Science1 publisher2 min readPublished
First people likely reached the Americas 14,000 to 20,000 years ago, Kansas-led review argues
Bethany Potter's University of Kansas team argues the evidence best fits people reaching the Americas 14,000 to 20,000 years ago. The paper reweighs published genetic, archaeological and climate evidence in a debate that still has no conclusive proof.
The Scientist · Science desk
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What happened
- Most genetic evidence shows key lineages splitting well before a land path through the ice opened, and the authors count that against a late arrival.
- Some archaeological sites hint at people before 25,000 years ago, but current genomic data show no population splits that old.
- Around 15,000 years ago, the authors say, a major branching split travelers into Northern and Southern Native American lineages that then spread in a series of migrations.
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Why it matters
- contradiction The review sides with genomes over the older archaeological sites, so the two lines of evidence it set out to combine still disagree about the earliest window.
- constraint Settling the earliest window depends on ancient DNA or firmer dates from the older sites, since genomic dating cannot reach populations that left no descendants.
- precedent New ancient DNA is what could test the early model, and the authors want that work rooted in questions from archaeologists and tribal historians and in descendant communities' priorities.
The paper is a review of published research [1]. Potter's group set each timeline against ancient DNA for when populations split, archaeological sites for signs of people, and environmental data for when ice and climate would have allowed travel [3]. A review can rank hypotheses against the existing record. It cannot add samples the record lacks, and the phys.org account does not say how many genomes or sites went into the comparison, or which route into the continent the authors favour.
The two rejections differ in kind. Against a late arrival, the genetic argument is positive evidence: the dated lineage splits themselves [6]. The Northern-Southern branching the authors identify, around 15,000 years ago [9], sits 1,000 years inside the younger edge of the favoured window [15].
Against an early arrival, the argument rests on missing evidence [7]. The authors offer three ways the older sites and the genomes could both be right: early groups may have left no descendants, their DNA may not have been found yet, or they may survive only as traces of ancestry in later populations [8]. A genetic tree is built from lineages that persisted or were sampled. It cannot date a population that did neither. I think the review makes a strong case against arrival after 14,000 years ago and a weaker one against arrival before 25,000.
The favoured window [5] is 6,000 years wide [14]. It puts arrival in a period when ice sheets blocked much of North America, the condition behind the hypothesis that early travelers followed Pacific coastal routes [4]. As phys.org summarises the models, the pre-glacial one covers anything older than 25,000 years and the glacial one starts at 20,000, leaving 5,000 years assigned to neither [16].
Phys.org's account notes that the paper will not be the last word [13]. Potter and her colleagues argue for combining the evidence. "Archaeological and genomic data can work in tandem to reveal subtle details of population history," they wrote [10]. Because the work concerns the ancestors of living people, they also set a condition on it. "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," they wrote [11].
What to watch
- Ancient DNA recovered from the archaeological sites that suggest people before 25,000 years ago, showing whether those groups left any ancestry in later populations.
- Revised dates for when the land path through the ice opened, the benchmark the late-arrival model is measured against.