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First Americans Used Two Distinct Routes, Genetic Study Suggests

NEW YORK (GenomeWeb News) – The first people to migrate from Beringia to the Americas did so via two routes — one along the Pacific Coast and another by an inland path east of the Rocky Mountains, according to research appearing online today in Current Biology.
A team of Italian and American researchers sequenced dozens of mitochondrial genomes from individuals belonging to two rare haplogroups. Based on their phylogenetic and phylogeographic analyses of these and other mitochondrial sequences, the researchers concluded that two groups of Paleo-Indians migrated from Beringia to the Americas roughly 15,000 to 17,000 years ago via different paths. And, they argue, that dual migration may shed light on everything from Native American linguistics to archeology.
“Our findings indicate that the first peopling of the Americas — the one carried out by the so-called Paleo-Indians (the first Americans) — was not the result of a single migratory event, but of at least two migrations,” senior author Antonio Torroni, a geneticist at Italy’s University of Pavia, said in an e-mail message.
“I think it’s the first evidence for genetic support for the use of both a land and sea route,” Connie Mulligan, a University of Florida geneticist and anthropologist who was not involved in the study, told GenomeWeb Daily News. But, she noted, the team will need to test their results statistically to be sure there aren’t alternative hypotheses that more accurately explain their data. Last year, Mulligan and her colleagues published work supporting a three-stage peopling of the Americas.
In an effort to better understand the timing and routes of human migration into the Americas, Torroni and his colleagues moved away from studying the major “common” mitochondrial haplogroups — A2, B2, C1, and D1 — frequently found in Native American populations in North, Central, and South America. Instead, they focused on two rare haplogroups: X2a and D4h3.
The X2a haplogroup, discovered just over a decade ago, is found relatively frequently in Ojibwa populations in northern Ontario. It has also turned up, in some Nuu-Chah-Nulth, Sioux, Yakima, and Navajo populations. To date, this haplotype has not been detected in modern Asian populations.
A bit more recently, researchers discovered the D4h3 minor mitochondrial haplogroup in Cayapa populations in Ecuador. Individuals from the D4h3 haplogroup appear to be mainly distributed along the Pacific coast.
In an effort to find more individuals from these groups, the researchers searched the Sorenson Molecular Genealogy Foundation mtDNA database for sequences characteristic of the X2a and D4h3 lineages. They then sequenced the roughly 17,000 base pair genomes of 55 individuals that they suspected of belonging to these rare haplogroups and did a phylogenetic analysis incorporating this data with existing sequence data on 14 X2a or D4h3 individuals and from 276 individuals from common haplogroups.
The researchers identified one Asian individual with the D4h3 haplotype — the first so far — and a handful of D4h3 individuals from Mexico and California. But most individuals with the D4h3 haplotype came from South America. In contrast, X2a and related individuals were found mainly in the Great Lakes and Great Plains regions of North America.
Based on their subsequent phylogeographic analyses, the researchers determined that there were probably at least two nearly simultaneous migrations of people from Beringia to different parts of the Americas.
They concluded that D4h3 individuals left Beringia and headed down the Pacific Coast, while X2 individuals picked their way along an ice-free strip of land east of the Rocky Mountains between the Laurentide and Cordilleran ice sheets. Torroni and his colleagues estimate that both migrations occurred roughly 15,000 to 17,000 years ago.
That’s similar to previous estimates, though the dual migration theory is new. Mulligan said the latest work doesn’t necessarily conflict with her team’s research, noting that estimated migration times depend on the mutation rates used in calculations.
While she agreed that the new paper offered compelling evidence that the first Americans may have arrived by both inland and coastal routes, Mulligan was skeptical about the group’s other suggestion: that Beringian populations were sub-structured in terms of language and culture.
Based on the migratory patterns they uncovered, Torroni and his colleagues speculated that there may have been non-homogenous populations in Beringia before the groups headed to the Americas. “[D]ifferent early industries and technologies observed in North American archeological sites may be related to the two different entry routes rather than being the result of in situ differentiation,” Torroni said in an e-mail.
The authors argued that the work could also have implications for understanding the controversial issue of Native American linguistic patterns. “Our data tend to support the possibility that there was more than one language for people who came to North America,” Torroni told GenomeWeb Daily News.
Mulligan, on the other hand, argued that these cultural and linguistic splits may just as easily have occurred after individuals left Beringia, with population bottlenecks offering one explanation for the distinct mitochondrial haplogroups existing today.
In the future, Torroni and his team plan to do more work aimed at dissecting the common or pan-American haplogroups into branches that could reveal more insights into the distribution of mitochondrial sub-groups across the Americas — an effort he predicts will reveal some overlapping distributions with X2a and D4h3 and uncover more yet-unknown human migrations across the Americas.

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