Ancient migration: Coming to America : Nature News & Comment
As caches of Clovis tools were uncovered across North America over subsequent decades, nearly all archaeologists signed on to the idea that the Clovis people were the first Americans. Any evidence of humans in the New World before the Clovis time was dismissed, sometimes harshly. T
hat was the case with the Washington-state mastodon kill, which was first described around 30 years ago1 but then largely ignored.
Intense criticism also rained down on competing theories of how people arrived, such as the idea that early Americans might have skirted the coastline in boats, avoiding the Bering land bridge entirely. “I was once warned not to write about coastal migration in my dissertation. My adviser said I would ruin my career,” says Jon Erlandson, an archaeologist at the University of Oregon in Eugene.
Overthrowing king clovis
It took a chance finding halfway around the world to set this reappraisal in motion. In the late 1970s, Tom Dillehay, an archaeologist at Vanderbilt University in Nashville, Tennessee, uncovered the remains of a large campsite in southern Chile, close to the tip of South America (see 'Routes to a new world').
Radiocarbon dating of wood and other organic remains suggested that the site was around 14,600 years old, implying that humans made it from Alaska to Chile more than 1,000 years before the oldest known Clovis tools2. But because the remote site was so hard for most researchers to examine, it would take nearly 20 years for Dillehay to convince his colleagues.
The case for pre-Clovis Americans has now gained more support, including from analyses of ancient DNA. One of the first bits of genetic evidence came from preserved faeces, or coprolites, that had been discovered in a cave in south-central Oregon by Dennis Jenkins, an archaeologist at the University of Oregon. Radiocarbon dating showed that the coprolites are between 14,300 and 14,000 years old, and DNA analysis confirmed that they are from humans3. The recovered DNA even shared genetic mutations with modern Native Americans.
Since the coprolite evidence emerged, in 2008, ancient DNA has also been used to reconstruct that long-ago mastodon hunt. Radiocarbon studies in the 1970s had suggested that the mastodon pre-dated the Clovis people, but some researchers explained that away by arguing that the animal had died in an accident. However, DNA studies last year4 showed that a fragment of bone embedded in the mastodon's rib had come from another mastodon — strong evidence that it was a spear point made by humans and not a shard that had chipped off a nearby bone in a fall.
DNA studies argue strongly against this hypothesis, and it gets little support from researchers. But some are hesitant to reject the idea outright, recognizing that the community was once before too conservative. “That's what happened with the Clovis paradigm,” says Dillehay.
Malhi and his colleagues also found hints that the first American colonists paused on their way out of Asia6, waiting out the peak of the last ice age on the exposed Bering land bridge for perhaps 5,000 years — long enough to become genetically distinct from other Asian populations. When the glaciers blocking their path into North America began to melt around 16,500 years ago, the Beringians made their way south over land or sea, passing those genetic differences on to their descendants in America.
Other researchers say that there is a major problem with relying on population genetics to answer questions about the peopling of the Americas. At least 80% of the New World's population was wiped out by disease, conflict or starvation after Europeans first arrived some five centuries ago. And the genes of many Native Americans today carry European and African markers, which confounds efforts to piece together the migration story. “If we look pre-contact, we're going to find a lot more indigenous diversity,” says Malhi.
