(CNN)Scientists have discovered evidence that may push back the timeline for humans living in North America from 13,000 years ago to 30,000 years ago, according to two new studies.
The studies about the evidence of human occupation and its timing and effect published Wednesday in the journal Nature.
The traditional understanding among scientists about the populating of North and South America is that humans arrived 13,000 years ago and were associated with the Clovis culture, as evidenced by the distinctive stone tools they left behind. But researchers tend to disagree on the exact timing and the path of migration humans took to reach the Americas.Some research in recent years has pushed the timeline back to about 18,000 years ago.
Regardless, their arrival in the Americas represented another wave of human expansion across the globe, so researchers are constantly trying to refine the information we have about that time and glean insights from evidence found at ancient sites in North and South America. The two studies provide a detailed look at a specific location, Chiquihuite Cave in Zacatecas in central Mexico, as well as a broader perspective on 42 archaeological sites across North America and Beringia. This region once joined Russia to North America and is still evident in Russia and Canada, albeit not as the land bridge it once was once the glaciers melted and buried it beneath water 11,600 years ago. Together, the authors of both studies determined that there is evidence for a human presence during, and possibly before, the Last Glacial Maximum (between 19,000 and 26,000 years ago), in North America. Even if it wasn't widely populated, humans were beginning to find their way and settle in areas.
Stone tools found in a cave in Mexico were dated to about 30,000 years ago.
And rather than take the Beringia route, which was likely blocked by glaciers or partially submerged underwater between 29,000 and 57,000 years ago, they may have found another way along the Pacific Coast, but the researchers have not determined that exact path yet."These are paradigm shifting results that shape our understanding of the initial dispersal of modern humans into Americas," said Lorena A. Becerra-Valdivia, an author on both studies and archaeological scientist at the University of Oxford and University of New South Wales, in an email. "They suggest exciting and interesting possibilities for what likely was a complex and dynamic process.
Across the 42 archaeological sites she and her colleagues analyzed, some had evidence of human burials, such as the Anzick site in Montana, while others, such as Chiquihuite Cave, contain evidence of stone tools. "Using a statistical tool called Bayesian age modelling, we were able to define the chronology at each of the 42 archaeological sites and link them together to study large-scale patterns," Becerra-Valdivia said. "This allowed us, for example, to identify that the three major stone tool traditions in the wider region began, roughly, at the same time, and that it was during this period that human populations expanded." The Last Glacial Maximum, which has often been viewed as a deterrent for humans settling in the Americas, was cold and dry and would have presented challenges. However, the period when humans expanded was much warmer.
"Once in North America, humans were present in low density groups before, during and immediately after the Last Glacial Maximum, until the global climate became warmer and they are able to thrive and expand," she said.
This expansion is connected to the disappearance and extinction of large land animals, like ancient types of camels, horses, mastodons, mammoths and elephants.
"It is likely that the growth of human populations had a negative effect on (them)," she said. "This could have been direct (hunting) or indirect (ecological impact).
"Secrets of the cave
When Ciprian Ardelean entered Chiquihuite Cave for the first time in May 2010, he found just enough tantalizing clues that motivated him to continue grueling and challenging excavations. At the time, Ardelean was working on his PhD. Now he's the lead study author for the research about the cave, and a professor and archaeologist at the Universidad Autónoma de Zacatecas in Mexico.
Information from local villagers initially led him to the cave after his search for hunter-gatherer sites in the region proved to be too recent and eroded by the passage of time. Caves preserve the past in a more intact way.
In the cave, Ardelean found stone flakes in a layer of the cave that dated to 30,000 years ago. Since that 2012 excavation, he conducted two larger follow-up excavations in 2016 and 2017, each lasting seven weeks. During these excavations, which took place deep in the cave and 164 feet away from its entrance, Ardelean and his colleagues found flaked stone tools that were shaped by humans who occupied the cave over a long period of time.
The tools were made from limestone, which is not naturally found in the cave, meaning it came from sources outside of it. The shaping of the tools is also completely unique from Clovis culture tools, which is often the case for artifacts from pre-Clovis sites. This represents a cultural and behavioral diversity between these groups of humans during the earliest days of settling in the Americas, Ardelean said.
Team members work inside Chiquihuite Cave.
"We discovered flake stone tools in a long succession of layered deposits that cover a long occupation span ranging from 31,000 years ago to 12,500 years ago," Ardelean said. "Indicators of constant visits to the cave from an ancient human group for an extremely long period of time, starting way before the commonly accepted human dispersal into the Americas."
The cave presented many challenges to the researchers. It's in a remote area affected by drug wars and instability, Ardelean said. The cave is also difficult to reach and required driving trucks up a mountain for 45 minutes to reach its location 3,280 feet above the village. Then, caravans of 40 donkeys and mules, as well as local villagers, helped transport all of their research equipment over the remaining steep slopes.
Ardelean's eight-person crew lived in the cave for seven weeks, bringing all of the food, water, gasoline for their generators and everything else they needed at the beginning of the journey to last for the duration. They worked during the winter because it's too wet and the storms make work impossible during the summer.
Sunlight only penetrates the cave for a very short time during winter, so they used the power from the generators to illuminate their work space. The team worked for 10 hours a day on excavations, fueled by strong coffee and their own cooking.
While the team did not recover human environmental DNA in the spots they sampled, they hope to be more successful in the future, he said. Who were the earliest humans in the Americas?
Although the cave has provided evidence of older dates associated with human occupation, it raises new questions for Ardelean, who sees a correlation between this and other ancient sites across America. The biggest questions remain. Who were these people and where did they come from?
Ardelean believes there were very few humans in the Americas during the Last Glacial Maximum.
"That makes them very difficult to find, and, when found, their traces are very scant," Ardelean said. "These ancient groups were nomads and they did not live for too long in the same location. They migrated in wide circular migration cycles, returning to a determined location after years or generations. They probably revisited Chiquihuite Cave every certain number of years and remained there for a few weeks only, maybe during winters, before moving on."
He also believes that many of these groups went extinct and died during their migrations, so their genetic and cultural legacy didn't pass on to the next generation. This makes them even more difficult to trace, he said.
The team members sampled the different cultural layers in the cave.
But the evidence from the cave has suggested an intriguing timeline. To reach the cave at its earliest reliable date, which is 27,000 years ago, humans still would have needed to reach America by 30,000 years ago to account for the distance.
"The antiquity of Chiquihuite Cave really changes the way we view the initial dispersal of humans into the continent," Becerra-Valdivia said. "Future research is required in South America. Only by unlocking the history of initial human occupation there will we be able to see the entire picture and understand the full migration pattern."
Ardelean suspects their findings will be considered controversial given that "American prehistory is highly paradigmatic and reticent to challenges."
"There will undoubtedly be challenges to this interpretation and close examination of the site data," wrote Ruth Gruhn, professor emerita in the department of anthropology at the University of Alberta, in an accompanying News & Views article. Gruhn was not involved in either study.
She cited six Brazilian archaeological sites that have been dated as older than 20,000 years ago, "although expertly excavated and analysed, are commonly disputed or simply ignored by most archaeologists as being much too old to be real. The findings at Chiquihuite Cave will bring about fresh consideration of this issue."
As to why no archaeological site of equivalent age to the cave has been found in the US, Gruhn suggested that the earliest sites may be submerged after the rise in sea level and that careful investigation for sites and traces of human presence, especially in previously discounted sites, "should intensify."
"The peopling of America is the ultimate battlefield in American archaeology and one of the last legitimate mysteries in the world of archaeology," Ardelean said. "Ice Age archaeology is full of real enigmas and there are new questions surging over and over again, constantly."