Humans were hunting large mammals in North America about 800 years earlier than previously thought, new analysis of a controversial mastodon specimen – with what appears to be a spear tip in its rib – seems to confirm.

The find suggests humans were hunting mastodons using tools made from bone about a thousand years before the start of the "Clovis culture", reputedly the first human culture in North America. Other evidence points to mammoth hunting using stone tools around this time, but the notion of pre-Clovis hunting has remained highly controversial.

The mastodon was found in 1977 by a farmer called Emanuel Manis. He contacted archaeologist Carl Gustafson, who excavated the skeleton and noticed a pointed object embedded in its rib. Gustafson took a fuzzy x-ray and interpreted the object as a projectile point made of bone or antler.

By dating organic matter around the fossil, he estimated that it was about 14,000 years old. Other archaeologists challenged Gustafson's dates and his interpretation of the fragment as a man-made point.

Decades later Professor Michael Waters from Texas A&M University contacted him about re-examining the specimen using modern technology. His analysis was published on Thursday in the journal Science.

Waters placed the mastodon in an industrial-grade CT scanner at the University of Texas. "It's more powerful than a hospital one. They're taking slices every 0.06mm, half the thickness of a piece of paper," he says. "The 3D rendering clearly showed that the object was sharpened to a tip. It was clearly the end of a bone projectile point."

Waters analysed collagen protein from the mastodon's rib and tusks to confirm that the animal died about 13,800 years ago, almost exactly as Gustafson predicted.

He also extracted DNA from the rib and the spear point. Analysis showed that both belonged to a mastodon, suggesting the animal had been killed with a weapon fashioned from the remains of its own kind.

"It should certainly be considered as evidence of hunting. You're bending over backwards to explain it as something else," said Prof Daniel Fisher from the University of Michigan.

Two other sites in Wisconsin appear to show people were hunting woolly mammoths and using stone tools between 14,200 and 14,800 years ago. The Manis specimen suggests they also hunted mastodons and used bone tools.

Together, the three sites provide strong evidence for pre-Clovis hunting. "They're incontrovertible," said Waters. "Clearly, people were hunting mammoths and mastodons again and again, playing a part in their ultimate demise."

He believes the beasts probably succumbed to gradual hunting pressure from humans, rather than a quick "Clovis blitzkrieg".

Despite Waters's efforts, the fragment in the Manis mastodon's rib is still stoking debate. "It's not definitely proven that it is a projectile point," said Prof Gary Haynes from the University of Nevada, Reno. "Elephants today push each other all the time and break each other's ribs so it could be a bone splinter that the animal just rolled on."

Waters does not credit this alternative hypothesis. "Ludicrous what-if stories are being made up to explain something people don't want to believe," he said. "We took the specimen to a bone pathologist, showed him the CT scans, and asked if there was any way it could be an internal injury. He said absolutely not."

Waters added: "If you break a bone, a splinter isn't going to magically rotate its way through a muscle and inject itself into your rib bone. Something needed to come at this thing with a lot of force to get it into the rib."

The spear thrower must have had a powerful arm, because the fragment would have punctured hair, skin and up to 30cm of mastodon muscle. "A bone projectile point is a really lethal weapon," said Waters. "It's sharpened to a needle point and little greater than the diameter of a pencil. It's like a bullet. It's designed to get deep into the elephant and hit a vital organ … I've seen these thrown through old cars."

What would it take to finally convince the sceptics? "I'm not going to dig my heels in and stonewall it," said Haynes. "If they yanked it out and showed that there are scraping marks and it's been polished and shaped, that would convince me."

Waters said he would not tamper with such an extraordinary specimen. "That would destroy it. I would be a pariah. It's unnecessary with modern technology. I could take 3D printers and recreate the object itself."