- So that is a human footprint.
- Yep, so there's a human footprint right there.
- Wow.
- Yeah, and so if you look.
- That is amazing.
- Here's the heel.
- Okay. - Right here.
(tense music)
- [Kirk] Scattered across the landscape are human footprints
from thousands of years ago.
Each track is a trace of an ancient person.
The shape of their bare feet locked in the sediment.
But how long ago were these people here?
In order to date the prints, the team has dug a trench.
(soft music)
It reveals layers of sediment deposited over many years
along the shore of this ancient lake.
Stamped on these buried surfaces are human prints.
And the farther down they are, the older they are.
But just how old are they?
- I can put 'em both on.
- Okay, sure.
- To help find the answer, David has been joined
by geologists Kathleen Springer and Jeff Bugatti
who is also an expert in radiocarbon dating.
(curious music)
You've cut a cross-section.
What are you trying to see with the cross-section?
- So the footprints themselves
are just an impression on a surface.
There's nothing to date.
It's an inorganic thing.
You have to find something organic
that you can date above and below the footprints
and get good dates on them
so that you can actually say
that footprint is between these two ages.
- [Kirk] In this trench, Kathleen and Jeff
have made a crucial find.
(gentle music)
Sandwiched in the layers, above and below the footprints
are scatterings of ancient seeds,
precious organic material which the team can date.
That way they can establish a window of time
for when the prints were made.
- There were actually plants growing on this,
on this surface when these critters were walking.
- So the same layers that have the tracks
will have the seeds. - Absolutely,
above and below them.
- Yeah. - Yes, so above
and below them, that way we can constrain in time.
- So then your seeds are effectively
little time pieces, right?
They're like little clocks
that somebody buried in the--
- They're little, little capsules, yeah,
and basically, these things are really resistant to decay.
And so they look like they were put down
on the landscape just yesterday.
But in fact, they might be tens, you know,
tens of thousands of years old.
- [Kirk] At the US Geological Survey Lab,
Jeff has been analyzing the seeds they found
in the sediment layers at White Sands.
He's been using radiocarbon dating to calculate the age
of the seeds, and from that, the age of the footprints.
(soft piano music) (steam blowing)
It's the moment of truth.
After more than a year and a half have Kathleen
and Jeff managed to find out the age of the footprints.
So tell me, what were the dates of those footprints?
- We were able to document that humans were
in White Sands National Park
between 23,000 years ago and about 21,000 years ago.
- [Kirk] 23,000 years ago.
That's that's way older than there's been good evidence
for humans in North America.
- It's about 10,000 years older
than sort of the established sort of thought
of when humans arrived in the Americas.
- [Kirk] And you got tracks at more
than one layer, which means it wasn't just one group
of people at one moment in time,
but it was many groups of people over a lot of time.
- [Kathleen] 2,000 years.
- [Kirk] I mean, 2000 years itself is
a long duration. - It is.
- [Kirk] But the fact that they were here 23,000 years ago.
- Yeah, crazy huh?
- [Kirk] Blows my mind, I mean--
- [Kathleen] It blew our mind.
(Kathleen chuckling)
- [Kirk] If these dates are correct
that would make the White Sands footprints
the earliest direct evidence of humans
ever found in North America.
(tranquil music)