When and how people first settled in America is a matter of considerable debate. In the 20th century, archaeologists believed that humans reached the interior of North America no earlier than about 14,000 years ago. But our new research found something different. Our latest research supports the idea that humans were in the Americas about 23,000 years ago.
Experts in the 20th century believed that the emergence of humans coincided with the formation of an ice-free corridor between two large ice sheets that crossed the territory of modern Canada and the northern United States. According to this idea, a corridor formed by melting at the end of the last ice age allowed humans to move from Alaska to the heart of North America.
Gradually this orthodoxy disintegrated. In recent years, the dates of the oldest evidence of human existence have changed from 14,000 years ago to 16,000 years ago. This is consistent with the fact that humans only reached the Americas when the last ice age ended.
We published an article in the journal in September 2021. ScienceThe fossils found in New Mexico date back to about 23,000 years ago, at the peak of the last ice age. These were made by a group of people passing by an ancient lake near what is now White Sands. The discovery rewrote America’s prehistory, adding 7,000 years to human history on the continent.
If humans had been in the Americas at the height of the last ice age, either the ice would not have prevented their passage, or humans had been there much longer. They may have reached the continent during an earlier melting period. Our findings have been criticized but we have now published evidence supporting the early dates.
pollen dating
For many people, the word pollen symbolizes a summer of allergies, sneezing, and misery. But fossilized pollen can be a powerful scientific tool. In our study in 2021, we radiocarbon-dated ordinary grass seeds found in the sedimentary layers above and below where the footprints were found. Radiocarbon dating is based on how a specific form (an isotope) of carbon (carbon-14) undergoes radioactive decay in organisms that died within the last 50,000 years.
Some researchers have argued that the 2021 radiocarbon dates in our study are too old due to exposure to the so-called “hard water” effect. Water contains carbonate salts and therefore carbon. Hard water is groundwater that has been isolated from the atmosphere for a while, meaning that some of the carbon-14 in it has already undergone radioactive decay.
Common grass is an aquatic plant, and critics claim that the plant’s seeds can consume old water, making palms appear older than they are. It is very right that they brought this issue to the agenda. This is the way science should proceed, with claims and counterclaims.
How did we test our claim?
Radiocarbon dating is a reliable and well-studied method. You can date any organic material this way if you have enough of it. So two members of our 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, usually around 0.005 millimeters in diameter, so you need a lot of pollen.
This has become a big problem: you need thousands of them to get enough carbon to date anything. You actually need 70,000 grains or more.
Medical science has found a surprising solution to our puzzle. To count and isolate fossil pollen for radiocarbon dating, we used a technique called flow cytometry, which is more commonly used to count and sample single human cells.
Flow cytometry uses the fluorescent properties of cells excited by laser. These cells move through the fluid flow. Fluorescence causes the gate to open, allowing individual cells to be removed from the liquid flow, sampled, and concentrated.
There are pollen grains in all the layers of sediment between the footprints at White Sands, allowing them to be dated. The most important benefit of having this much pollen is that you can harvest plants like pines that are not affected by old water. Our samples were processed by flow cytometry for pollen concentrations.
After a year or more of painstaking and expensive laboratory work, we were rewarded with pine pollen-based dates that confirmed the original road chronology. They also showed the absence of ancient water impacts in the area.
Pollen also allowed us to reconstruct the vegetation that grew when humans made pressures. We found in New Mexico exactly the kind of plants we expected to be there during the Ice Age.
We also used another dating technique called optically stimulated luminescence (OSL) as an independent control. OSL is based on the accumulation of energy within hidden quartz grains over time. This energy comes from the background radiation that surrounds us.
The more energy we find, the older we can consider the quartz grains. This energy is released when quartz is exposed to light, so what you see is the last time the quartz grains saw sunlight.
To obtain samples of embedded quartz, you insert metal tubes into the sediment and carefully remove them, avoiding exposure to light. You take quartz grains from the middle of the tube, expose them to light in the laboratory, and measure the light emitted by the grains. This reveals their age. The dates from OSL confirmed the dates we obtained using other methods.
The humble pollen grain and surprising medical technology helped us confirm the dates when footprints were made and humans reached the Americas.
Source: Port Altele
