Markings on fossils from an extinct elephant relative called a mastodon are at the centre of a row over when humans reached the Americas.Credit: Kate Johnson, SDNHM

Archaeologists are taking aim at a controversial study that claimed to rewrite theories about when humans first reached the Americas, one of the biggest questions in palaeoanthropology.

When researchers made the astonishing suggestion last year that early humans settled the Americas 100,000 years earlier than thought, they asked doubters to keep an open mind and consider the evidence backing their claim. But their study1, which proposed that mastodon bones from California were broken by an as-yet-unidentified group of early humans 130,000 years ago, was instantly questioned by archaeologists. Most researchers agree that humans settled the Americas around 15,000 years ago.

Nearly a year later, the sceptics are still not convinced. In a rebuttal to the work, published on 7 February in Nature2, archaeologists say that modern construction equipment better explains the mastodon bone damage than does the handiwork of ancient hominins. They present an analysis of mammoth bones from Texas that, they say, have similar-looking damage, which was caused by natural wear and tear and heavy equipment.

“It calls into question the basis for their paper,” says Joseph Ferraro at Baylor University’s Institute of Archaeology in Waco, Texas. He says his team began their critique soon after the original claims were published in Nature in April 2017.

Telltale signs

In the original study, a team co-led by Tom Deméré, a palaeontologist at the San Diego Natural History Museum in California, examined bone fragments of a mastodon (Mammut americanum), an extinct relative of elephants, that had been found during roadworks in suburban San Diego in the 1990s. Deméré and archaeologist Steven Holen at the Center for American Paleolithic Research in Hot Springs, South Dakota, contended that the remains bore telltale fractures seen in bones struck by the stone tools of early humans. No obvious stone tools or human remains were found at the site.

Deméré’s team also established that the mastodon bones were around 130,000 years old, and suggested that an unknown hominin species had reached California by that time. Current scientific consensus on settlement of the Americas is that early humans from Asia crossed the Bering land bridge into Alaska around 20,000 years ago, a theory based on archaeological research and studies of modern and ancient DNA.

To rebut the mastodon claim, Ferraro’s team examined a site in Waco containing the remains of at least 26 mammoths that died about 60,000 years ago. Archaeologists have previously looked for evidence of humans at the site and found none. According to Ferraro, some of the mammoth bones were battered and broken in the same way as the bones from the San Diego site.

Ferraro thinks that construction work — some of the Waco mammoth bones were found during a building project — and natural wear can explain the similarities. One type of bone break found at both sites, a spiral fracture, has been seen as far back as the Triassic period. “A dinosaur would break a leg. It happens. There are natural processes that could reasonably explain spiral fractures,” says Ferraro.

Damage questions

In a response published3 alongside the critique, Deméré and his colleagues stand by their assertion, and say the resemblance between the bones from the sites is merely superficial, and that comparing the sites is not appropriate. “We’re really quite familiar with what kind of damage is caused by heavy equipment,” Deméré adds. He asks doubters to come to San Diego to look at the material in person before making a judgement.

David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas, who co-wrote an earlier critique4 of the 2017 study, is glad to see other groups questioning the strength of the evidence. Meltzer says that he is open to the idea that humans reached the Americas more than 100,000 years before he thought — just not on the basis of such equivocal data.

“Given everything we know, it makes no sense,” he says. “You’re not going to flip people’s opinion 180 degrees unless you’ve got absolutely unimpeachable evidence, and this ain’t it.”