If confirmed, the findings would shed new light on what might have caused the extinction of giant Ice Age herbivores, a question that is assuming greater importance as humans confront rising extinction rates among many of the animals with whom we share the planet. Some experts have argued that early humans hunted mammoths and mastodons to the brink of extinction. Others have cited environmental changes, a strike by a comet or comet fragment, or some combination of factors as the most likely reasons they vanished from the landscape.

“In Africa, [large mammals] are quickly being eradicated from most areas by poaching and habitat loss,” Moore said. “These extinctions will have profound and permanent effects on the ecosystem in Africa by removing the large herbivores. The same ecosystem transformation happened in North America 13,000 years ago.”

Other experts disputed the report, saying it is unlikely blood residue could have survived thousands of years without being contaminated.

“A lot of researchers, including myself, doubt the validity of some of the blood residue studies they do,” said Joseph Gingerich, associate professor of anthropology at Ohio University, who has been researching Clovis people for 20 years. He commended the study authors for examining a large sample size of stones but said the analysis technique they used has produced flawed results before.

Clovis people, once thought to have been the first humans in North America, were named for the New Mexican city close to where lance-shaped flint spearheads were found in the 1930s. More recently, the growing consensus among researchers is that there were “pre-Clovis people” in North America, Moore said. Even so, Clovis people appear to have left behind a strong genetic imprint.

A 2014 study mapped the genetic blueprint of a Clovis male skeleton and concluded that “some 80 percent of all present-day Native American populations on the two American continents are direct descendants of the Clovis boy’s family.”

In what is now the western United States, archaeologists have found evidence that Clovis people hunted mastodons and mammoths, but Moore said that until now, archaeologists knew “almost nothing” about the diet of Clovis people in the eastern part of the country.

“That’s our big problem in terms of understanding the Clovis people in South Carolina, North Carolina and surrounding states,” he said. “Did they hunt? Were the [large mammals] here? Were they hunting them?”

To find that evidence, Moore traveled across South Carolina collecting Clovis-era points from museums, private collections and military bases. He arranged to have all of the artifacts tested at a lab in Portland, Ore., then packed them in a protective case. At the airport, staff from the Transportation Security Administration inspected the prehistoric weapons before allowing Moore to board his plane.

The artifacts were washed in an ultrasonic bath with a weak ammonia solution to remove the protein residues. Lab workers then tested the residues using a technique called crossover immunoelectrophoresis, which was once used to identify blood or semen from crime scenes. The method takes advantage of the way the immune system responds to foreign substances, or antigens, by confronting them with proteins called antibodies.

The lab tested the residues against antibodies taken from various families of animals. Antibodies only respond if they come from the same animal family as the blood residue.

Researchers got five matches for Proboscidea, an order of large mammals that includes modern elephants as well as extinct mastodons, mammoths and gomphotheres, which were elephant-like animals with special teeth for grazing. Residues from other stones matched animals in the horse family and the family that includes ruminant mammals, such as bison.

While the horses 13,000 years ago were smaller than those today (roughly the size of zebras), Moore said bison were about twice as large as the ones now seen.

The residue-testing method used by Moore and his colleagues “is an older technique that isn’t used much in current [criminal] casework,” said Cynthia Zeller, an associate professor of chemistry in Towson University’s Human Remains Identification Laboratory. “The last time I used it was at least 20 years ago.”

Zeller, who worked in the Maryland State Police Forensic Sciences Division about 18 years ago, said the technique was reliable, but takes about half a day to yield results and has been replaced by faster methods.

Some researchers have expressed doubt that residues found on stone knives or spearheads could have survived thousands of years without being washed away by rain or contaminated by urine or feces from animals. However, Moore said the protein residues accumulate in micro-cracks in the stone and become sealed over and protected by clay and other sediment.

For her part, Zeller found the residue tests convincing because they revealed proteins from elephant-like mammals, “and elephants haven’t been roaming around the U.S. in quite some time.”

Gingerich said he believes the residue tests will be replaced by more precise methods.

“I think the new frontier in this — and it’s being used more commonly now — is DNA-based analysis and amino acid studies, which can identify specific proteins that are unique to a species,” he said.

Studying DNA from the blood residue would allow researchers to determine not just whether the residue came from an animal in the same family as elephants, but whether that animal was a mammoth, mastodon or gomphothere.