A team of scientists in Chile say they've found a human footprint that dates back more than 15,000 years -- the oldest one ever found in the Americas.
The discovery challenges the previous timeline and map of human migration into South America. Most available evidence held that humans did not reach the Patagonia region at the southern tip of South America until 12,000 years ago, Karen Moreno, one of the co-authors of the research, told CNN.
The scientists believe the trace fossil is an impression of the bare right foot of an adult human, Moreno said. The research was published last week in the scientific journal PLOS-ONE .
The footprint was found in 2010 at a paleo-archaeological site in the city of Osorno in the southern part of the country, at the edge of a town home development. But it took years for scientists to confirm the age of the fossil through carbon dating.
Scientists were alerted to the richness of the area after construction workers, digging to build houses, began unearthing treasures hidden below the surface.
A new wave of research has been ongoing there since 1986 and has uncovered the remains of large animals, ranging from mastodons to horses to a paleo-llama, a larger species of llama that is now extinct.
But the ancient human footprint was maybe the biggest prize.
Confirming the footprint's age took eight years
Moreno's colleague, Mario Pino, discovered the footprint just before Christmas in 2010.
To pinpoint the age of the footprint, the team used radiocarbon-dating techniques to find the age of wood, seeds, and bones found around the impression. They also found evidence of primitive stone tools in the area around the fossil.
They identified the species as Hominipes modernus, which is closely related to Homo sapiens.
The experiments on the organic material around the footprint created a range of possible dates, with the median age pinning the print at around 15,600 years old.
But more than eight years passed between spotting the footprint and publishing the confirmation of its age. Moreno said the team spent that time "trying to convince our colleagues that it was a footprint."
Over time the evidence became undeniable, she said. "We confirmed, and checked and re-checked."
Moreno said there were "a lot of difficulties" in convincing colleagues how it's possible for humans to have been in the area 3,000 years earlier than previous traces of the nomadic Clovis culture that existed in the area.
Eventually the peer reviewers were satisfied.
The find rewrites the human migratory map
Human footprints as old as 3.6 million years old have been found in Laetoli, Tanzania. In 1978 scientists discovered a trail of footprints made there, where three early humans are believed to have trod through wet volcanic ash.
It took millions of years for human feet to march to the Americas. They eventually crossed over the Bering Strait from modern-day Russia into Alaska during the last ice age.
Last year , scientists found a footprint trail in British Columbia dating to 13,000 years ago. But those footprints were much younger and much closer to the Bering Strait than this new announcement in Patagonia.
"If we have the evidence of humans before then, we have have to figure out how they got there," Moreno said.
But much of that evidence, if it exists, is now probably at the bottom of the sea. Sea levels were lower 15,000 years ago.
"Most of the evidence is underwater or has been eroded by glaciers," Moreno said.