In late 1992, scientists were monitoring a construction site, as the California Transportation Department was making improvements to a San Diego highway.

They were collecting fossils of mammals that dated roughly to the Ice Age, when they came upon a site that had partial skeletal remains of an American Mastodon.

That discovery led to a recent paper in the journal Nature, which posits that humans showed up in North America roughly 115,000 years earlier than scientists had previously thought.

With me to talk about this is Tom Demere, curator of Paleontology at the San Diego Natural History Museum.