A shape that appears to be a footprint was made 1,000 years before the oldest previous evidence of humans in South America. The finding provides further evidence that people reached the Americas long before the end of the Ice Age.
In 2011 a student from Austral University of Chile found an impression in a sandy-peat layer at a dig site in Osorno, southern Chile. Since then Dr Karen Moreno has been trying to confirm if this really is a human footprint, and measure its age. In PLOS ONE, Moreno has provided evidence for the print's human origin, finding that it is 15,600 years old. If both claims are right, this could be the oldest known footprint anywhere in the Americas, and the oldest evidence of human habitation in South America.
The Osorno site contains the remains of mastodons and horses. Hints these animals were hunted, and flakes of stone that might have been from tools created suspicions humans might have lived there in very ancient times.
Moreno conducted nine experiments to see how sediments in which the print was found would have responded to different sorts of pressure. She found the shape is consistent with the right foot of a barefoot adult man weighing 70 kilograms (155 pounds). No South American animals would produce a similar print, and Moreno is skeptical the shape formed by chance.
Although the footprint itself cannot be dated, the same layer of sediment contains seeds, wood, and a piece of mastodon skull, which provide a consistent age.
Osorno lies 100 kilometers (60 miles) from Monte Verde where stone tools provide evidence of human habitation 14,600 years ago.
Moreno attributed the footprint to Hominipes modernus, a classification given to prints that could be from modern humans or one of our nearest relatives. If found in a time or place where other members of the human family co-existed with Homo Sapiens, it would be impossible to determine which of several human species made a footprint like this. However, there is little support for theories that humans other than our own species made it to the Americas, making this almost certainly one of ours.
Similar-looking prints have been found in Mexico and dated as at least 40,000 years old. However, these are so much older than other evidence for human presence in the Americas they have remained deeply controversial. The Osorno discovery, on the other hand, represents an incremental development on Monte Verde and butchered 14,000-year-old animal bones in Argentina making it a more believable step in the timing of America's settlement.
Signs of human presence dating to 16-20,000 years ago have been found in Texas, but nothing quite this old has previously been seen in South America.
The timing fits well with the opening of a coastal route to the Americas.