Human evolution rewritten after strange monkey tooth discovery: ‘We need to reassess!’

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Many ancient hominin specimens feature unusual patterns of wear on their teeth — specifically in the form of large scratches on their front teeth and grooves on their back teeth. These have long been taken by anthropologists as evidence for early tool use, with the scratches thought to have been accidentally made during a behaviour called “stuff and cut”, in which items like animal hides would have been gripped between the teeth and sliced with a stone tool. The damage at the rear of the mouth, meanwhile — so-called toothpick grooves — were, as the name suggests, thought to have been caused by the insertion of tools between the back teeth to either remove trapped food debris or to relieve pain.

These long-held assumptions are being challenged, however, by dental researcher Dr Ian Towle of the University of Otago and his colleagues.

The team have been studying the wear and tear on the teeth of a group of wild Japanese macaques living on Koshima Island, off the coast of Kyushu, southwest Japan.

They found both toothpick grooves and large scratches on the front teeth of the monkeys — but these marks certainly were not caused by tool usage.

Instead, the team reports, the macaques appear to have damaged their teeth by accidentally chewing on grit and sand with their food, as well as by eating tasty shellfish.

Dr Towle, who has been studying tooth wear and other pathologies across a wide variety of primate species, said that he was “extremely surprised” when he found these unusual marks in a group of wild monkeys.

The macaques, which have a population of around 100 individuals living on the island, are well-known for their remarkable behaviours.

These include catching and eating fish and washing their food in water before they eat it.

However, at no point in the more than 70 years that researchers have been observing their behaviour have they been seen using any tools that could create either of the wear patterns.

Dr Towle said: “Unusual wear on our fossil ancestors’ teeth is thought to be unique to humans and demonstrates specific types of tool use.

“These types of wear have also been considered some of the earliest evidence of cultural habits for our ancestors.

“However, our research suggests this idea may need reconsidering, since we describe identical tooth wear in a group of wild monkeys that do not use tools.

“Although this does not mean hominins were not placing tools in their mouths, our study suggests the accidental ingestion of grit and/or normal food processing behaviours could also be responsible for these atypical wear patterns.”

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Dr Towle added: “This research raises questions for our understanding of cultural changes during human evolution and suggests we may need to reassess early evidence of cultural habits.

“We are so used to trying to prove that humans are unique, that similarities with other primates are often neglected.

“Studying living primates today may offer crucial clues that have been overlooked in the past.”

The full findings of the study were published in the American Journal of Biological Anthropology.

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