NEW YORK (GenomeWeb News) – The first people to arrive in the Americas migrated from Asia in three stages, with a long pause on land now covered by the Bering Strait, according to the latest genetic research.
Florida researchers used mitochondrial DNA and Bayesian skyline plot analyses to track the size of ancient populations over time. Based on the population growth patterns, combined with non-genetic evidence, they believe the founder group arriving in the New World was larger than previously suspected. Their results, published in PLoS ONE online today, also suggest these founders descended from a population that left Asia roughly 40,000 years ago and arrived in the Americas about 15,000 years ago — with a 20,000 year layover in Beringia on the way.
This was “a much longer period of time than anyone previously thought,” senior author Connie Mulligan, a University of Florida anthropologist, told GenomeWeb Daily News.
Mulligan and her team did multiple sequence alignments of 77 published Native American mitochondrial genomes, representing five American haplogroups. They also analyzed 812 hypervariable I and II sequences and a combination of analysis using mtDNA genomes, autosomal loci, and X and Y loci from Native American, Asian, and other New World populations.
They then estimated proto-Native American population changes over time using Bayesian skyline plots, which estimate the population size at every point where there is a change in mitochondrial sequence. These analyses “provide essentially a video of how population size has changed,” Mulligan said.
What they found was two distinct increases in population separated by tens of thousands of years. The first, they concluded, represents a population of roughly 640 people that left Asia about 40,000 years ago and slowly expanded to around 4,400 people as it migrated through Siberia and into Beringia.
During the second growth spurt, the population sprung up from about 4,000 to an estimated 64,000 individuals. Because this coincides with other evidence suggesting an ice-free corridor to North America opened up around this time, the researchers propose that this second expansion represents the peopling of the Americas.
“At the time, North America became accessible, all of a sudden a population moved into North America and rapidly expanded,” Mulligan said.
But evidence for a long, stable population in between Asia and the Americas is surprising. So far, no one has discovered archeological evidence of ancient populations living somewhere en route between Asia and the Americas for 20,000 years. The team’s conclusion: that period of human history may be submerged.
“Our theory predicts much of the archeological evidence is underwater,” lead author Andrew Kitchen, a graduate student in the University of Florida’s anthropology department, said in a statement. “That may explain why scientists hadn’t really considered a long-term occupation of Beringia.”
Mulligan and her colleagues also suspect that the New World founder group was larger than previously suspected. In contrast to theories that had a hundred or so individuals first peopling the Americas, the researchers’ data predicts that the founder population consisted of at least ten times this many people — between 1,000 and 5,000 individuals.
“Their technique of reading population history by using coalescence rates to analyze genetic data is very impressive,” Henry Harpending, University of Utah’s chairman of anthropology, who was not involved in the research, said in a statement. “The idea that people were stuck in Beringia for a long time is obvious in retrospect, but it has never been promulgated.”