Ancient DNA evidence narrows down when people first reached the Americas. Researchers led by Bethany Potter at the University of Kansas reviewed migration models to find answers. They said the evidence best fits a timeline of 14,000 to 20,000 years ago, as genetic data showed a major branching event occurred later.
One of the unresolved mysteries about the history of the Americas is when the first people arrived. Despite extensive debate centering on genetic, archaeological and environmental evidence, there is still no consensus or conclusive proof. Did they enter before, during or after the coldest period of the last ice age, known as the Last Glacial Maximum? Evidence from recent analysis of ancient DNA is providing fresh clues.
Researchers led by Bethany Potter at the University of Kansas reviewed published research to reevaluate the three main migration models. These propose that the ancestors of today's Indigenous Americans arrived before the Last Glacial Maximum (more than 25,000 years ago), during it (between 20,000 and 14,000 years ago) or after it ended (less than 14,000 years ago).
Searching for clues
To test these three scenarios, the researchers evaluated how well each timeline fits three lines of evidence. They examined findings from ancient DNA for genetic clues about when populations split, checked archaeological sites for evidence of human activity and considered environmental data to see when ice sheets and climate conditions would have been favorable for travel. For example, ice sheets blocked much of North America during the Last Glacial Maximum, which is why some scientists believe that early travelers followed Pacific coastal routes.
As the researchers write in a paper published in the journal Current Opinion in Genetics & Development, the evidence best fits a timeline of 14,000 to 20,000 years ago for several reasons.
DNA narrows down the timeline
Arriving after 14,000 years ago is at odds with most of the genetic evidence, which shows that key population lineages split well before a land path through the ice opened up. The authors argue this makes it more likely that people had already entered the Americas by this point.
Arriving before 25,000 years ago also runs into problems. While some archaeological sites suggest earlier human activity, current genomic data show no population splits stretching that far back. Any groups living in the Americas back then may have left no descendants today, or scientists may just not have found their DNA yet. Alternatively, they may have only left traces of ancestry in later populations.
What the research suggests is that a major branching event occurred around 15,000 years ago, which divided the ancient travelers into Northern and Southern Native American lineages. From that point, their descendants would have expanded throughout North and South America in a series of migrations rather than a single movement.
Of course, this research won't be the last word on the matter, but Potter and her colleagues believe that combining several lines of evidence is ultimately going to be beneficial. "Archaeological and genomic data can work in tandem to reveal subtle details of population history," they note.
And because studies of this nature involve the ancestors of living people, ethical collaboration is essential. "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."