The tale is embedded in the footprints. Along the edges of a vanished ice age lake are the fossilized tracks of people who lived among the mammoths, giant ground sloths and other Pleistocene mammals of ancient New Mexico. There were so many prehistoric pedestrians here that their feet pressed the seeds of a local plant called spiral ditch grass into their tracks, and these plant remnants are what has given archaeologists a possible time for when people lived here. Radiocarbon dating puts the age of the seeds at between 23,000 and 21,000 years ago—far older than expected.
Tracking the peopling of the Americas has been a difficult task, and archaeologists disagree over how and when humans arrived. Most agree people were present on these continents by 13,000 years ago, represented by what researchers refer to as the Clovis culture. But evidence from potentially older archaeological sites is often controversial and can be difficult to verify. The radiocarbon dates for the seeds from the New Mexico tracks would thus be the clearest evidence yet that people were making ancient North America their home more than 20,000 years ago.
The tracks, reported on Wednesday in Science by geologist Matthew Bennett of Bournemouth University in England and his colleagues, dot the flat lake deposits in New Mexico’s White Sands National Park. At least seven footprint sites have been found, including one with 37 prints. Most are from smaller-statured people with foot anatomy just like that of modern humans. This indicates, Bennett and his co-authors hypothesize, that most of the tracks were left by adolescents and children.