The argument over when people first reached North America took a turn this week with a report dating deer and rabbit bones found at the bottom of a Mexican rock shelter to more than 30,000 years ago. It remains to be shown, however, that the deceased animals were there because of human activity.
The aim of Andrew Somerville from Iowa State University and colleagues in the project, reported this week in Latin American Antiquity, had nothing to do with the colonialization of the Americas. It was to study the start of agriculture in that part of the world, and narrow down the earliest human occupation of Coxcatlan Cave.
To achieve this, they revisited animal bones found half a century ago at the deepest level in the cave, which is 30 meters (almost 100 feet) and 8 meters (26 feet) deep.
The Coxcatlan rock shelter is part of a cave system in the Tehuacán Valley of south-central Mexico, where the exceptionally arid conditions enabled the preservation of bones and other organic material for tens of thousands of years. In previous work, archaeologists found and identified early domesticated plants, from chili peppers to squash, beans and maize.
However, the dating of all this was debated, for reasons we won’t get into, and the chronology of human arrival and the start of agriculture in the valley remained entirely unclear.
It bears adding that the first-known culture in that part of the world, called the Ajuereado, is believed to have relied on hunting wild horses, antelopes and deer, and largely eschewing the plant world for sustenance.
The rub for the investigators of paleo-agriculture is that quite quickly, by the tail end of the Ajuereado period around 10,000 years ago, the biggest herbivores in the area had gone extinct. They may well have been hunted to oblivion, but in any case the people had to resort to plants. It was in that next period, called the El Riego phase, that we find people in this area not only hunting for whatever still survived, but using mortars to grind stuff, and that evidence of early plant domestication appears – avocados, chilies and so on.
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Somerville explains that the team hadn’t been trying to weigh in on the debate over the peopling of the Americas: “We were just trying to situate our agricultural study with a firmer timeline,” he explains.
But the team discovered that deer and rabbit bones originally found in the 1960s at the base of the cave ranged from 33,448 to 28,279 years old, based on radiocarbon dating. That was a surprise, he says, and it certain does touch on the debate.
Stone tools or just rocks?
If the dating is accurate and if the deer and rabbits were brought there by humans, then humankind reached the Americas almost 20,000 years earlier than is commonly thought. Present thinking is that the earliest occupation of the Tehuacán Valley was by small groups of preceramic hunter-gatherers (“micro-bands”) about 11,500 years ago.
We add that other bones found in the cave and dated, correspond to dates from 11,500 years and afterward, which fits the conventional narrative. It was the bottom-most layer that sprung the surprise.
But maybe rabbits romped and deer played, and died there, 30,000 years ago irrespective of human intervention? They do get about, animals with legs.
Could be. Finding cut marks on these ancient bones would be more definitive. Indeed, Somerville and his colleague Matthew Hill say they plan to take a closer look at the bones for evidence of marks indicating that the animals were butchered using stone tools, or charring indicative of cooking. Preceramic peoples would likely have barbecued the beasts on open fires, not having clay pots yet to make stews and soups.
Somerville adds that stone tools found in the early levels of the cave may also be indicative, and therefore bear closer examination. He himself stresses that they need to see whether ostensible tools from early layers of the cave are actually products of human manufacture or if they were just naturally chipped stones.
That has been the argument about other sites in the Americas claimed by some to be pre-Clovis: If indeed stone tools in the region date to more than, say, 15,000 years ago, then people arrived earlier. But if they’re not tools at all and just rocks, that’s another story.
In 2020, a team published an article in Nature arguing that humans had reached central America as long as 33,000 to 31,000 years ago – well before the Last Glacial Maximum (about 26,000 years ago) that rendered much of the northern hemisphere uninhabitable.
That study was based on stone tools found at Mexico’s Chiquihuite Cave. Yet some argue the 1,900 stone tools identified there are nothing of the sort – they’re just rocks.
By 12,000 years ago, there’s no argument that people were in Mexico. A separate team reported on not mere occupation but ocher-mining in Mexico’s Quintana Roo cave system that long ago.
So, a great deal depends on further study of the deer and rabbit bones found in Coxcatlan.
“I think a lot more research needs to be done. It sounds as if even the authors don’t know what to make of what they’ve got,” commented Prof. Metin Eren of Kent State University, who was not involved with the research. “We’ll see what happens with future research, but my prediction is that – like all claims for a pre-Last Glacial Maximum occupation of North America – this will turn out to be nothing.”
Eren qualifies that, of course, his prediction could be wrong. But the fact remains that none of the pre-Clovis putative human sites in the Americas have been robustly confirmed.
Somerville points out that if humans did reach North America more than 30,000 years ago, they couldn’t have walked there. It’s commonly assumed that the first Americans reached there via the land bridge connecting Siberia and North America, but the land was covered in ice during the Last Glacial Maximum. If people came then, they had to have come by boat. That in and of itself wouldn’t necessarily be a surprise. The discovery of ancient hominins on islands that hadn't been connected to the mainland even at the lowest point of sea level, speaks volumes.