Were humans in Mexico 30,000 years ago?
New testing from a cave in Mexico has offered evidence that humans were in North America around 30,000 years ago—that’s more than twice as early as many previous estimates.
But the finding is, so far, not backed by genetic evidence, leading some to doubt it.
The discovery was announced in Nature. Researchers said excavations at Chiquihuite Cave, which is located at 2,740 meters in the Astillero Mountains of Zacatecas, revealed about 1,900 stone artefacts “within a 3-m-deep stratified sequence, revealing a previously unknown lithic industry that underwent only minor changes over millennia.”
More than 200 of those objects, noted an accompanying article, were set in a gravel layer that was carbon dated to between 25,000 and 32,000 years old. Researchers speculated that the cave was visited infrequently, perhaps every few decades during severe winters—a plausible scenario, since this would have been around the time of the last Ice Age maximum.
“The tools belong to a type of material culture never before seen in the Americas, with no evident similarities to any other cultural complexes,” researcher Lorena Becerra-Valdivia wrote in The Conversation.
But the results are not infallible. Numerous events could have deposited the tools in that layer, including animal digging. And it’s not entirely clear that what is being called a “tool” was actually a tool—many of the artefacts look simply like broken rocks and not something with telltale chips removed from the edge. A search for human DNA in the cave’s dirt returned zilch.
“There continues to be no convincing genetic evidence of a pre-15,000-years-ago human presence in the Americas,” Harvard Medical School geneticist David Reich told Nature.
The announcement of the findings in Mexico came on the heels of another pre-Clovis discovery—human waste in Oregon.
The finding came from coprolite in Paisley Caves, a series of four caves on the edge of the desert in south-central Oregon. In this study, authored by English and American researchers and published in Science, researchers used fecal lipid biomarkers to date the coprolite to 14,000 years ago.
Previously, a mitochondrial DNA analysis found the coprolite was human, but the results were questioned because dog DNA was also found, which raised the possibility of contamination, reported Gizmodo.
“Our study addresses persistent criticisms of the DNA evidence for the earliest human occupation of the Paisley Caves,” said John C. Blong, one of the study’s authors. “We address issues of potential DNA contamination through fecal lipid biomarker analysis, providing evidence that there likely was DNA moving from younger human occupations into older cave sediments and coprolites, but also confirming that people were camping at the caves as early as 14,200 years ago.”