Ancient shifts shaped migrants long before they crossed.

The latest genetic work has forced researchers to reconsider what happened to early humans before they stepped into the Western Hemisphere. Samples pulled from Siberian caves, ancient teeth and long buried bone fragments show that the ancestors of the first Americans did not arrive as a single unchanged lineage. They carried the marks of three major evolutionary shifts that shaped their biology, resilience and survival strategies long before they crossed into the Americas. Each shift left clues in their DNA that researchers can now finally read.



