Humans
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The question of when people first arrived in the Americas has gained another fascinating piece of the puzzle, but don’t expect it to be the last say on the matter.
The "Clovis First” hypothesis is old news
For decades, the dominant idea was that prehistoric people arrived at the end of the last Ice Age, around 13,500 years ago, by walking across a long-gone land bridge between the northeastern tip of Eurasia and North America, then heading south through an ice-free corridor.
Known as the "Clovis First” hypothesis, this neat little story has since come under heavy scrutiny in recent years after several archaeological sites have shown that humans were likely in the Americas long before this time. However, exactly how much earlier has remained hotly debated.
Three possible ETAs
In the latest study, scientists from the University of Kansas reviewed previous genetic and archaeological work to assess three leading models for the first arrival: before the peak of the last Ice Age (more than 25,000 years ago), between 20,000 and 14,000 years ago, and towards the end of the Ice Age (less than 14,000 years ago).
The researchers suggest the current body of evidence favors the view that the direct ancestors of Native Americans – AKA the First Peoples – arrived between 20,000 and 14,000 years ago.
However, like many things related to genetics and ancestry, it gets complicated, and the team argues further genetic evidence could upend this theory.
Ancient DNA hints at one story
By tracing through ancient DNA, it's possible to see Native Americans descend from a mix of Ancient North Eurasians and ancient East Asians. This group made the trek across to North America and appears to have lived in isolation for thousands of years during the Ice Age, a period known as the "Beringian Standstill", perhaps because massive ice sheets blocked their movements.
Around this time, they diverged into distinct lineages, including Ancient Native Americans, Ancient Beringians, and "ghost populations" known only from faint genetic traces. Once temperatures warmed up, the groups were free to travel farther south and spread further across the Americas.
Crucially, however, a genetic schism between northern and southern Native American branches appears to have occurred around 15,000 years ago, somewhere south of the ice sheets.
It's not clear how exactly this occurred, but it's a fairly solid indication that people had arrived in the Americas much earlier than 13,000 years ago, the researchers argue.
Archaeology tells another tale
Some of the older archaeological sites go against the arrival window of 20,000 and 14,000 years ago. However, the study argues these exceptional old sites are very much the outliers, noting there's more archaeological evidence that people arrived in the Americas about 14,500 years ago than there is for an arrival around 25,000 years ago.
For instance, sites such as Cerutti Mastodon in California, where potentially butchered mastodon bones were dated to about 130,000 years ago, remain highly contested. Modern humans hadn’t even left Africa at this point, plus there’s no indication of their presence in any genomes.
Nevertheless, a smattering of other sites, like the 21,000-year-old site in South America that has evidence of giant armadillos being chopped up, do back up the idea of humans roving around the Americas during or before the Ice Age’s peak.
If that view is accurate, these people either left no living descendants, belonged to lineages not yet sampled by scientists, or left only faint traces in the genomes of later generations.
Gaps in knowledge
Indeed, the researchers acknowledge that frustratingly little ancient DNA has been recovered from the Americas, especially North America where this story most likely begins.
If we are to understand how the American continents were first settled, they say, it will require far more genetic data and rigorous archaeological digs, as well as insights from tribal historians and Indigenous knowledge.
“It is critical that paleogenomics research be rooted in questions articulated by archaeologists and tribal historians, and that such research be attuned to the needs and priorities of descendant communities,” the researchers concluded.
"We see several scenarios that could be compatible with a pre-LGM [Last Glacial Maximum] peopling scenario, but they require further characterization of patterns of Native American genetic variation in the past to truly assess. Such characterization is highly dependent on developing a more ethical and inclusive approach to paleogenomics," they added.
The study is published in the journal Current Opinion in Genetics & Development.
