Between 26,500 and 19,000 years ago, massive sheets of ice covered Earth’s Northern hemisphere. During this so-called last glacial maximum, the ice sequestered water, causing a drop in sea level and exposing land that connected northeast Asia and northwest North America near present-day Alaska. In what is now Canada, two glaciers merged and covered the region with ice thousands of feet thick that stretched from the Atlantic to the Pacific. At least 13,000 years ago, those glaciers started to recede, opening up an ice-free corridor that is thought to have been used by early humans who came down from northeast Asia and populated what is now the United States.
In a study published today (July 22) in Nature, researchers describe stone artifacts found in Chiquihuite Cave in northern Mexico that date to the last glacial maximum. For humans to be present in the region then, they would have had to traverse Canada before the northern-most part of the continent was a wall of ice—perhaps as far back as 33,000 years ago. Or they might have entered North America via the Pacific coast.
However, critics of the new study call into question whether or not the stone samples were truly made by humans.
The overall picture “that you have older settlements than those that have previously been known” is clear, says Wolfgang Stinnesbeck, a paleontologist at the University of Heidelberg in Germany who was not involved in the study. In contrast to earlier studies that have made similar claims but without much data, he tells The Scientist, “here you have all the details and you can discuss methods . . . so that’s very good.”
Ciprian Ardelean, an archaeologist at the Autonomous University of Zacatecas in Mexico, first excavated Chiquihuite Cave as part of his PhD thesis in 2012. At that point, he says he realized there were very old layers that seemed to contain some human-related evidence, particularly stone tools. A few years later, he returned with an international team to dig more extensively.
In the new study, he and his colleagues determined 46 radiocarbon dates from samples of sediments, animal bones, and charcoal. The dates span about 20,000 years, starting approximately 33,000 years ago. They also found ancient DNA from plants and animals in the sediments, the age of which was consistent with their radiocarbon dates. They did not report any human DNA in this study because the chance of finding human DNA in the sampled sediments is quite low, Ardelean explains.
The researchers also describe 1,930 stone artifacts, unearthed from multiple layers of the cave. While most of the artifacts were found in layers of the cave determined to be between 16,000 and 13,000 years old, 239 of them were found in the oldest excavated layer, dating to between 33,000 and 31,000 years ago—before the last glacial maximum.
There are ways to differentiate naturally flaked stones and proper artifacts, Ardelean says, which include identifying clear marks from other stone tools used to make them, repeated patterns, and similar shapes. Based on these factors and the dating, he and his colleagues conclude that the cave was occupied repeatedly, though not constantly, over a 20,000-year period.
“The implications of the peopling of the Americas during a time period before the last glacial period are, of course, very interesting,” says Loren Davis, an archaeologist at Oregon State University who did not participate in the work. However, “humans don’t have a monopoly on the narrow range of physics required to break rocks,” he adds. “You have to basically demonstrate beyond a shadow of doubt that these things can’t be made naturally, and when I look at the images—and that’s all I have to work with—I’m not convinced.”
“One of the things that struck me is there does seem to be some patterning,” says Vance Holliday, an archaeologist at the University of Arizona who was not involved in the study. For instance, there are teardrop-shaped rocks that appear in even the oldest layers. “At least in outline, they look like stone tools,” he tells The Scientist, but without clearer images than those provided in the paper, detailed drawings, or three-dimensional renderings, it’s difficult to determine whether humans shaped them or not.
Another issue Holliday raises is that the artifact styles don’t appear to change over the 20,000 years of occupation the authors propose. In other parts of the world, it’s possible to see differences arise in tools made by ancient people over that period of time, he says.
“With modern humans, one expects to see evidence of technological and cultural change over such a long span of time, especially given the changes in the climate and environment they’ve detected in the ancient DNA,” agrees David Meltzer, an archaeologist at Southern Methodist University in Dallas who did not participate in the study, writing in an email to The Scientist. “So why is it unchanged, and apparently localized to just this spot? With a stone tool tradition that long lasting, one expects it would have been far more widespread in the region, raising the question of why that technology hasn’t been spotted elsewhere in the region.”
Ardelean and colleagues continue to work at the cave. He says they’ve only excavated a small portion, far from areas that could show other indications of human occupation, such as animal bones with evidence of butchering or having been made into tools or spots that would be appropriate for burning fires for cooking or warmth.
If the evidence for spatial patterning of activity areas or butchering is uncovered, or the authors present other evidence that these are artifacts, then Davis is “open to being convinced,” he says.
C.F. Ardelean et al., “Evidence of human occupation in Mexico around the Last Glacial Maximum,” Nature, doi:10.1038/s41586-020-2509-0, 2020.