1037 GMT January 22, 2020
An unidentified Homo species used stone tools to crack apart mastodon bones, teeth and tusks approximately 130,700 years ago at a site near what’s now San Diego, sciencenews.org reported.
This unsettling claim upending the scientific debate over the settling of the Americas comes from a team led by archaeologist Steven Holen of the Center for American Paleolithic Research in Hot Springs, South Dakota, and paleontologist Thomas Deméré of the San Diego Natural History Museum. If true, it means the Cerutti Mastodon site contains the oldest known evidence, by more than 100,000 years, of human or humanlike colonists in the New World, the researchers reported in Nature.
Around 130,000 years ago, the researchers say, a relatively warm and wet climate would have submerged any land connection between northeastern Asia and what’s now Alaska. So ancient colonizers of North America must have reached the continent in canoes or other vessels and traveled down the Pacific coast, they propose.
Candidates for southern California’s mastodon bone breakers include Neandertals, Denisovans and Homo erectus, all of which inhabited northeastern Asia around 130,000 years ago. A less likely possibility, Holen says, is Homo sapiens, which reached southern China between 80,000 and 120,000 years ago. No hominid fossils have turned up among the mastodon remains.
Whatever Homo species reached the Cerutti Mastodon site probably broke apart the huge beast’s bones to obtain nutritious marrow and claim limb fragments suitable for fashioning into tools, the scientists suspect. Hominids probably scavenged the mastodon’s carcass, since its bones contain no stone tool incisions produced when an animal is butchered, they add.
Researchers already disagree about whether humans reached the Americas more than 20,000 years ago, so it’s unsurprising that the new report is controversial. Critics quickly questioned its findings.
Excavation of the mastodon site occurred in 1992 and 1993 following its partial exposure during a construction project. Backhoes and other heavy construction equipment can cause the same damage to mastodon bones that the new report attributes to an ancient Homo species, says archaeologist Gary Haynes of the University of Nevada, Reno.
The ancient southern California landscape also may have included streams that could have washed broken mastodon bones and large stones from separate areas to the spot where they were eventually unearthed, says archaeologist Vance Holliday of the University of Arizona in Tucson. Perhaps hominids used these stones to break bones, but the new study doesn’t rule out other possibilities, such as trampling by animals at locations where the bones may have originated, he says. “Making a case for [hominids] on this side of the Pacific Ocean at 130,000 years ago is a very heavy lift, and this site doesn’t make it.”
Nothing that clearly qualifies as a stone tool has been found at the Cerutti Mastodon site, says archaeologist Michael Waters of Texas A&M University in College Station. Mounting genetic evidence indicates that the first people to reach the Americas and give rise to present-day Native Americans arrived no earlier than about 25,000 years ago, Waters adds.
But study coauthor Richard Fullagar of the University of Wollongong Australia argues that “the evidence is incontrovertible.” Measurements of natural uranium and its decay products in mastodon bone fragments enabled scientists to estimate their age.
A sediment layer at the San Diego site contained pieces of a mastodon’s limb bones, molar teeth and tusks bearing marks consistent with repeated battering by large stones, the team says. Ends of some bones were broken off, suggesting marrow had been removed.
Mastodon bones lay in two clusters. One set of bones was associated with two large stones. The other bone cluster was spread around three large stones. These lumps of rock range from 10 to 30 centimeters in diameter.
Holen’s team used comparable stones lashed to branches to break elephant bones resting on large rocks. Damage to experimental stones employed as hammers resembled that on three stones at the mastodon site. The researchers conclude that those stones were used to bash mastodon bones. Rocks used as anvils in the experiments incurred damage similar to that observed on the other two excavated stones.
Construction machinery produces distinctive damage to large bones that does not appear on mastodon remains at the California site, Holen says. Excavations of the bones and stones reached about three meters below the area originally exposed by heavy equipment.
Analyses of sediment at the mastodon site indicate that streams did not wash in bones and stones from elsewhere, the scientists hold. It’s also unlikely that trampling or gnawing by animals or the fossilization process created the types of bone damage observed, they say.