Ruppia seeds are small, identifiable, and widespread in lakebed sediments from this kind of environment.

They are also organic, meaning they contain carbon that can be analyzed using accelerator mass spectrometry radiocarbon dating — the most precise available method for dating organic material up to about 50,000 years old.

The radiocarbon dates from the Ruppia seeds returned ages in the range of approximately 21,000 to 23,000 years before the present.

The team published their findings in the journal Science in September 2021.

The title of the paper was understated in the manner of scientific publishing: “Evidence of humans in North America during the Last Glacial Maximum.” The findings were not understated at all.

Why 23,000 Years Was Explosive