This new paper in Nature (click on screenshot, pdf here, reference at bottom) has the potential to be the big human-paleobiology story of the last several years. It reports finding human occupancy of a high-altitude cave in Mexico during the last glacial maximum (LGM): about 26,000 years ago. And that, say the authors, implies that humans have been in the New World since more than 30,000 years ago—more than doubling the time we thought they’d been here. Previously, the best guess was that humans crossed the Bering Strait from Siberia about 15,000 years ago, and then spread through the Americas.
Click on the screenshot below to get the paper (free through the legal app Unpaywall, or you can make a judicious inquiry).
Before we accept these results as overturning the received wisdom about humans in the New World, though, there has been some criticism of the paper, as you can see in a precis in Science by Andrew Curry.
The cave where the finds were made sits atoop a remote mountain in the Mexican state of Zacatecas, about 2,740 m high, and has been studied since 2012. Although dry and barren now, it was thought to be verdant during the LGM, with water, plants, and plenty of edible animals nearby. Researchers worked there for a month at a time, camping in the cave and hauling water and food by donkey from the nearest town.
What made the researchers suppose that the cave was occupied by humans were several things, most prominently 2000 specimens of what looked like sculpted tools. Here’s a figure showing some of these putatively manufactured objects:
Now I would have thought that by now paleoanthropologists would be able to distinguish non-human rock artifacts from real, chipped tools, but apparently that’s not the case. As one critic says in the Science writeup:
Critics point out that the tools are simple and don’t resemble other toolkits from the Americas, raising the possibility they’re the product of natural breakage. “They look like they could be artifacts, but why aren’t they found anywhere else in the landscape?” wonders David Meltzer, an archaeologist at Southern Methodist University. The tools’ consistency is also remarkable, he says. “If these tools are real, why are they only found—so far at least—in this one spot over a 10,000-year period? Humans adapt and adopt new technology.”
The tool-making conclusion, at least, must remain tentative. There was also burned wood that was radiocarbon dated, implying human campfires, but the critics again say that this could derive be from “wind-blown” wildfires. The researchers also used OSL dating of quartz from the sediments, which tells you when the mineral was last exposed to light, ergo when it was laid down.
Finally, the researchers trawled the cave for DNA, which they could sequence to see what kind of animals and plants were there. The fauna included bats, mice and other rodents, marmots, goats, and sheep, as well as birds, though this could have come from more recent occupancy. Plant DNA included forest species like spruce, pines, grasses, and palms. The disappearance of cold-adapted species and forest trees that gave way later to Joshua trees and grasses suggests that the sediments in the cave did go through the late Glacial Maximum, which was followed by a period of dryness.
Notably, no human or humanoid DNA was found in the cave, which would have gotten people much more excited about this find.
How strong is the evidence for human presence in the Americas beginning 30,000 years ago? The 30,000 years is a guess by the authors, derived from guessing how long it would take humans to get to a 26,000-year-old cave residence in Mexico after crossing from Siberia. In terms of the age of the cave itself, that seems reasonable, but the evidence for human occupation is largely the “tools”, and their provenance is doubtful. And if humans inhabited the cave continuously for millennia, as the authors suppose, then why wasn’t human DNA found there? My judgment, and I’m a tyro here, is that the evidence is intriguing but not terribly strong. A lot hinges on whether the “tool-like” stone artifacts really were chipped by hominin hands.
On the other hand, the Science article says that there is a cave in the Yukon that’s yielded dates as old as the Mexican cave (about 24,000 years), but although it contains thousands of animal bones, there are “few stone tools or cut marks.” But other researchers are beginning to think that people came to America earlier than we thought, and could have spread quickly by traveling along the West coast by boat, avoiding the largely frozen interior. Here’s a tweet (h/t: Matthew) showing sites where there could have been earlier habitations:
News & Views: Two studies in Nature report evidence that the initial human settlement of the Americas happened earlier than is widely accepted. Some of the evidence suggests settlement began at least 10,000 years earlier than was generally suspected. https://t.co/OBuIQzBI7v
— Nature (@nature) July 23, 2020
How good are the dating methods? From what I read, they seem fairly reasonable, and they used at least two methods that give about the same dates. The question is not how old the cave is, but whether humans lived there and made the tools and charcoal.
What happened to the people? Part of the reason we think humans have been in the New World for only 15,000 years is not just evidence from habitation, but from DNA of Native Americans (note: there are some older estimates). If that’s the case, why doesn’t the DNA give a consistent age of 30,000 years from when Native Americans branches off from East Asians? One possibility is that the early arrivers went extinct without leaving descendants, so we wouldn’t find a genetic signature of their existence. Given that some paleoanthropologists see evidence of an early arrival from other sites, like that in the Yukon, the possibility of extinction seems unlikely.
All in all, this is an exciting finding, and may well be right, but we’ll have to let the experts fight it out.
Excavating in the cave, a photo from the Science precis:
h/t: Matthew Cobb
Ardelean, C.F., Becerra-Valdivia, L., Pedersen, M.W. et al. 2020. Evidence of human occupation in Mexico around the Last Glacial Maximum. Nature (2020). https://doi.org/10.1038/s41586-020-2509-0