Each and every scientific discovery sends a small ripple through academia – it changes what we know about the world around us in a fundamental but usually quite subtle way. A breathtaking new study in Nature, however, is more of a tidal wave, a revolution in the way we understand the story of humanity.

The general consensus has been that humans arrived in North America no later than 24,000 years ago, at the earliest. A startling archaeological discovery of ancient human activity in California, however, has possibly moved this date back to 131,000 years, and in the process has potentially rewritten the history books.

“Extraordinary claims require extraordinary evidence,” co-author Richard Fullagar, Honorary Principal Fellow at the University of Wollongong, told IFLScience on a conference call. “The dates are truly remarkable, but it’s hard to argue with the clear evidence we see. It’s incontrovertible.”

A deeply-buried site in coastal San Diego County contained the remains of a mastodon, an extinct creature distally related to elephants, including several tusks with curious indentations on them. On closer inspection, these marks appeared to perfectly match up with various hammerstones, anvils, and tools found nearby.

No actual human remains were found, which means that this study is likely to generate some criticism from other archaeologists. Nevertheless, these tools resembled those being used by humans and their ancestors all over the planet, before and since. Using a series of experiments that recreated the hammering and cutting activity the tools were presumably used for on the mastodon remains, the team perfectly recreated the indentations.

^{A mastodon skull. Brooke Crigger/Shutterstock}

“What’s truly remarkable here is that you can match the hammers to the anvils to the stones – it really does demonstrate human interference,” Fullagar noted.

State-of-the-art uranium dating techniques revealed without question that these tusks were 131,000 years old, as were the marks on them. No known carnivore or geological process could have made such precise scratches on them, and the site itself had remained undisturbed by erosional processes since it appeared.

Ruling everything else out, and approaching their assessment of the find as carefully and as conservatively as possible, the team concluded that this was an archaeological site.

^{The First Americans. Nature Video via YouTube}

A mastodon was killed and some of its remains were moved here, where ancient humans or human ancestors began carving up the tusks for use as tools, ornaments or to extract the bone marrow for food – an ancient human activity that dates back at least 1.5 million years to African settlements.

The date completely tears up everything we knew about human migration across the world.

“I expect there will be some extraordinary claims about how they got there,” co-author Steven Holen, Co-Director of the Center for American Paleolithic Research, added. “We expect criticism, and we are ready for it. I was skeptical when I first looked at it myself, but it’s definitely an archaeological site.

^{Some of the possible migration routes. Note, starting points do not necessarily indicate speciation points. Chris Jones/IFLScience}

Here’s a peculiar thought for you: The first Americans may have appeared more than 130,000 years before the Declaration of Independence was signed – and, most tantalizingly, we can’t be sure who they even were or quite what species of hominid they belonged to.

A far cry from the melting pot of diversity that America is today, these settlers pre-date Native Americans and even the enigmatic Clovis people. The most likely possibilities, according to the team, are that they were anatomically modern humans, Homo sapiens sapiens, Homo neanderthalensis, or, perhaps, something else.

“There are Neanderthal sites in Siberia, and unquestionably, they could have made the journey across during the last interglacial when sea levels were lower,” Fullagar commented, mentioning Beringia, the now-submerged land bridge between Siberia and what is now Alaska.

This would have been quite the journey. They would have made it all the way from Europe, through to Asia, and up, across and around Siberia before making their way down the western seaboard of the US. Remarkably, Fullagar also suggests that these Neanderthals “could have even used boats.”

If these travelers were Neanderthals, this would be the first evidence of them found outside of Europe. Interestingly, Native Americans do have relatively high numbers of Neanderthal genes in their genomes.

^{Excavation of the site. NPG Press via YouTube}

They could have been modern humans too, or even the mysterious Denisovans, who started their epic migrations from East Africa (or perhaps China), and South-East Asia, respectively. Fullagar suggests the site could have been populated by “meta-populations” of humans, a “hybrid mix” of different species.

It’s almost certain, though, that whoever made these marks will never be identified.

“Human remains are very rare in NA back at the time of the Clovis,” co-author James Paces, research geologist at the United States Geological Survey, told reporters. “The possibility of finding remains dating back to 130,000 years would be a truly exceptional find, but not very likely, unfortunately.”

^{The disappearance of the Bering land bridge. NOAA}

Whomever the First Americans were, they certainly settled in a rather pristine environment. “There were mastodon, capybara, deer, dire wolf there,” Paces added. “It was close to a river along the coast – a nice place to live.”

In any case, the migration from Eurasia to North America would have been incredibly dangerous. Ancient America, then, was certainly the land of the free and home of the brave.

^{A timeline of human evolution, featuring its major lineages. Smithsonian Museum of Natural History}