Researchers have shown that human footprints in North America are 21,000 to 23,000 years old. This means people were in North America thousands of years earlier than previously thought. Previously, these researchers were criticized for the reliability of their dating, but now they bolster their claim with two new sources.
In 2021, researchers found evidence for the first time of ancient footprints in White Sands National Park in New Mexico, USA, dating back 21,000 to 23,000 years. Due to the research methodology, these results are controversial. Researchers have now come up with new evidence which confirms that the footprints are actually more than 20,000 years old.
In 2021 census The researchers used the seeds of an aquatic plant Ruppia cirrhosa To determine how old the footprints are by carbon dating. They found these seeds in layers of soil around fossil molds. However, because aquatic plants absorb ‘old carbon’ from the water, using this method can make the plants appear older than they actually are.
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In Follow-up researchRecently released Science, US Geological Survey researchers used pollen from the land plant conifers for carbon dating. This pollen does not suffer from the problems encountered when dating aquatic plants.
Additionally, the researchers dated the footprints using a different technique. They determined the time-dependent damage to the structure of the minerals they found in the footprints.
Carbon dating
Carbon dating is a technique by which researchers determine how old a fossil is based on the proportion of a specific type of carbon. There are different types of carbon in the air, and the nucleus has different numbers of neutrons: twelve, thirteen, or fourteen. Carbon-12 is the most common (over 98 percent), carbon-13 is much less common, and carbon-14 is even rarer.
The latter, carbon-14, is essential for dating fossils. An important property of carbon-14 is that it is radioactive. This means that it is not stable and changes over time to stable nitrogen-14. At the same time, new carbon-14 is formed in the atmosphere, keeping the concentration in the air the same.
During their lifetime, organisms absorb carbon from the air, a small portion of which is carbon-14. When an organism, in this case a plant, dies, it cannot absorb carbon. But already absorbed carbon-14 can still convert to nitrogen-14. Researchers can measure how much carbon-14 a fossil contains, thereby determining how old it is: the less carbon-14, the less carbon-14 it has already turned into nitrogen-14, so the organism lived a long time ago.
Dated water
So in 2021, researchers used the seeds of an aquatic plant for carbon dating. Aquatic plants extract carbon from water instead of air.
Water contains relatively little carbon-14 because its exchange with new carbon from the air is slow. How quickly this exchange takes place depends on, among other things, how deep the water is, how much air there is and how long it is covered with ice. Supplying meltwater from ‘old’ groundwater or glaciers also reduces the share of carbon-14 in water.
If aquatic plants absorb carbon from the water—and a relatively large portion of the carbon-14 is converted to nitrogen-14—the seeds appear older than they actually are. This is also known as the reservoir effect.
land plant
So the researchers used land plants in a follow-up study. They isolated about 75,000 conifer pollen grains for each dated sample.
They collected these pollen grains from the same soil layers as the seeds of the aquatic plant in 2021 so they could directly compare the measurements. The age of the pollen always corresponds to the age they determined for the seed of the aquatic plant in 2021.
According to the chemist Matthew Boudin, who works at KIK-IRPA’s Carbon Dating Laboratory in Brussels and is not involved in the research, said it was good that the researchers made new measurements with land plants. However, he believes more dating is needed to confirm that the footprints are that old. ‘Criticism of dating aquatic plants is justified because of the reservoir effect. The new dating with land plants is a good step, but researchers need to take more measurements. They have only made three dates with land plants and the measurements have a large margin of error. This is because the researchers had little material to research.’
More evidence
Another dating technique used by the researchers removes some of the remaining doubts surrounding the age of the footprints. In addition to carbon dating, the so-called group is used Optically induced luminescence method (OSL). Using the OSL method, the researchers were able to determine when the mineral grains around the footprints last saw sunlight, and thus how long they had been underground.
Exposure to natural radiation damages the crystal structure of minerals such as quartz. As long as the mineral grains are above ground, this radiation damage is released by sunlight. But if grains end up underground and don’t see sunlight, radiation damage accumulates.
From measurements of this damage, the researchers were able to determine that the footprints must be at least 21,000 years old. According to the chemist Dimitri Vandenberghe From the University of Kent, not involved in the research, the researchers mainly have a strong story because they used different dating techniques and materials that produced consistent results.
Also an archaeologist Luck Amcrutz Leiden University and the National Museum of Antiquities say the new dating adds almost 10,000 years of history more reliably. The dates indicate that humans arrived in the Americas during the peak of the last Ice Age.
Additional sites with firm dates are needed to determine migration and routes. ‘Footprints in white sand are not ancient. Or perhaps these traces are not from a true beginning, but from a failed attempt. “How exactly this migration went says a lot about not just America, but about us as a species, how we populated the world from Africa and what technologies played a part in this,” says Amcrutz.
