NPR's Up First is the news you need to start your day. The three biggest stories of the day, with reporting and analysis from NPR News — in 10 minutes. Available weekdays at 6:30 a.m. ET, with hosts ...

Fossilized footprints, preserved in gypsum mud that hardened over time, are estimated to be 23,000-21,000 years old. NPS / Alamy Ancient human footprints, preserved in a dry lakebed at White Sands ...