The peopling of the Americas
New archaeological and genetic evidence is fleshing out the story. Dyani Lewis reports.
Exactly when and how the first people populated the final continental frontiers of North and South America has been a subject of debate for more than a century. We might be getting closer to an answer, however.
Recent technological advances have seen genetic and archaeological evidence converge on an increasingly consistent narrative, according to a review in the journal Science by anthropologist Michael Waters from Texas A&M University, US.
“It is an exciting time to be in this field, with new archaeological and genetic discoveries being published at an ever-increasing rate,” he says.
An early consensus emerged in the twentieth century that people arrived in the Americas around 13,000 years ago, during the Pleistocene.
This was largely based on the discovery of stone spearheads known as Clovis points from sites across North America. Radiocarbon dating put these sites at roughly 13,000-12,700 years old.
The assumption that Clovis sites were the oldest on the continent was on shaky ground by the late 1990s, however.
Artefacts including stone projectiles, wooden tools and hearths discovered at the Monte Verde site in southern Chile dated the site to a pre-Clovis age of 14,200 years old.
A smattering of pre-Clovis sites across North America have subsequently turned up. As the number of these sites has grown, so too has the accuracy of dating methods. There are now undisputed ages stretching back more that 15,000 years.
Genetic studies of modern Indigenous populations and prehistoric individuals is also fleshing out the complex narrative.
Studies of mitochondrial DNA and Y chromosome DNA from modern Indigenous people suggest that the ancestors of all contemporary Indigenous Americans are descended from just five maternal and two paternal lines.
This tiny founding population has its roots in Asia. Its ancestors split with the ancestors of East Asians around 36,000 years ago. Intermingling between the ancestral Indigenous American population in northern Asia and ancient peoples of East Asia and Siberia continued until around 20,000 years ago.
As the ancestors of Indigenous American moved across the land bridge connecting modern-day Siberia and Alaska – a region known as Beringia – populations split and split again.
Two branches of ancestral Americans eventually migrated south into the unglaciated areas of North America.
Precisely when this occurred is difficult to pinpoint, however, it probably coincided with the melting of ice sheets that blocked the passage south from Beringia.
Once south of the ice sheets, the “northern” branch of Indigenous Americans populated regions of North America, whereas the “southern” branch spread throughout North and South America.
Mitochondrial genomes place the arrival of ancestral Indigenous Americans on unglaciated lands south of the ice sheet at around 16,000 years ago. The genetics of dogs travelling with humans gives a similar estimate (16,500-13,700 years ago).
Ancient DNA, extracted from the remains of long-buried people, has been particularly transformative for the field. “In the last 10 years ancient genomic studies have revolutionised the study of the first Americans,” says Waters.
Scientists are still picking through the complex web of interconnections between geographically close – and sometimes distant – people that recent studies reveal.
For instance, ancient genomes from 10,000-year-old Lagoa Santa individuals in Brazil and some contemporary Amazonian tribes share an unexpected connection with Indigenous peoples from New Guinea, Australia and the Andaman Islands.
By around 13,000 years ago, regional stone tool technologies, such as Clovis points, Fishtail points and Stemmed points have been established, but the genetics don’t match these defined cultures.
Understanding the origins of these technologies and how they emerged is just one of the many avenues of research that is ongoing.
As additional older sites and additional ancient genomes are uncovered, the specifics of how the Americas were first settled will come into even sharper focus.