A Mastodon tusk (partially reassembled) from the Page-Ladson site. DC Fisher

It took decades, and a countless number of SCUBA tanks. Now, the painstaking excavation of an underwater archaeological site in northern Florida may change our understanding of when humans first populated North America.

A team of archaeologists led by Jessi Halligan—an anthropologist who specializes in underwater archaeology at Florida State University—just completed an aquatic dig of the oldest archaeological site in the American Southeast. It's a deep sinkhole called the Page-Ladson Archaeological Site located just beyond the southeastern skirts of Tallahassee in the Aucilla River. Halligan's team found stone knives and mastodon bones, tusks and dung, leading the scientists to believe the mastodon was either butchered or scavenged at the site by humans. Most interestingly, 71 individual radiocarbon dates show that the site is at least 14,550 years old—a full 1,500 years before many scientists recently believed humans first populated North America. The underwater dig was outlined today in the journal Science Advances.

This new find is important, because many archaeologists had long believed that 13,000-year-old stone spearheads and other remains found in the 1920s in Clovis, New Mexico, represented the first wave of human settlers in North America. "For over 60 years, archaeologists accepted that Clovis were the first people to occupy the Americas... Today, this viewpoint is changing," says Michael Waters, an anthropologist at Texas A&M University who's part of the team. "The Page-Ladson site provides unequivocal evidence of human occupation that predates Clovis by over 1,500 years."

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"For over 60 years, archaeologists accepted that Clovis were the first people to occupy the Americas... Today, this viewpoint is changing."

Halligan and her colleagues are not the first to root around in the waters of the Page-Ladson Archaeological Site, which is the "oldest [underwater] site yet discovered in the new world," Halligan says. "A recreational diver and a vocational archaeologist by the name of Buddy Page reported the site in the early 1980s to a team of paleontologists and divers... that team excavated this site for several seasons throughout the 1980s and 1990s, and made the amazing discovery of an adult mastodon tusk that appeared to have human-made cut marks. In the same geological layer, they found several possible stone artifacts," she says, as well as the remains of a dog.

An initial round of radiocarbon dating—a method where scientists date material by observing the decay of carbon atoms in organic matter—was done in the early '90s and placed the age of the remains roughly 14,400 years ago. But back then, many assumed that date was a fluke and dismissed it. "It was an impossible age for the scientific community to accept at the time," says Halligan, because the resounding agreement was that the humans hadn't made it to North America until around 13,000 years ago.

Neil Puckett, a Ph.D. student from Texas A&M University involved in the excavations, surfaces with the limb bone of a juvenile mastodon. Brendan Fenerty

Because underwater archaeological digs are notoriously messy and inaccessible, until the last few years no one had been able to rigorously vet those earlier findings. Today they have. "We used underwater lasers to control our depth of excavation, all of which allowed us to be very precise and to see items as we expose them in place," says Halligan.

Because the discovered mastodon dung "consisted of millions of fragments of chewed plant matter that was perfectly preserved, that allowed us to collect more than 70 radiocarbon samples from the site, she says. "All of the samples from this layer dated to more than 14,400 years ago and samples associated with the knife dated to 14,550 years old." In addition, Halligan's team confirmed that distinct markings on the tusks of the mastodon—a species that was hunted to extinction around 12,600 years ago—are chop marks from the stone tools.

According to Waters at Texas A&M, the big takeaway is that these newly confirmed discoveries "contribute significantly to the debate over the timing and complexity of the peopling of the Americas in several ways." Since the 1990's, archaeology has seen a boon of newly discovered sites similar to Page-Ladson across North and South America, many of which have pushed back our understanding of when humans first entered the two continents.

A schematic showing underwater excavation methodology at Page-Ladson, and location of artifact. J. Halligan

"First, Page-Ladson is essentially the same age as the Monte Verde site in Chile and these two sites show that people were living in both hemispheres of the Americas by at least 14,500 years ago," Waters says. "Second, prehistoric people at Page-Ladson were not alone. [Other recent] archaeological evidence shows us that people were also present between 14,000 and 15,000 years ago in what are now the states of Texas, Oregon, Washington, Pennsylvania and Wisconsin."

A biface found in situ at Page-Ladson in 14,550-year old sediments. CFSA

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