It has been commonly believed that humans started to expand into the Americas from Siberia as the last ice age ended about 13,000 years ago, but new findings are igniting debate about this timeline. Thousands of human footprints dating back to 23,000 years ago were discovered in dried lake beds in White Sands National Park in New Mexico, according to a study published in Science. The prints were discovered in 2009, but it took more than a decade for researchers to reliably date the prints with the help of ditch grass seeds that were found in the rock above and below the foot prints. “Using radiocarbon dating, we can determine how old those seeds are, and determine the age above and below, and the tracks date from around 21,000 to 23,000 (years),” says Matthew Bennett, lead author of the study.