How long have humans been in North America? New Mexico footprints are rewriting history.

New research has provided more evidence that prehistoric human footprints in New Mexico are likely the oldest direct evidence of human presence in the Americas, a long-studied and surprisingly controversial topic.

The question went mainstream in 2021, when scientists said they had found the oldest human footprints ever discovered in North America, about 23,000 years, sparking debate and questions about methods used. Until then, the earliest known dates of humanity’s colonization of North America was about 15,000 years ago, after the last ice age.

Now the original researchers have responded with more evidence and explanations in a study published Thursday in the journal Science.

What was the original discovery?

A study published in September 2021 in Science said the oldest known human footprints in North America had been discovered at White Sands National Park in New Mexico. Researchers identified about 60 fossilized footprints buried in layers of gypsum soil on a large playa in the Tularosa Basin. By carbon dating seeds embedded in the footprints, the U.S. Geological Survey estimated the prints were up to 23,000 years old. 

The 2021 study said humans could have crossed from Asia into the Americas 26,000 to 19,000 years ago, through land connecting what is now Russia and Alaska, during the last ice age.

This discovery upset other archeological theories of how human beings came to populate the American continent, and how long ago. The reported age of the footprints challenges the once-conventional wisdom that humans didn’t reach the Americas until a few thousand years before rising sea levels covered the Bering land bridge between Russia and Alaska about 15,000 years ago.

'Incredible:'Oldest known human footprints in North America discovered at national park

The debate about the footprints was all about the way the age of the seeds was determined – and a separate group of scientists in 2022 published a study that said the seeds were much younger than 23,000 years.

Questions focused on whether seeds of aquatic plants used for the original dating may have absorbed ancient carbon from the lake – which could, in theory, throw off radiocarbon dating by thousands of years.

What's the latest news?

In the study released Thursday, scientists from the 2021 study said their methodology and findings were valid, which they say verifies that humans were indeed present in North America 23,000 years ago.

The new study undertook multiple independent age estimates of the White Sands footprints, which all supported their previous study’s claims. “We always knew that we would have to independently evaluate the accuracy of our ages to convince the archaeological community that the peopling of the Americas occurred far earlier than traditionally thought,” said Jeff Pigati, a research geologist with the U.S. Geological Survey.

According to the new study, with three separate lines of evidence – involving samples of pollen grains and two different sediments – pointing to the same approximate age, "it is highly unlikely that they are all incorrect or biased and, taken together, provide strong support for the 21,000 to 23,000-year age range for the footprints."

Kathleen Springer, USGS research geologist and co-lead author of the new study, said in a statement that "even as the original work was being published, we were forging ahead to test our results with multiple lines of evidence and independent chronologic techniques.”

“Although we were confident in the original seed ages, we wanted to develop community confidence in them as well. Our new ages, combined with the strong geologic, hydrologic, and stratigraphic evidence, unequivocally support the conclusion that humans were present in North America during the last Glacial Maximum,” she said.

Thomas Stafford, an independent archaeological geologist in Albuquerque, New Mexico, who was not involved in the study, said he “was a bit skeptical before” but now is convinced.

“If three totally different methods converge around a single age range, that’s really significant,” he said.

Is the debate settled?

USA TODAY contacted two of the authors of the 2022 study that questioned the earlier findings, and both said some questions still remain about the new research released Thursday.

"The dating issue is not yet resolved because we don’t know when the footprints were buried," said Loren Davis of Oregon State University, who added there are methods available that can determine that. "Until that information is available, the issue will remain unresolved."

Contributing: Algernon D'Ammassa, Las Cruces Sun-News; The Associated Press