Bridge to the New World

How and when humans first came to the Americas has long been a topic of intense debate. Theories to explain the colonization of the New World—the last great habitable landmass to be occupied by humans—focus on the Bering land bridge, or Beringia, which emerged between Siberia and Alaska during the last ice age. Rising from seas drained by the water-locking effect of spreading ice caps, Beringia is said to have given passage to the forebears of Native Americans anywhere between 40,000 and 15,000 years ago.

The earliest date, based on a footprint left in volcanic ash in Mexico, might be wildly off track, according to recent studies that put the migration at the opposite end of the timescale. While older archaeological evidence exists, 13,000 years is the date put on the oldest known human remains. This fits with the findings of DNA studies that indicate that the genetic ancestors of the Native Americans consisted of a few dozen individuals from eastern Siberia who ventured across Beringia less than 20,000 years ago.

The gap between their departure and arrival (the length of this time gap varies depending on the study) is explained by the presence of two massive ice sheets, the Laurentide and Cordilleran, that blocked entry to North America. The hunter-gatherers waited out the ice age in Beringia. (A significant landmass, if a temporary one, Beringia supported large herds of mammoth, reindeer, and other substantial prey.) When conditions eventually warmed, and the glaciers began to melt, the migrants were up and running again.

Or were they? Another debate centers on the migrants’ mode of transport. Some researchers believe Beringia was flooded by rising sea levels before its inhabitants had the chance to cross into Alaska. Challenging the idea that the land bridge was the only route in, they say groups must have ferried themselves in boats at least part of the way. This seafaring theory is supported by evidence of human occupation in Chile by 14,500 years ago. Such rapid progress from the Arctic, all the way down through South America, is difficult to explain if the colonizers had to wait for corridors to open up in the ice sheets that overlaid the land route through Canada. Southward excursions down the Pacific Coast by boat would have been a much quicker way of covering the distance.

Backing for this argument has been found in the form of ancient DNA extracted from a 10,300-year-old tooth from Alaska’s Prince of Wales Island. Researchers found that only one percent of modern indigenous people they sampled matched the tooth’s genetic markers, but those who did lived primarily along the Pacific Coast, between Chile and California.

Such findings haven’t helped the pioneer reputation of the Clovis people, long feted as the earliest occupiers of America. A culture characterized by fluted stone spear points, often found with the bones of bison and other large animals, the Clovis people are described from sites dating back 13,000 years in New Mexico and Colorado. But that’s too recent, and too far inland to retain their claim as founders of the New World.