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Holliday and a graduate student spent several days examining geologic layers in trenches, dug by previous researchers, to piece together a timeline for the area. They had no idea that, about 100 yards away, there were footprints, preserved in ancient clay and buried under gypsum, that would help spark a wholly new theory about when humans arrived in the Americas.

Researchers from Bournemouth University in the United Kingdom and the U.S. National Park Service excavated those footprints in 2019 and published their paper in 2021. Holliday did not participate in the excavation but became a co-author after some of his 2012 data helped date the footprints.

The tracks showed human activity in the area occurred between 23,000 and 21,000 years ago—a timeline that would upend anthropologists' understanding of when cultures developed in North America. It would make the prints about 10,000 years older than remains found 90 years ago at a site near Clovis, New Mexico, which gave its name to an artifact assemblage long understood by archaeologists to represent the earliest known culture in North America.

Critics have spent the last four years questioning the 2021 findings, largely arguing that the ancient seeds and pollen in the soil used to date the footprints were unreliable markers.

Now, Holliday leads a new study that supports the 2021 findings—this time relying on ancient mud to radiocarbon date the footprints, not seeds and pollen, and an independent lab to make the analysis. The paper was published in the journal Science Advances.

Specifically, the new paper finds that the mud is between 20,700 and 22,400 years old—which correlates with the original finding that the footprints are between 21,000 and 23,000 years old. The new study now marks the third type of material—mud in addition to seeds and pollen—used to date the footprints, and by three different labs. Two separate research groups now have a total of 55 consistent radiocarbon dates.

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