We know that humans were in North America at least 13,000 years ago because of findings like those from the Clovis culture. A few other findings have nudged this date back somewhat, but the prevailing thought up to now has been that it would be difficult for humans to have made it here much before 16,500 years ago, because glacial coverage was too extensive up to that time.
But some fossilized human footprints that were originally found in New Mexico back in 2009, and conjectured to be perhaps 10,000 years old at the time, have gotten another, more-careful look. The prints were found in between layers of seed beds, and sometimes the seeds were embedded in the footprints themselves. The seeds are from the shallow-water aquatic grass Ruppia cirrhosa, and they have now been dated to between 23,000 and 21,000 years ago, suggesting that humans have been here since well before the retreat of the last glacial maximum.
The finding is reported by Dr. Matthew R. Bennett of Bournemouth University (Poole, England) and several others in the September 24 issue of Science. If it stands up to further scrutiny, it would continue the trend toward redefining our thinking of the peopling of the Americas:
Sometime between 23,000 and 21,000 years ago, human beings were in what is today New Mexico, according to a new report in the journal Science. If so, it is a dramatic development in our understanding of how the Americas were populated and would blow the "Clovis First" theory out of the water once and for all.
It has been met with great enthusiasm by some so far...
“I think this is probably the biggest discovery about the peopling of America in a hundred years,” said Ciprian Ardelean, an archaeologist at Autonomous University of Zacatecas in Mexico who was not involved in the work. “I don’t know what gods they prayed to, but this is a dream find.” [...]
“This is a bombshell,” said Ruth Gruhn, an archaeologist at the University of Alberta who was not involved in the study. “On the face of it, it’s very hard to disprove.”
...while others aren’t quite convinced yet. Ben Potter, an archaeologist at the Arctic Studies Center at Liaocheng University in China, has been skeptical of the dating of finds much older than Clovis, such as the July 2020 report in Nature of humans potentially in present-day Mexico well prior to the last glacial retreat that caused quite a stir but was not convincing to everyone. Potter said he agrees that these footprints do represent the strongest such case yet, but...
“I’d like to see stronger data, and I don’t know if it’s possible to get stronger data from this particular site. If it’s true, then it really has some profound implications.”
The main impediment to human migration to North America prior to that window of 13,000 to 16,500 years ago was the fact that ice sheets covered northern North America almost entirely.
The Laurentide and Cordilleran Ice Sheets at their maximum extent:
According to Patrick J. Lynch in A Field Guide to Cape Cod, Yale University Press, 2019:
At its maximum extent during the Wisconsinan Glacial Episode 25,000 years ago, the Laurentide Ice Sheet, a single, massive glacier, covered most of northeastern, eastern, and north-central North America. […] Beginning about 20,000 years ago the Earth’s climate began to warm very quickly. The Laurentide Ice Sheet melted back, and the Wisconsinan Glacial Episode effectively ended in the Outer Lands (Cape Cod and Long Island) about 16,500 years ago.
One of the very last remnants of the Laurentide Ice Sheet is Barnes Ice Cap on Baffin Island in Canada, with ice over 20,000 years old, although it is projected to disappear within 300 years.
The advance and retreat of the Laurentide Ice Sheet left us some other goodies as well, such as the largest glacial boulder in the U.S. in Madison, New Hampshire:
All of this ice would seem to have made it difficult for humans to have arrived in North America via Alaska prior to that last retreat 16,500 years ago. There was a thin gap between the ice sheets, but it’s thought not to have opened up enough to allow passage by land until at least about 16,000 years ago. There was also, of course, a potential ocean route:
So it was by no means impossible. Just difficult.
Let’s take a closer look at the area where the footprints were found. Tularosa Basin (outlined in red below), is surrounded by mountain ranges. Surface runoff (which includes gypsum, or hydrated calcium sulfate) drains into the basin from all sides. The lowest ground within the basin used to be full of water perenially but variably, and the approximate maximum extent of the water is now called Lake Otero (the largest of the blue regions below).
Zooming in and looking at the present-day features: the former Lake Otero is now Alkali Flat, a gypsum playa, and downwind of that are the gypsum sand dunes, the “white sands” made of windblown sediment from old Lake Otero. The very lowest point of the basin is called Lake Lucero, shown in dark blue below, which still fills with water intermittently today, and whose sediment still creates new dunes. The actual site of the footprint find is just east of the first letter “e” in “Lake Lucero” in the figure below:
The footprints (61 in all) were found in what would have been mud surrounding the former Lake Otero, mud apparently thick enough to preserve a footprint quite nicely. They were found within layers of excavated ground covering about 2,000 years’ time:
When the grass seeds interspersed within those same layers were dated with carbon-14 analysis, they were found to be between 23,000 and 21,000 years old, seeming to indicate a human presence for at least that span of 2,000 years.
“From the dating perspective I think the authors have done a very sound job,” says Tom Higham, a leading radiocarbon dating expert at the University of Oxford.
But [Oregon State University archaeologist Loren] Davis suggests a nagging possibility: that the seeds are older than the footprints because they eroded out of older sediments, then sifted into the mud the team excavated. He’d like to see the team try optically stimulated luminescence dating, a method that reveals when quartz grains were last exposed to light, to date when the sediment around the footprints was buried.
The authors also addressed the possibility of a “hard-water” effect:
Aquatic plants, including R. cirrhosa, obtain their carbon from the dissolved inorganic carbon (DIC) in the host water, and can yield 14C ages that are too old if the 14C activity of the DIC is lower than concurrent atmospheric values. This is referred to as the “hard-water” or “reservoir” effect, and the magnitude of the resulting age offset can be highly variable both spatially and temporally.
If lake water is isolated from the atmosphere (not the case here), or if a lot of much-older groundwater wells up, it can throw off the measured age of plant material. But the shallowness and openness of the water would likely negate any effects like that, and more variability in layer dating and in different materials within a single layer would have been seen if this were the case. It’s therefore unlikely (but not impossible!) that the dating is incorrect.
Most of the footprints appear to be of teenagers and children, although assorted with a few larger, adultlike prints. Were the younger people tasked with carrying water? Were they just playing by the lake? Seeing their footprints in the sediment as if they were made yesterday certainly gives us much more of a sense of kinship with these people, our very early predecessors on this continent, wouldn’t you say?
So just when and how did those people get here? Could it have even been before the glacial maximum, before the ice closed off land routes? As always, the search for more data and a better interpretation continues…..
A closing note: Much thanks to Professor Bennett for sending me a reprint of the Science article.
And a few more closing notes from Jai Uttal: