Human evolution: handedness origin (Introduction)

by David Turell , Saturday, April 06, 2024, 19:23 (229 days ago) @ David Turell

All theory:

https://www.newscientist.com/article/2425718-left-handed-monkeys-prompt-rethink-about-e...

"Monkeys that adopted an urban lifestyle in India are mostly left-handed – in contrast to humans and many other primates that live on the ground.

"The findings clash with long-standing claims that primates that come down from the trees generally evolve a tendency to be right-handed, raising questions about what really drives this trait.

"The Hanuman langurs (Semnopithecus entellus) – named in honour of Lord Hanuman of Indian mythology – gave up their natural homes in the trees at least 165 years ago and now live off food offerings from people who revere them as gods.

“'Hanumans have a high amount of deity value among the Indians, irrespective of religion,” says Akash Dutta at the University of Calcutta in India. Only 9 per cent of the animals’ natural habitat remains following massive expansion of human settlements, he adds.

"Realising that the monkeys’ behaviours and actions have been evolving over time, Dutta started wondering about the animals’ laterality, or handedness. In particular, he questioned how they might fit into what is known as the postural origin theory.

"This popular, but often debated, concept suggests that the earliest branches of primates lived in trees and were left-handed, using their left hand to grab food or branches while holding onto the tree with their right. Those that later adapted to life on the ground – like humans, 90 per cent of whom are right-handed – switched to having a predominantly right-sided laterality as they could now use their free right hand to handle tools and other objects. This then stimulated the motor skills region in the left side of the brain, which controls the right side of the body – in turn promoting more use of the right hand.

"Studies show that chimpanzees, gorillas, baboons, bonobos and ring-tailed lemurs are mostly right-handed, for example, whereas Sichuan snub-nosed monkeys, deBrazza’s monkeys and orangutans – all of which live in trees – are mostly left-handed.

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"The researchers handed each monkey a sweet bun trapped inside an open, transparent bottle. Only 27 per cent of the monkeys showed right-handedness in getting the bun out of the bottle. Fifty-three per cent were left-handed, and the rest were ambidextrous or tried using their mouths to extract the bun.

"The results came as a surprise to Dutta. He thinks the explanation may be that the monkeys’ DNA still programs for left-handedness, even after dozens of generations in their new environment. “The ancestral tendency is still there,” he says. He predicts that the urban langurs will shift to right-handedness over time.

"However, Kai Caspar at Heinrich Heine University Düsseldorf in Germany believes the study provides further evidence that the tree-related idea for the evolution of handedness is wrong. “I frankly don’t understand why it still maintains its high status in the research community,” he says.

"His team has already strongly questioned the validity of the postural origins theory – as well as other ideas proposing that the dominance of right-handedness in humans is related to walking erect or developing language. For Caspar, the explanation for handedness has yet to be uncovered."

Comment: I saw an article recently that said there is evidence that handedness is about 25% inherited. That is the first time I've ever seen such a reference. I have special interest in the subject as I am strongly left-handed. One grandfather was said to have had some lefty characteristics.