Until not too long ago, the consensus among archaeologists and anthropologists used to be  that the first humans arrived in the Americas by crossing a narrow land bridge between Siberia and glacier-covered Alaska some 14,000 years ago towards the end of the Last Glacial Period. However, research carried out over the past 15 years suggests that the timeline of human settlement in the Americas stretches further back in time than previously believed.

Two new studies published this week suggest that the first humans set foot in the lower 48 states as early as 30,000 years ago, drawing from extensive fieldwork across dozens of archaeological sites in North and Central America.

What’s more, this newly refined timeline of human dispersal also casts doubt over the accepted migratory path. Rather than entering North America from Asia via Beringia (the now-submerged land bridge between Eurasia and Alaska), the new findings point towards a route along the Pacific Coast as a more likely entry and dispersal point.

The human journey into the Americas may have first begun at least 10,000 years earlier than previously thought

There is still much uncertainty regarding the timeline of the migration and the divergence between the northern and southern Amerindian populations.

The first natives to the Americas were widely believed to be the Clovis, named so after the town of Clovis, New Mexico, where archeologists found “matted masses of mammoth bones” and slender, finger-long spear points during the 1930s. These weapons — known as Clovis points due to their characteristic flaking technique — have been found across more than 1,500 sites in the southeast and central contiguous United States, dating as far back as 13,000-14,000 years ago.

However, growing archaeological and genetic evidence since the early 2000s disputes the notion that the Americas were originally populated by Clovis people. Instead, there were likely multiple migrations of people from Asia.

Ciprian Ardelean, a researcher and lecturer at the Universidad Autónoma de Zacatecas, and colleagues performed excavations at a cave in Zacatecas, in central Mexico. They found a treasure trove of stone tool artifacts, as well as plant remains. Dating suggests that some of the flaked stone tools cover a long occupation ranging from 31,000 years ago to 12,500 years ago, predating the Clovis culture by many thousands of years.

“The peopling of America is the ultimate battlefield in American archaeology and one of the last legitimate mysteries in world archaeology. I became obsessed with these questions over the years. Doing Mayan archaeology for several years (1999-2006) showed me that there were few true mysteries left to address in Mayan culture, that most discoveries were basically repetitions of already known finds so that was not challenging enough. Ice Age archaeology, instead, is full of real enigmas and there are new questions surging over and over again, constantly. That attracted me to the point that I completely changed the direction of my professional life,” Ardelean told ZME Science.

Ardelean has been investigating hunter-gatherer sites in the region since 2009, while he was still a Ph.D. candidate. Previously, the archaeological sites he had access to were too recent for his interests, spanning only the last 1,500 years. They were also too extremely eroded for any meaningful work. This is why he focused his attention on caves, which can preserve remains and artifacts better. Caves are also excellent natural shelters that would have been extremely appealing to early humans.

“Following information from local villagers, one colleague and I visited Chiquihuite Cave for the first time in May 2010. The first test excavation was done in 2012 for my PhD thesis. I managed to find a few stone flakes in a layer that was dated to almost 30,000 years ago. That was sufficient motivation to search for funding and grants and start excavations on a larger scale,” said Ardelean.

With funding secured, Ardelean returned to Chiquihuite Cave between 2015 and 2017, where he began major excavations — but also had to face innumerable challenges that come with the territory of conducting fieldwork in an area constantly besieged by “drug wars and profound instability”.

To reach the high-altitude cave, perched more than 1,000 meters above the closest village, the archeologists had to drive 4×4 trucks up the mountain, before continuing the rest of their journey up the steep slopes to the site on the back of donkeys and mules. Over 40 pack animals and 40 people, including local villagers, formed huge caravans carrying equipment and retrieved artifacts during each excavation season.

“We lived inside the cave, for 7 weeks in a row each time, without leaving it. All the food, water, gasoline (for power plants) was carried from the beginning of the season, calculated to last for 7 weeks. We camped inside the cave and cooked at the entrance of the cave. We never took showers or bathed during the field season; only minor intimate hygiene was possible. We always worked during winters, at 2740 m of altitude. In summers, it’s wet and stormy, one cannot work up there; it has to be winter, which is the dry season here. Sometimes, we had snow, hail, cold rain and strong winds; sometimes, the temperature outside dropped to -10ºC. But, inside the cave, the temperature always stays still at +12ºC, day and night. This makes the cave a potential perfect shelter for spending winters during the harsh Last Glacial Maximum (LGM) conditions,” said Ardelean.

“The excavation was done far inside the cave, at 50 m away from the entrance. The ancient entrance was buried under tonnes of rocks fallen from the cliffs around 12,000 years ago, blocking the access. It is possible to excavate along the ancient entrance (drip line), which also prevents us from discovering many important things, such as the fireplaces and diet remains. So far inside the cave, it’s difficult to hope for something other than stone tools and debitage.”

The fieldwork, which was described in a study published in Naturetoday, indicate that the cave was constantly occupied by ancient human groups over a period of nearly 20,000 years.

This timeline runs counter to the currently established narrative among scholars. Although the paradigm of the earliest human dispersal in the Americas has shifted towards a pre-Clovis presence, the threshold is set to around 18,000 years ago.

Although earlier artifacts than this threshold have been reported in the scientific literature in the past, the establishment is reluctant to accept earlier timelines. According to Ardelean, it is widely believed that the climatic conditions at the end of the Last Glacial Maximum (LGM) would have made it impossible for hunter-gatherers to cross Asia into Alaska due to massive ice caps that occupied the entire territory of Canada from coast to coast.

“The finds will be considered controversial and I will obviously receive many attacks and rejections. That is normal in American prehistory, which is highly paradigmatic and reticent to challenges,” Ardelean said.

“In my case, the discovery is based almost exclusively on flaked stone tools. Unfortunately, until now, we have not found undeniable modified bone or other sort of indicators. However, the tools are human-made, beyond any doubt. I have spent years with them in my hands, in successive rounds of analysis, so I know what and why I am saying. Most of them speak by themselves in the published figures. However, they have a few characteristics that make this assemblage suitable for criticism. “

The study’s main focus lies almost exclusively on flaked stone tools retrieved from the Mexican cave. Unlike Clovis points that are made from chert or obsidian, the tools found at Chiquihuite are made from a particular variety of limestone — “greenish and finely grained, recrystallized, which makes it behave just like chert,” Ardelean said.

Judging from the geology of the cave, the limestone for these tools must have been sourced elsewhere, outside the cave. Limestone is a rather abundant raw material for stone tool manufacturing, so it is not at all unusual to see it there.

“The Chiquihuite assemblage could be defined as a microlithic technology, dominated by microblades, microlith fragments and transversal flakes. Also, by transversal points made by the superficial modification of transversal flakes,” said Ardelean.

As for what these early hunter-gatherer populations must have looked and socialized like, there is not much to draw inferences from, unfortunately. The researchers were not able to extract human-environmental DNA (eDNA) from the Mexican cave, which would have helped pinpoint the lineage of the cave’s inhabitants.

“I think there were very few humans in the Americas during the LGM and before. Literally a few small groups scattered across such a large territory. That makes them very difficult to find, and, when found, their traces are very scant. 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,” Ardelean said.

“I also think that many LGM human groups went extinct, they were lost migrations that died out and did not pass their genetic and cultural legacy to the later populations. That makes them even more difficult to find and trace,” he added.

More than 40 archaeological sites in North America and Beringia point to pre-Clovis human presence4

Chiquihuite cave is just the latest addition to a roster of archaeological sites that support a much earlier human dispersal into the Americas than it is currently established.

Another study published today in Nature, led by Lorena Becerra-Valdivia, an archaeological scientist at the University of New South Wales and the University of Oxford, and Thomas Hingham from the Research Laboratory for Archaeology and History of Art at the University of Oxford, shows that by at least 15,000 years ago, the North American continent was already widely settled.

The authors’ statistical analysis and radiocarbon dating, which required years of research and were very laborious, imply that some archaeological sites were settled even far earlier than that.

For instance, six Brazilian sites — five in the state of Piaui and the other one in Moto Grosso — are more than 20,000 years old. Clearly, humans must have migrated from Asia much earlier than that.

“The results of our study show that whilst there were humans in North America before, during and immediately after the Last Glacial Maximum, populations expanded significantly across the continent much later, during a period of abrupt global climate warming at the end of the Ice Age, beginning at around 14,700 years ago. This is based on the synchronous start of three major stone tool traditions (Clovis, Western Stemmed and Beringian), a spike in archaeological sites and chronological data, as well as genetic evidence pointing to marked population growth,” Becerra-Valdivia told ZME Science.

Becerra-Valdivia, who is Ardelean’s co-author in the accompanying Nature paper, also visited Chiquihuite cave as part of her research, which encompassed 42 archaeological sites in North America and Beringia.

“One of my highlights was a visit to Chiquihuite Cave, after a four-hour-long foot journey. Having the opportunity to see the site and evidence first hand is invaluable to a radiocarbon expert. It helps us better understand the archaeological site and makes us appreciative of the work that archaeologists do in the field,” she said.

“The accompanying paper is a marvelous piece of work that I enjoyed very much. The authors are also co-authors of my paper. Lorena and Tom are very good colleagues and dear friends of mine, and I have known them for years. I have a profound respect for their talent and geniality with radiocarbon dating. Their paper gives us the best image of the accurate dates of the early sites and it will be a referent for years to come,” Ardelean said.

For most of the last major glacial advance, the entry point through the Bering Strait was blocked by ice sheets. This is the main reason why a much earlier timeline of pre-Clovis migration has been hard to swallow for the research community at large.

Becerra-Valdivia and Hingham have proposed an alternative route down the Pacific coast, which is supported by archaeological findings in coastal zones. However, validating this model will prove challenging in the future as it is likely that the earliest coastal-entry archaeological sites are now submerged offshore.

“Given that humans were already present in the continent between 33 and 31 thousand years ago (at Chiquihuite Cave), the initial crossing from Asia must have occurred earlier. We suggest that this likely occurred at the end of a period called Marine Isotope Stage 3 (between 57 and 29 thousand years ago), when ancient Beringia was either completely or partially submerged under water. Therefore, the journey must have required a degree of maritime or littoral (coastal) adaptation,” Becerra-Valdivia told me in an email.

The highly precise radiocarbon dating of the archaeological sites overlaps with the latest dates of the appearance of 18 now-extinct fauna genera. The end of the last ice age was a period of intense climate change, which is believed to have brought the demise of many species. But the role that human intervention had in these extinctions may have been important.

“The expansion of humans during GI-1 seems to have played a role in the decline of large megafauna, including types of camels, horses and mammoths, who disappear within this and an immediately preceding, colder period. The contribution of climate change in faunal extinctions cannot be excluded, however,” said have required a degree of maritime or littoral (coastal) adaptation,” Becerra-Valdivia.

Both studies make valuable contributions to the ongoing debate of how humans first settled the Americas. They are also likely to be met with quite a bit of skepticism — and rightfully so. There’s no other site that comes close to the early timeline of the Chiquihuite Cave, and this is a problem. However, perhaps other similarly ancient sites may be discovered in due time in North America as more investigations are performed.

“We are starting now to analyze the materials from the very last excavation at the cave, done in 2019. We will study new artefacts and other materials with new techniques and involving new proxys. Hopefully, within a year or so, we’ll be able to submit a second paper about Chiquihuite, with new data and with an improved resolution,” Ardelean 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,” Becerra-Valdivia added. “These are paradigm-shifting results that shape our understanding of the initial dispersal of modern humans into Americas. They suggest exciting and interesting possibilities for what likely was a complex and dynamic process.”