New research, supported by advanced dating techniques, shows that humans settled in the Americas approximately 23,000 years ago, questioning an earlier timeline of 14,000 years. When and how humans first settled in the Americas is a controversial topic. In the 20th century, archaeologists believed that humans arrived in the interior of North America no earlier than about 14,000 years ago. But our new research finds a different story. Our latest research supports the idea that humans were present in the Americas around 23,000 years ago.
Ice-free corridors and previous assumptions
Experts in the 20th century believed that the emergence of humans coincided with the formation of an ice-free corridor between two huge ice sheets spanning what is now Canada and the northern United States. According to this view, this corridor allowed humans to hike from Alaska into the heart of North America due to melting at the end of the last Ice Age.
This orthodoxy gradually disintegrated. In recent decades, the earliest evidence of humans has been dated back from 14,000 to 16,000 years ago. This is still consistent with the idea that humans did not reach the Americas until the end of the last ice age.
In September 2021, we published a paper in Science that dated fossil footprints unearthed in New Mexico to about 23,000 years ago, the height of the last Ice Age. The footprints were left by a group of people passing by an ancient lake near what is now White Sands. This discovery added 7,000 years to the historical record of humans on the American continent and rewrote American prehistory.
If humans arrived in the Americas at the peak of the last ice age, either ice and snow posed little obstacle to their passage, or humans had been living there much longer. Perhaps they arrived in the Americas during the early stages of melting glaciers.
Our conclusions have been criticized, however, we now publish evidence that confirms the earlier date.
Determining the age of footprints and the pollen mystery
Pollen is a useful tool for dating evidence of human settlement. For many people, the word pollen conjures up images of allergies, sneezing, and miserable summers. But fossilized pollen can be a powerful scientific tool.
In the 2021 study, we performed radiocarbon dates on common ditchweed seeds in the sedimentary layers above and below where the footprints were found. Radiocarbon dating is based on how a specific form of carbon (carbon-14) -- called an isotope -- radioactively decays in organisms that have died within the past 50,000 years.
Some researchers claim that the radiocarbons in our 2021 study are too old because they were affected by something called a "hard water" effect. Water contains carbonates and therefore carbon. Hard water is groundwater that has been isolated from the atmosphere for a period of time, meaning some of the carbon-14 in it has undergone radioactive decay.
Common ditchweed is an aquatic plant, and critics say the plant's seeds may have consumed old water, thereby disturbing the date and making it appear earlier than it actually is. They are quite right to ask this question. This is how science should be done, making claims and rebutting them.
The role of advanced technology and conclusive findings
Radiocarbon dating is reliable and easy to understand. Any type of organic matter can be determined in this way as long as there is enough of it. So two members of the team, Kathleen Springer and Jeff Pigati of the U.S. Geological Survey, set out to date the pollen grains. However, pollen grains are very small, typically around 0.005 mm in diameter, so you need a lot of them.
This presents a difficult challenge: You need thousands of these to get enough carbon to date something. In fact, you'll need 70,000 pills or more.
Medical science provides extraordinary solutions to our problems. We used a technique called flow cytometry, which is more commonly used to count and sample individual human cells, to count and isolate fossilized pollen for radiocarbon dating.
Flow cytometry exploits the fluorescence properties of cells under laser stimulation. These cells move in a fluid stream. The fluorescence causes the gates to open, allowing individual cells in the liquid stream to be transferred, sampled and concentrated.
The researchers found pollen grains in all the layers of sediment between the white sand footprints, allowing us to date them. The main advantage of having so much pollen is that plants such as pine trees that are not affected by old water can be harvested. The samples were processed using flow cytometry to concentrate the pollen in them.
After a year or more of labor-intensive and expensive laboratory work, the researchers obtained a date based on pine pollen, verifying the original age of the tracks. They also showed that ancient water effects were not present at the site.
Pollen can recreate the vegetation that was growing when people left their footprints. The types of plants we got were exactly what we would expect from the Ice Age in New Mexico.
After a different dating technique called optically stimulated luminescence (OSL) as an independent check. OSL relies on the accumulation of energy over time within buried quartz grains. This energy comes from our background radiation.
The more energy, the older the quartz grains can be assumed to be. This energy is released when quartz is exposed to light, so the date measured is the last time the quartz grain saw sunlight.
To sample buried quartz, a metal tube is inserted into the sediment and then carefully removed to avoid exposing it to light. Remove quartz grains from the center of the tube, expose them to light in the laboratory and measure the light emitted by the grains. This betrays their age. OSL's dates support dates obtained by researchers using other techniques.
Humble pollen grains and some amazing medical techniques have helped us date the footprints and when people arrived in the Americas.