19 February 2017

Early humans didn't just "walk across Beringia"

Most well-informed people understand that the Americas were populated by early humans who crossed from Asia via the glacial-age land-bridge at Beringia.  But too many people have the misconception that once the land bridge formed (when sea levels dropped because oceanic water was tied up in glaciers), humans just marched across to the new world.  The process of crossing probably extended over thousands of years - and a new paper reports that this may have been long enough for the human genome to have evolved during the process.
In fact, the word ‘bridge’ definitely conjures up the wrong image. It was a geographic region, often called Beringia, and people lived there for so long that it probably would have been ludicrous to them that we could think of their home as transient. Current estimates suggest that people lived there for between 5,000 and 8,000 years, starting about 23,000 years ago.

That is a long enough time for natural selection to have had an effect on the genome of people who lived there, according to a paper in PNAS this week. The Beringians would have faced distinct diseases, food constraints, and climate conditions, and natural selection would have helped those with the right genetic adaptations to thrive in that environment. According to the new paper, we can see evidence of that natural selection in modern Native American populations...

Specifically, there’s evidence to suggest that three genes involved in metabolizing fatty acids (called the fatty acid desaturases, or FADS, genes) show changes that might be the result of adaptation to a diet high in protein and fats. That sort of diet tends to be one of the side-effects of living in the Arctic.
I don't mean to oversimplify the process.  The crossing may also have involved watercraft, and some subpopulations may have traversed the region more quickly.  And the Beringia crossing may have been supplemented by trans-Pacific voyages and/or by trans-Atlantic migrants from Solutrean regions.  But the PNAS paper does suggest that the time spent in the subarctic crossing may have influenced the Native American genome.

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