Ancient footprints in New Mexico raise questions about when humans inhabited North America

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• Judy Woodruff:

  When humans first populated North America and how they arrived has long been a matter of spirited debate.

  As Stephanie Sy reports, a recent study dig detailing what archaeologists believe are the oldest known footprints in the United States is sparking new questions and upending long-held assumptions.

• Stephanie Sy:

  Within the sprawling expanse of gypsum sand dunes in dry and dry lake beds in New Mexico's White Sands National Park, researchers have spent years examining ancient footprints.

  David Bustos says is the park's resource program manager. He and a team of scientists discovered ancient animal tracks here over a decade ago from giant ground sloths, ancient camels and mammoths. Previously buried under layers of sand and clay the sequence of footprints called a trackway were revealed after a flood.

  Matthew Bennett is a footprint expert.

• Matthew Bennett, Bournemouth University:

  It's probably the most important track site in the Americas, both in terms of scale, geographical scale, but also in the frequency attracts. And that's what's really special about it.

• Stephanie Sy:

  In 2017, the team confirmed they had found human footprints.

• David Bustos, White Sands National Park:

  We were brushing out a set of sloth prints, and Matthew found the human print right inside the middle of the sloth print. And that's sort of what sealed the deal. Oh, yes, you definitely have the megafauna and humans together. So that's sort of where the human side of the story all began.

• Stephanie Sy:

  The footprints show how humans coexisted with large wild animals. Many are of children and reveal a story about everyday life and play.

• Matthew Bennett:

  The stories of children jumping in a puddle created by a sloth track, for example, that's one of the most fantastic things. Children love jumping in puddles everywhere.

• Stephanie Sy:

  But one big unanswered question remained, how old the human footprints were, and whether they showed if humans inhabited North America earlier than previously thought.

• Matthew Bennett:

  The peopling of the Americas is one of the most controversial archaeological debates, from an indigenous perspective of having always been here, from a more traditional archaeological perspective saying that peopling the Americas was quite a recent event.

  And in a controversy, one of the issues is a lack of good data points.

• Stephanie Sy:

  That's where Jeff Pigati, an expert in radiocarbon dating, and Kathleen Springer, a geologist and paleontologist, came in.

• Jeff Pigati, U.S. Geological Survey:

  And so we need to be able to find an area where the footprints are in the layers of sediment where we can find something to date below and above, so we can basically constrain the age of those trackways.

• Kathleen Springer, U.S. Geological Survey:

  You need to sort of carve out a big trench to sort of, what I always say, reveal the belly of the beast and get inside.

• Stephanie Sy:

  It's like a cross-section.

• Kathleen Springer:

  It's a cross-section, exactly.

  So then the hope is, OK, we found tracks in cross-section. And then the hope is, there better be something that is suitable for radiocarbon dating.

• Stephanie Sy:

  It turns out there was, layers of seeds from aquatic grasses that grew near the trackway.

• Kathleen Springer:

  Some places, there's seeds underneath these human footprints. So we know that those seeds were and those plants were actually actively living and dying there.

• Stephanie Sy:

  They used radiocarbon dating to determine the age of the seeds.

• Jeff Pigati:

  And so if we measure the amount of radiocarbon that's in a seed, for example, and we know how fast it decays, we can calculate how old the plant actually is.

• Stephanie Sy:

  Once they crunched the numbers, what they found was astounding.

• Jeff Pigati:

  At the very bottom, where people were starting to walk around and where we have our lowest seed layer age, it's about 23,000 years old. And at the top, where the people were still walking around, and where we have our highest seed layer, it's about 21,000 years old. So we basically documented 2,000 years of human occupation in this area at White Sands a long, long time ago.

• Stephanie Sy:

  Archaeologists have long believed that humans arrived in North America 13,500 to 16,000 years ago, after a period of warming had melted massive glaciers, opening up a land passage from Asia to North America.

• Jeff Pigati:

  This is just so much older. And then our first reaction is, oh, my God, we better check everything, because these better be right.

• Stephanie Sy:

  Jeff Pigati says he has a 95 percent confidence level in the accuracy of the dating.

• Jeff Pigati:

  We're as sure as you can possibly be scientifically that that's actually the case.

• Stephanie Sy:

  But the evidence isn't rock-solid, say some archaeologists.

• David Meltzer, Southern Methodist University:

  The critical issues here are, is the dating reliable?

• Stephanie Sy:

  David Meltzer is an archaeologist at Southern Methodist University in Texas. He believes dating seeds from an aquatic plant to tell the age of the footprints is potentially problematic, and says it's just too early to be confident.

• David Meltzer:

  The people that are doing this, that are doing the work, they're pros. They know what they're doing. They know what they're about. But here's the thing. Nature has a mischievous streak. And nature has fooled us before.

  So the motto here is trust, but verify.

• Stephanie Sy:

  Jeff Pigati and Kathleen Springer with the U.S. Geological Survey expected scrutiny and are now working to radiocarbon date pollen found in the rock layers. And even then, there's so much yet to uncover.

• Kathleen Springer:

  So we want to expand the story to not just occupation for 2,000 years, between 23 and 21, but what if it looks more like this, people were here for much longer?

• Stephanie Sy:

  If the findings hold up, it could spark a reexamination of similar dry lake bed sites in the Southwest. And that could reveal even older evidence of humanity's foothold in North America.

  For the "PBS NewsHour," I'm Stephanie Sy.