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Stepping Back

NATIONAL PARK SERVICE/AP

Scientists have analyzed footprints made thousands of years ago in White Sands National Park, in New Mexico. The tracks show that humans lived in North America some 5,000 years earlier than previous evidence suggested.

The fossilized prints were preserved in the mud of an ancient lakeshore. Researchers say they are between 21,000 and 23,000 years old. Findings were published last month in the journal Science.

Over time, thousands of footprints have been found across 125 square miles of the park. One path shows a woman walking with a toddler. Another reveals people hunting a giant sloth.

Researchers also found ancient grass seeds. They were able to estimate the age of the prints by measuring the carbon in those seeds.