Confirmed – Humans reached the Americas much earlier than thought

New research has now confirmed that humans reached the Americas at least 7,000 years earlier than thought – between 21,000 to 23,000 years ago.

Conventional wisdom has been that humans first settled in the Americas around 14,000 years ago. However, this wisdom has also been mired in controversy, with some researchers suggesting an earlier time period for Homo sapiens’ arrival in the New World.

Then in September 2021, a study was published in the journal Science presenting evidence that humans were in North America about 23,000 years ago. This is significantly earlier – by 7,000 years – than the previously accepted date of human arrival in the Americas1(4,000 years ago).

The earlier thinking regarding human arrival in the Americas has been based on the formation of an ice-free corridor between two gigantic ice sheets between Canada and the northern US. The thinking was that this corridor, which was caused by the melting of ice at the end of the last ice-age, allowed human beings to move from Alaska to the rest of North (and South) America. Subsequently, the date of human arrival did move from 14,000 years ago to 16,000 years ago. However, this was still within the time-frame of the end of the last ice-age.

The 2021 study was based on the analysis of fossilized footprints found in White Sands National Park in New Mexico. The footprints were found in a layer of sediment that was dated to between 21,000 and 23,000 years ago, which was the height (and not the end) of the last ice-age. They were also found alongside the tracks of other animals, such as mammoths and camels, which suggests that they were made by early humans who were living in the area at the time.

This three-dimensional model highlights footprints across one of the excavated surfaces. The height of the area corresponds to the color, changing from cool to warm colors with increasing elevation.
PHOTOGRAPH BY DAVID BUSTOS

The study’s authors argue that the footprints provide strong evidence that humans were in North America much earlier than previously thought. They also suggest that the footprints may have been made by people who crossed over into the Americas from Asia during a period of time when the Bering Land Bridge was exposed. This discovery has meant an addition of 7,000 years to the human record and can result in rewriting American history because if they were there at the height of the last ice-age, how did they get there? Perhaps the ice was not as big a barrier as we had thought or perhaps they had been there even longer, reaching during an earlier melting phase.

The study was met with some skepticism from other scientists, who argued that more evidence is needed to confirm the date of the footprints. However, the study’s findings have also been praised by other scientists, who say that they provide strong new evidence for an early human presence in the Americas.

Now the researchers have published new evidence confirming the early dates.

In the earlier study they had used radiocarbon dating on common ditch grass seeds found in sediment layers above and below where the footprints were found. As the researchers said to The Conversation: “In our 2021 study, we carried out radiocarbon dating on common ditch grass seeds found in sediment layers above and below where the footprints were found. Radiocarbon dating is based on how a particular form – called an isotope – of carbon (carbon-14) undergoes radioactive decay in organisms that have died within the last 50,000 years.”

This was criticised by other researchers: “Some researchers claimed that the radiocarbon dates in our 2021 research were too old because they were subject to something called the “hard water” effect. Water contains carbonate salts and therefore carbon. Hard water is groundwater that has been isolated from the atmosphere for some period of time, meaning that some of its carbon-14 has already undergone radioactive decay. Common ditch grass is an aquatic plant and the critics said seeds from this plant could have consumed old water, scrambling the dates in a way that made them seem older than they were.”

The scientists then used flow cytometry to count and isolate fossil pollen for radiocarbon dating. This is a technique used for counting and sampling human cells.

According to their statement: “Flow cytometry uses the fluorescent properties of cells, stimulated by a laser. These cells move through a stream of liquid. Fluorescence causes a gate to open, allowing individual cells in the flow of liquid to be diverted, sampled, and concentrated.”

A year of extensive analysis showed that the original date of the footprints (between 21,000 to 23,000 years ago) was correct. Furthermore, old water effects were absent at this site.

The scientists say in the study: “We present new calibrated ¹⁴C ages of terrestrial pollen collected from the same stratigraphic horizons as those of the Ruppia seeds, along with optically stimulated luminescence ages of sediments from within the human footprint–bearing sequence, to evaluate the veracity of the seed ages. The results show that the chronologic framework originally established for the White Sands footprints is robust and reaffirm that humans were present in North America during the Last Glacial Maximum.”

The pollen was also used to reconstruct the vegetation that was prevalent at the time and this revealed exactly the kinds of plants that would have been expected to be there during the Ice Age in New Mexico.

Finally, they also used a technique known as called optically stimulated luminescence (OSL) as an additional check. This also confirmed thedates the footprints were made, and when people reached the Americas i.e between 21,000 and 23,000 years ago.

It is now increasingly becoming clear that human beings had arrived in the Americas earlier than thought. Now the question remains how did they get there?

The two studies can be found here and here.

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Saima Baig

I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.