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Of all the long-running disputes in archaeology, few roil scholars more than the question of when humans arrived in the Americas. For much of the past century, the reigning theory was that in or around 11,500 years ago big-game hunters from Asia trudged to North America across a land bridge spanning the Bering Strait, hung a right through a corridor between glaciers and, in less than a millennium, reached the tip of South America.

Over the past three decades, however, archaeological research has made it increasingly clear that the hunters were preceded by much earlier cultures that colonized the Americas between 24,500 and 16,000 years ago.

This week a new academic study upended even those migration timelines by proposing that what is now central-west Brazil was settled as early as 27,000 years ago, a finding that bolsters the theory that our ancestors inhabited the continent during the Pleistocene Epoch, which ended around 11,700 years ago. The period is also called the Ice Age because of its numerous cycles of glacial formation and melting.

The conclusions of the paper, published in the journal Proceedings of the Royal Society B, are based on an analysis of an improbable source: three bones from an extinct giant ground sloth. Excavated 28 years ago in the Santa Elina rock shelter, the fossils — similar to the hard, scaly plates, called osteoderms, that armor the skin of present-day armadillos — showed signs of having been modified into primordial pendants, with notches and holes that researchers said could only have been created by people.

“This is a really significant study because it adds to a growing body of data on the antiquity of human occupation in the Americas,” said April Nowell, a Paleolithic archaeologist at the University of Victoria who was not involved in the project. “It also shows the importance of personal ornaments.”

The giant ground sloth first appeared in South America 35 million years ago. Some species were as hefty as modern elephants and, rearing up on their hind legs, stood more than 10 feet tall. The hulking herbivore, a distant relative of today’s much smaller tree sloth, had massive jaws and powerful clawed limbs, and it may have served as inspiration for the mapinguari, a mythical beast that, in Amazonian legend, had the nasty habit of twisting off the heads of humans and devouring them. The giant sloth disappeared from the continent some 11,000 years ago, but fossil remains abound.

Three dating methods, applied to three layers of sediment, osteoderms and charcoal particles at Santa Elina, indicated that humans first left a mark on the oldest and deepest layer around 27,000 to 23,000 years ago. Since then, people have occupied the shelter at different times: from 17,000 to 13,000 years ago in the middle layer and after 6,000 years ago in the top layer, researchers say. “The big question is, were those artifacts made by humans during their coexistence with the sloths?” said Mirian Liza Alves Forancelli Pacheco, an author of the study and an archaeologist at the Federal University of São Carlos in Brazil.

Shaped like triangles and teardrops, the three peculiar sloth bones found in the deepest layer appeared to have been smoothed and perforated. “Complete or partial holes were clearly drilled near the edges, as if they had been designed to be threaded on a string,” Dr. Pacheco said.

Microscopic markings suggested that the osteoderms, and even their holes, had been buffed by human hands. Neither natural abrasion nor animal bites could explain their texture and shape, said Thais Rabito Pansani, a paleontologist at the Federal University of São Carlos and first author of the paper. Further analysis revealed scratches going in different directions and stone-tool gouges made a few days to a few years after the sloths had died, but before the bones had fossilized.

“In our view, the early humans who lived in the shelter fashioned the bones into personal ornaments, possibly pendants, that over time became worn from heavy use,” Dr. Pansani said. This would make them the oldest known jewelry unearthed in the Americas and the only trinkets in the archaeological record known to have been made from giant sloth bone.

“The authors show very compelling evidence for an anthropogenic modification of the sloth bones,” said Mercedes Okumura, an archaeologist at the University of São Paulo. “Such a study can help to shed light on the use of adornments by the early Americans, as well as about the interaction between past humans and megafauna in the Americas.”

For thousands of years, Dr. Nowell noted, the human body has been a site for the creation and expression of individual and group identity, and relics like giant sloth baubles play a vital role in that process. “I love the fact that these beads are heavily worn from being strung or from rubbing against the skin, cloth or other beads,” she said. “That speaks to the value of these objects; it suggests they were worn for a long time.”

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