this represents approximately 1 percent of the effective size of the estimated ancestral Asian population."

"Effective size" in population genetics is often thought of as the number of adults of reproductive age. One rule of thumb is the effective size might be about one third of the 'census population size' which, in this case, comes out to about 200 people.

In addition to population size, Hey's rigorous and complex methodology also generated historical estimates of when the divergence occurred. His dates are consistent with much of the archaeological record ? in the range of 12,000-14,000 years ago.

He was also able to discern changes in population size and the extent of gene flow between populations, potentially representing renewed contact. Hey used nine genes in which sequences and frequencies were well documented in the scientific literature.

"The beauty of the new methodology is that it uses actual DNA sequences collected from Asian peoples and Native Americans, an approach that can provide a detailed portrait of historical populations," Hey said. The method doesn't use summary statistics or averages as some approaches do, but gleans as much information as possible directly from the genetic data.

Hey focused on the genetics of Amerind-speaking populations, one of three major language groups in the New World representing the earliest migrants who extended deep into the Americas. The other groups, the more recent Athabascan speakers and the even more recent Eskimos and Aleuts, had less comprehensive genetic information available and were not included in Hey's study.

And the paper is:

Hey, Jody. 2005. On the number of New World founders: A population genetic portrait of the peopling of the Americas. PLoS Biol 3(6): e193.

Abstract:
The founding of New World populations by Asian peoples is the focus of considerable archaeological and genetic research, and there persist important questions on when and how these events occurred. Genetic data offer great potential for the study of human population history, but there are significant challenges in discerning distinct demographic processes. A new method for the study of diverging populations was applied to questions on the founding and history of Amerind-speaking Native American populations. The model permits estimation of founding population sizes, changes in population size, time of population formation, and gene flow. Analyses of data from nine loci are consistent with the general portrait that has emerged from archaeological and other kinds of evidence. The estimated effective size of the founding population for the New World is fewer than 80 individuals, approximately 1% of the effective size of the estimated ancestral Asian population. By adding a splitting parameter to population divergence models it becomes possible to develop detailed portraits of human demographic history. Analyses of Asian and New World data support a model of a recent founding of the New World by a population of quite small effective size.

Click HERE for the full article.

I’ll try to “digest”, but I already suspect that the apparent elegance (to me, anyway) of the model doesn’t do justice to the complex realities of the New World early palaeoanthropological record.

Jacques Cinq-Mars

[Jacques Cinq-Mars]

All,

Here is a first media follow up on the Hey’s article mentioned in my earlier post. I should add that Michael Hopkin, the author of this Nature piece, should be commended for his rather accurate summary of an otherwise, (palaeo)anthropologically -- in an interdisciplinary sense -- inane paper.

Jacques Cinq-Mars

Americas had seventy 'founding fathers'
Michael Hopkin
news@nature.com --Published online: 24 May 2005

Gene study counts the first humans to reach the New World.

The first people to colonize the Americas were a band of just 70 hardy explorers and their families, a genetic study suggests. Analysis of Native Americans' genes shows that their ancestors represented just a tiny fraction of the Asian population at the time.

This intrepid group is thought to have made the arduous journey across a long-lost land bridge between Siberia and Alaska about 14,000 years ago. The research suggests that this entire group might have numbered just 200 people, since experts generally expect populations to be about three times the size of the group that ultimately pass on their genes.

"The number of founders might be a surprise to some, although we knew that there was a bottleneck of some magnitude," says Jody Hey of Rutgers, the State University of New Jersey, who carried out the study. The fact that the most plausible way on to the American continent involved a trek through the frozen north probably meant that few attempted the journey.

To test the theory, Hey analysed data from DNA samples previously collected from Amerind-speaking Native Americans, and from populations in northeast Asia. He compared the amount of variation in nine sections of their DNA to estimate the size of the founding population.

At the time, the Asian group had an 'effective population' (the number of reproducing adults) of around 9,000, Hey calculates. But the effective population of the Amerind group was less than 1% of this, he reports in the journal PLoS Biology1.

Far and wide

Once established in the New World, humans are thought to have made short work of spreading across its continents. Archaeological evidence from Monte Verde in Chile suggests that humans reached South America within a few centuries of their arrival in North America2.

The idea that they did this from such modest beginnings makes that achievement all the more impressive. But Hey points out that his study may not tell the whole story. His analysis did not include samples from other Native American populations in the northern reaches of the continent such as Aleuts, who are believed to have arrived there much later than the Asian immigrants.

He is also unable to say whether any of the New World's founders travelled back to mate with Asian people, or whether the original band were followed generations later by subsequent waves of migrants. But he does know that, once the first group reached America, their numbers boomed in just a few generations, and the effective population increased by an order of magnitude.

Hey hopes that his model can be used to investigate other situations in which a human population broke off to colonize a new area, such as the peopling of Australasia. "This approach could provide a detailed portrait of historical populations," he says.

References:

1. Hey J., et al. PLoS Biology, 3. e193 published online at plosbiology.org (2005).
2. Fiedel S. J. J., et al. Archaeol. Res., 8. 39 - 103 (2000).

Jacques Cinq-Mars

[Daryl Habel]

I'm in general agreement general with Jacques' apparent indigestion (!!).  John Hawks' weblog has a lengthy and detailed critical review of the current Hey paper.  Hawks specifically points to a number of problems with the conclusion given.  To readers (like me) who have trouble recognizing the limitations of genetic analyses,  and lacking the knowledge to properly critique papers dealing with genetic data, I'd recommend reading the entire Hawks review.  The permanent link Hawks provides for access to the review is:
CLICK HERE

Thanks John,
Dar