The conventional story of the peopling of the Americas held for most of the twentieth century: humans crossed a land bridge from Siberia to Alaska roughly 13,000 years ago, spread rapidly south, and became the ancestors of all Indigenous peoples of the Western Hemisphere. It was a clean narrative, supported by the oldest well-documented archaeological sites and the genetic data available at the time.
Ancient DNA has been dismantling it steadily for a decade. A new study, analyzing genomes extracted from sediment cores at three sites in South America, has pushed the picture back further than any previous analysis — and introduced a complexity that the old model cannot accommodate.
What the sediments contain
Extracting human DNA from sediment, rather than from bone, is a relatively new technique. Cells shed from skin, saliva, and other tissues settle into lake beds and cave floors, where, under the right conditions of cold and acidity, they can survive for tens of thousands of years. The three sites analyzed in the new study — in Patagonia, coastal Peru, and the Colombian highlands — have yielded genomic material dating to between 18,000 and 22,000 years ago.
The genomes do not match any known population. They share ancestry with East Asians and with the ancestral population that gave rise to most Indigenous Americans, but they also carry a signal from a third, unidentified source — a ghost population that left no other genomic trace in the record.
What it means for the model
The simplest interpretation is that multiple migrations reached the Americas before the end of the last ice age, by routes that did not require an ice-free corridor. Coastal migration along the Pacific — moving quickly from island to island and headland to headland — could have brought people to South America far earlier than the land bridge would allow.
This does not erase the Clovis-first model so much as demote it: one migration among several, perhaps the one that left the most visible archaeological signature, but not the first. The people whose DNA is in those sediment cores were already in Patagonia when the Clovis culture was still millennia in the future.
