The story of how humans spread across the globe is constantly being rewritten. Dating back to about 13,000 years ago, the "Clovis culture" was long thought to be the earliest human inhabitants of North America, but throughout the 21st century new finds consistently suggest the presence of an older culture. Now, archaeologists have uncovered stone tools at a dig site in Texas that could be as old as 20,000 years.

The Clovis culture is so named because the first examples were discovered near the town of Clovis, New Mexico, in the 1920s. These distinctive pointed tools were dated to be between 11,000 and 13,000 years old, and after others began turning up around the continent, the prevailing theory was that they belonged to the first settlers of the Americas. These people were believed to have crossed over into Alaska from Siberia during the last ice age, when the Bering Strait had dried up, before spreading out across North and South America.

But since the year 2000 or so, older artifacts have pushed the date of settlement back to between 13,000 and 16,000 years. The new finds, made in the Gault Site in Central Texas, might trace their roots even further back.

Plenty of Clovis tools have turned up in the Gault site over the years, but the archaeologists on the new study dug a little deeper – literally. In lower layers of sediment, the team discovered older tools including a previously unknown type of projectile point.

"These projectile points are unique," says Tom Williams, lead author of the study. "Combine that with the ages and the fact that it underlies a Clovis component, and the Gault site provides a fantastic opportunity to study the earliest human occupants in the Americas."

In trying to get a timeline for the tools, the scientists found that the regular radiocarbon dating wasn't going to work. The sediments they were deposited in were most likely a riverbed, which doesn't allow for the preservation of organic materials all that well.

Instead, the team turned to a dating process called optically stimulated luminescence (OSL), which works by exposing the artifacts to light or heat until they release trapped electrons. The photons given off by these emissions can tell scientists how long it's been since the materials were last exposed to that kind of light or heat, indicating how long they've remained buried.

Using this technique, the researchers estimate that the artifacts are between 16,000 and 20,000 years old. That's older than the general consensus on North American occupation, but lines up reasonably well with other evidence. In some cases, that goes back to a ridiculous 130,000 years ago, with the discovery of mastodon bones that showed signs of having been broken by stone tools.

The research was published in the journal Science Advances.

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