WERE PEOPLE OCCUPYING A CAVE IN MEXICO 30,000 YEARS AGO?

There are few scientific questions as controversial as when humans first entered the Americas. For decades widely-spread groups using the distinctive Clovis toolkit held that title with a date of first entry around 13,000 years ago. In recent years a few hotly contested findings have pushed that date back by a few thousand years--for example there's now generally accepted evidence of humans living at Monte Verde, Chile, near the tip of South America, 14,800 years ago. 

Now, new evidence from the depths of a cave near Puebla, Mexico may double the length of human occupation of the Americas--to 30,000 years or more. If these new dates and the signs of human activity associated with them hold up to the intense scrutiny they are sure to provoke, it will place humans in North America before rather than after the most recent glacial maximum, dramatically re-writing the prehistory of the Americas.

Coxcatlan Cave and its location--Credit Andrew Somerville/ISU/Latin American Antiquity

Paleoecologist Andrew Somerville, at Iowa State University in Ames, Iowa, did not set out to make a potentially paradigm-breaking discovery. He and his colleagues, Isabel Casar (UNAM*) and Joaquin Arroyo-Cabrales (INAH**) just wanted to clarify the dating of the previously excavated and studied Coxcatlan Cave in the Tehuacan Valley of Mexico, with a focus on the development of agriculture.

Earlier researchers had excavated and described 28 distinct levels of deposits in the cave extending over thousands of years, most of which included signs of human habitation. The strata revealed a progression of material culture starting with slightly worked stone blades, scrapers and choppers through the development of carefully crafted stone tools, ceramics, woven materials, grinding stones, and early signs of agriculture.

Somerville and his colleagues used accelerator mass spectrometry (ACM) to perform precision carbon-14 dating on deer, hare and rabbit bones from various levels of the cave. While their findings corroborated the dating of the site's more recent levels, they were surprised when bones from the deepest deposits consistently dated to between 33,448 and 28,279 years before the present. 

Since respected previous researchers had found that those deep levels showed signs of human habitation--for example slightly worked stone tools including flint that had to be imported to the cave, and bones showing signs of heating and butchering, Somerville and his colleagues realized that they had stumbled upon a potentially revolutionary find.

"We weren't trying to weigh in on this debate, or even find really old samples," Somerville says. "We were surprised to find these really old dates at the bottom of the cave, and it means we need to take a closer look at the artifacts recovered from those levels."

Somerville is teaming up with Matthew Hill, also at Iowa State, to re-examine the stones and bones  from the deepest levels of the cave to see if they can prove or disprove that they were worked or modified by humans. "Determining whether the stone artifacts were products of human manufacture or it they were just naturally chipped stones would be one way to get to the bottom of this," Somerville says. Similarly, microscopic examination of the bone fragments may reveal proof of heating, or cut marks that could only come from stone tools wielded by human hands. 

Unfortunately, the Covid-19 pandemic has so far prevented Somerville and his colleagues from re-examining the bones. They think they will be able to access them during the summer of 2022. While the bones were carefully identified and categorized in the 1960s by Kent Flannery, a highly respected archaeologist, he did not study them at the level of detail Somerville plans. 

"Documenting and quantifying modifications to the bones, such as the patterns of breakage, the presence of cut marks, and thermal alterations to the bones were not part of the original study," Somerville explains. "Our plan is to return to the collection with a digital microscope and to systematically analyze each specimen to document this information. The goal is to ascertain whether or not humans had actually hunted and consumed these animals."

Somerville is also working with the Peabody Museum, in Andover, Massachusetts, to locate and re-study the possible stone tools from the deepest levels of the Coxcatlan Cave.

According to Somerville, Flannery and his colleague, the equally highly respected archaeologist Richard S. MacNeish, were fairly sure that the bones and stones from the deepest levels of the cave were left there by humans. As a result, Somerville thinks that its likely that further study with current techniques will confirm that. "It wouldn't surprise me if they were right all along," he writes. "However the possibility remains that these levels were natural accumulations of bones and stones, so I'm remaining skeptical but with an open mind until we have more data."

Somerville and his colleagues are thoroughly aware of how contentious a finding of a 30,000 year old human presence in the Americas would be, and accordingly have presented their current research very cautiously. "Because we very intentionally tried not to oversell our results, I think we have been able to avoid any serious criticism or backlash so far," he writes.

"So far" are the key words. American archaeology is notorious for the large number of sites at which researchers have found what they thought was evidence of human habitation dating back 20,000 years or more, only to find their work dissected, discounted and eventually dismissed by other researchers. 

While we eagerly await the next round of findings from professors Somerville and Hill, it's clear from the history of the field that they will have to produce extremely convincing proof before the needle marking when people first occupied the New World budges from its current position. But if it does swing from after to before the last glacial maximum, it would mean a revolution in our understanding of the peopling of the Americas.

REA

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You can access the original, open-source Latin American Antiquity article reporting these findings here

*Institute of Physics, National Autonomous University of Mexico

**Subdirectory of Laboratories and Academic Support, National Institute of of Anthropology and History

300,000-YEAR-OLD BONES FROM MOROCCO REVISE HUMAN EVOLUTION

For the past 20 years, researchers have believed that Homo sapiens--humans like us--appeared somewhere in the heart of Africa around 200,000 years ago and gradually spread from there into Eurasia and later to the rest of the world.

New discoveries from Jebel Irhoud, a mountainous site 100 km (62 miles) west of Marrakesh, Morocco, may require a rewrite of the textbooks.

300,000-year-old Homo Sapiens from Jebel Irhoud

composite reconstruction

Credit: Philipp Gunz, MPI EVA Leipzig

An international team of paleoanthropologists uncovered and studied fossilized remains of five individuals who lived there around 300,000 years ago and whose mixture of advanced and more primitive features places them, according to lead author Jean-Jacques Hublin, ". . . at the very root of our species, the oldest members of our species ever found in Africa or anywhere."

The Jebel Irhoud bones are the oldest securely dated fossils of our own species, pushing the origin of our species back a full 100,000 years.

Finding full-fledged Homo sapiens living so long ago and so far north in Africa leaves the location where our species emerged up for grabs. Eastern, sub-Saharan Africa, where 200,000-year-old human fossils have been found, remains a likely possibility, but now not the only one.

Hublin and colleagues now think that the early evolution of Homo sapiens may have taken place in a widely dispersed population. They point out that there were repeated periods when what is now the Sahara desert was open savanna with scattered trees, lakes and rivers. During those wetter epochs, early human groups could have interacted, interbred and evolved. "Long before the out-of-Africa dispersal of Homo sapiens, there was dispersal within Africa," says Hublin.

"The green Sahara happened several times," Hublin says. "During those periods, there could be exchanges of innovations, and also of genes.  Any favorable mutation would spread. There was no one Garden of Eden in Africa. If there is a Garden of Eden, it's all of Africa--the Garden of Eden is the size of Africa."

Hublin and his colleagues found the Jebel Irhoud fossils along with flaked-stone tools and the remains of game animals in "a pocket of reddish sediments" preserving what once was the floor of a cave on the flank of a mountain. The bones--skulls, teeth and long bones--come from at least five individuals; three adults, an adolescent and a child of 7 or 8. According to the researchers, they show a kaleidoscopic mixture of ancient and modern features, but with enough typical Homo sapiens characteristics to make them the earliest known representatives of our species.

"They contained a surprising combination of very advanced features, especially the face and the dentition, and more archaic features such as the shape of the brain case and the brain," says Hublin. "Their faces are the faces of people you could cross in the street today."

The human fossils were found in the same layers as the bones of animals they hunted, including gazelles, zebras and wildebeest, along with charcoal, and flaked stone tools typical of the Middle Stone Age.

Shannon McPherron, an expert on ancient tool use, emphasizes these early ancestors' control and use of fire, and the craft they brought to toolmaking. "The flint they used came from 25 kilometers away," he says. "They sought out high-quality raw-material locations, collected the flint, carried it around with them, and at this site, re-tooled it for their weaponry."

Some of the 300,000-year-old flint artefacts

found at Jebel Irhoud

Credit: Richter, et al.,

doi:10.1038/nature22335

It was the flint tools, many of which had been burned, that allowed accurate dating of the site using thermoluminescence dating (TL). TL allows scientists to date materials that have once been heated by measuring the amount of radiation they have been exposed to since that time. McPherron found a range from 280,000 to 350,000 years ago. "The average TL ages all point towards 300,000," he says, "so 300,000 is the best estimate for the fossils and for the middle stone age artefacts. Things are all falling together around this age."

When asked why he believes that the fossils represent true members of Homo Sapiens, given their mixture of typical and more archaic traits, Hublin explains that evolution happens trait by trait and over time. He also differentiates between our entire species, which has evolved over time, and fully modern humans, who represent just the current cohort of Homo sapiens.

"There's no reason why representatives of our species living 300,000 years ago would be just like us," he explains. "We are not saying that these are modern humans—people having our morphology.  We prefer to use the term Homo sapiens for the whole lineage leading to us, but not necessarily looking like us."

The bottom line according to Hublin and his colleagues is that our clan, Homo sapiens, has roots--albeit still in Africa--that are much deeper in time and far more widespread than anyone thought until now.

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An online version of the June 8 Nature paper can be found at: http://nature.com/articles/doi:10.1038/nature22336

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