Dating the White Sands Footprints

by Brian Dunning
September 2, 2025

This episode was sponsored by Jay and Carrie from Middleton, WI.

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The greatest strength of science is its self-correcting nature. Whenever new evidence arises that doesn't fit our current theories, we put that evidence to the test. If it passes and other lines of evidence support doing so, we update our theories. If it doesn't, we discard it. In nearly every scientific field, there is something like this going on practically at any given time. And there's one we're going to focus on today that comes to us from the annals of paleoanthropology. It comes in the form of a specific challenge to the accepted date range that early humans first descended into North America from Beringia.

In 2009, a set of fossilized human footprints were found in White Sands, New Mexico. They are located in the dry Tularosa Basin, which was once underwater in the extinct Lake Otero. Thousands and thousands of years ago, there were periods of intense drought; and it was during one such period that Lake Otero receded significantly, exposing vast swaths of lake margin. Over a period of some 2,000 years, all sorts of megafauna left their tracks in this soft ground. These included ground sloths, mammoths, American lions, dire wolves, and humans. So far 61 human footprints have been found in eight layers of sediments, named TH1 through TH8. The prints are beautiful — distinct, sharp, diverse, and even lively. So, with no meaningful doubt, early humans walked there when those sediments were fresh.

Meanwhile, let's peek in on what our other ancestors were doing at the time. Early humans from Asia moved into Beringia during the last ice age — Beringia being the land that's now at the bottom of the sea in the Bering Strait. The ice age had done two things that made sea level drop: First, ocean temperatures were cooler, so thermal contraction shrunk the volume of water; and second, a lot of water was sequestered onshore in glaciers. This left Beringia high and dry. And it was dry, it was not snow and ice. Beringia was dry and fertile, and we believe it had been settled until the ice age ended and it once again was covered by rising sea levels. But somewhere in that time, the curious migrated east, and then south, and discovered a new continent: the Americas.

The question of when this took place has been an active field of study for almost 400 years. The latest and greatest evidence tells us this probably began something less than 20,000 years ago, and we've derived this from three primary lines of evidence:

1. Archaeological evidence. Familiar to almost everyone is the Clovis culture, defined by the so-called Clovis points, finely fluted stone spear points. The many Clovis sites — so far at least 1500 are catalogued — date as far back as around 13,500 years ago. However, since then, we've found more and more pre-Clovis sites that prove humans were here even longer. These sites push the earliest dates of human habitation beyond 15,000 years ago.
   
   Most of these pre-Clovis sites are not controversial, with their dates being widely accepted. However, some of the very oldest (notably the Bluefish Caves at 24,000 years old, and the White Sands Footprints at 23,000 years old) are absolutely controversial, and debate continues — both scientific and otherwise.
   
   
2. Genetic evidence. When we find ancient skeletal remains, we do DNA analysis that lets us identify certain genetic lineages, called haplogroups. Early humans spent enough time in Beringia — probably some 4-5,000 years — that they formed a genetically distinct population, which has left genetic markers in all these haplogroups. Combining the fields of population genetics and paleogenomics, we know that the founding population of all Native Americans first formed in northeast Asia some 36,000 years ago. We know that North American and South American populations split from one another between 14,000 and 17,000 years ago. And we know that it took less than 2,000 years for the first humans to populate the southern tip of South America to get there all the way from Beringia. So we know a lot, and we have good boundaries for most of it; but we don't know everything. Exactly when did the first adventurers head south from Beringia? Probably between 13,000 and 22,000 years ago — which is a pretty big range.
   
   
3. Environmental and Climatic Evidence. We know from both paleoclimatology and paleobotany that the route early humans would have taken was not viable until relatively recently: the most recent end of that scale around 13,000 years ago. This was the so-called "ice-free corridor" through Alaska and Canada down into the 48 contiguous United States; a north-south gap between the Cordilleran Ice Sheet on the Pacific Coast and the Laurentide Ice Sheet that covered most of Canada and the Northeastern United States. This tells us that for any pre-Clovis peoples to have been in the Americas, they had to migrate along the coastline, along the very edge of the Cordilleran Ice Sheet. Access to this coastal route from Beringia was available since before any of these suspected pre-Clovis peoples; however it would have been a difficult route indeed.

So with all of that in mind, let's head back on down south to New Mexico, and see what dates are indicated for the footprints; see how well it fits with our current understanding; and see if that date is even possible considering the constraints on the routes south from Beringia.

We'll start with the thing many of you are probably wondering: How the heck can you tell when a footprint was made? Well, we can't. There is no way to determine that. However, we can determine when the mud was wet, and that tells us when it was possible for footprints to be made. Wet mud contains stuff from its environment, notably including (in this case) seeds from the aquatic plant Ruppia cirrhosa, aka ditch grass. Those seeds are robust enough to stick around, and absolutely can be carbon dated.

And so this was done by Bennett et al., the original scientists who worked on the site. And the dates came back as ranging from 21,000 to 23,000 years old — older than almost anyone had dared speculate before!

Now, nearly all paleoanthropologists are happy to acknowledge that we don't have a solid boundary on just how long ago the oldest pre-Clovis cultures were here. But numbers in excess of the teens simply represented too big of a shift. On the one hand, if there were cultures that old, there would have been more sites with corroborating evidence; but on the other hand, we should expect that the older evidence is, the less of it we'll be able to find. Nevertheless, the radiocarbon dates from the ditch grass seeds seemed like too much of an outlier. And lots of scientists had potential explanations for this.

For one thing, the ditch grass seeds were likely exhibiting something called the "hard water effect". Carbon dating works because when that plant was alive it was metabolizing carbon dioxide from the air, and we know the half lives of the various isotopes of carbon. But if hard water is present — for example, containing dissolved limestone, whose carbon would be ancient — that ancient carbon could have been absorbed by the seeds from the water. This would throw off the ratios of the isotopes and make it appear that the seeds were much older than they really are.

Many scientists were quick to react, and within only months the paper "A critical assessment of claims that human footprints in the Lake Otero basin, New Mexico date to the Last Glacial Maximum" appeared in the journal Quaternary Research. One of its authors said:

While the researchers recognize the problem, they underestimate the basic biology of the plant. For the most part, it’s using the carbon it finds in the lake waters. And in most cases, that means it’s taking in carbon from sources other than the contemporary atmosphere — sources which are usually pretty old.

And to prove their point, they took ditch grass seeds known to have been collected in 1947, carbon dated them, and got a date of 7,400 years! And it turns out that's a pretty interesting number, because that would correct the age range of the White Sands seeds down to 12,600-15,600 years old, which is right smack in line with the predominantly accepted date range for human population of the Americas.

This wasn't the only potential problem with the dating. The seeds themselves could be correctly dated, but may have eroded from older deposits and washed to their current location by a storm, and were not necessarily the same age as the people making footprints. It's also been suggested that the samples were not taken from the right places, and may have been from a layer of mud not at the surface, but at the bottom of the footprint, and could have been much older.

And so the original researchers went back to the drawing board and performed two new tests: they dated pollen from conifer trees found in the mud at the same layer as the footprints, and they performed a test called optically stimulated luminescence dating on the mud sediments themselves.

• The conifer pollen, which was not susceptible to the hard water effect like the ditch grass seeds were, was carbon dated and yielded an age of 21,000-23,000 years old: exactly the same as the original results.
  
  
• Optical stimulated luminescence dating depends on the fact that all sediments contain some amount of various radioactive elements. When these elements are exposed to the air, the decay particles radiate away; but when the sediment is buried and confined, those decay products accumulate and build up over time. Thus, this type of dating can tell us when some sediment was last exposed to the air. This testing gave a result of 21,500 years — again within the range established by the original ditch grass seed dating.

Then in 2025, a University of Arizona archaeologist and geologist published his own independent analysis in which he radiocarbon dated organic matter in the mud itself, and made sure to have the testing done at labs different from those used by all the previous researchers.

• Published in the journal Science Advances, the mud was given a date range of 20,700 to 22,400 years old — the last time that mud was moist enough for living organic matter. Once again, it was nearly a perfect overlap of the original researchers' dates of 21-23,000 years.

So there's a lot of head-scratching going on in the field right now. The data seems robust, and now it's a matter for paleoanthropologists, paleogeneticists, and paleogeologists to see if their standard models can fit this new data. Today, as of the date of this writing, there is no consensus, but everyone involved is eager to find one, and we'll probably have one soon enough.

So to summarize, this crazy early dating of the White Sands Footprints might be an outlier, but it's no crackpot notion. It's supported by solid data that's repeatedly been replicated. Will it turn out to be the new accepted date? It already is, by some; not at all by others; and still up in the air for many.

Some have pointed to the case of the White Sands Footprints as proof that science itself is fatally flawed. If all of these people were following the same method, how could they have gotten different answers? And if yesterday the answer was 13,000 and today it's 20,000, then what reason do we have to think the newer one is correct? Maybe tomorrow it will be 25,000. Therefore science knows nothing and is 100% unreliable, they say.

Well, in a way, they're right. Science can be described as "100% unreliable" but only if we wrongly redefine science as a set of facts and claims. But it never was that. It's a process, it's a method, for finding new information and incorporating it into our best and latest theories, which makes them even better. The "Peopling of the Americas" is the general name for our theory of how the first Americans got here: when they did it, how they did it, how did they live, what did they do. There has not been a single year in your lifetime in which this theory was stable and did not improve or change slightly.

Does that make science itself fundamentally flawed? Only with a truly distorted definition of flawed. The daily changing and improvement of the state of our knowledge is what makes science the most powerful tool in the universe.

By Brian Dunning
Follow @BrianDunning