Unexpectedly early signs of Americans

Abstract

Humans are thought to have reached the Americas less than 15,000 years ago. But evidence of stone tool use on an animal carcass excavated in California points to a much earlier arrival of human relatives from the genus Homo. See Letter p.479 Around 130,000 years ago, a mastodon died near what is now San Diego, California. Although this seems uncontroversial, Thomas Demere and colleagues present evidence that the carcass had been modified by human beings. Stone hammers and anvils were found alongside mammoth bones and teeth that show signs of having been broken by percussion, presumably to extract bone marrow. Dating the site has been problematic because the bones preserved too little collagen for radiocarbon dating, and optically stimulated luminescence dating put the age at over 60,000–70,000 years. Dates based on the decay of uranium, constrained by the movement of uranium between the environment and the bone, now give an age of around 130,000 years. If confirmed, this would extend tenfold the time that human beings are known to have been present in the Americas and predate the time that modern humans are thought to have first left Africa. The identity of the hominin species—if any—remains unknown.