Looking back, it was pretty dumb to think that people got to the Americas from Asia once, or that a single group braved the ice bridge to the new world. A cool new study published online today in the journal Current Biology may bury that simplistic assumption once and for all: according to the evidence of mitochondrial DNA, the first Americans arrived in at least two separate migrations, at about the same time, about 15,000 to 17,000 years ago.
To be fair, anthropologists have been tiptoeing up to the idea of multiple migrations for a while. When scientists at the Center for the Study of the First Americansrecalculated the age of the Clovis culture, named for a characteristic tool technology and considered to be the oldest irrefutable evidence of humans in the Americas, they got dates of 11,050 to 10,800 years ago—a range implying that “humans already lived in the Americas before Clovis” (since non-Clovis sites are older than that). And as far back as 2004, anthropologist Theodore Schurr of the University of Pennsylvania wrote that while the first migrants from Siberia crossed over to North America 20,000 to 14,000 years ago, probably following a coastal route, “a second migration that may have come from the same Siberian region entered the Americas somewhat later, possibly using an interior route.”
New genetic evidence makes that case more solid. Although the human genome project hasn’t been the promised godsend for human health and medicine (that link is to a 2005 article, but nothing much has changed in three years), it’s definitely been a boon to studies of human evolution and prehistory.
The new genetic analysis looks at mitochondrial DNA, which is passed from mothers to their children. Specifically, it looks at two rare mitochondrial DNA types that share a maternal ancestor. People carrying the type called D4h3 entered North America from Siberia and hugged the Pacific coastline, from which ice had mostly disappeared after the glacial maximum, the scientists conclude; they eventually reached Tierra del Fuego. People carrying the other mitochondrial form, X2a, set off inland, following a land corridor between two ice sheets which led them into the region east of the Rocky Mountains. They remained in North America. These first Americans were the ancestors of almost all modern Native Americans in North, Central, and South America (but not the Na-Dene or the Eskimos-Aleuts of the arctic).
“Two almost concomitant paths of migration . . . about 15,000 to 17,000 years ago led to the dispersal of Paleo-Indians, the first Americans,” said Antonio Torroni of the University of Pavia in Italy, who led the study (he made a splash last year when another genetic analysis suggested that a mere six women in the migration were the ancestors of all the Native Americans). That means the most likely scenario is that, within a few centuries, there were several arrivals into the Americas from Siberia. Did the earliest arrivals send back a scout to tell the stay-at-homes that this new world was worth a look? Did difficult conditions or population pressure in Siberia nudge people to seek greener pastures?