Primates with longer thumbs tend to have bigger brains, research finds | Evolution

Large hands can mean large feet, but it seems that long inches are linked to large brains – at least in primates.

The researchers say that the results suggest that the brain has been co-evolved with manual dexterity in these mammals.

“We imagine an evolutionary scenario in which a primate or a human has become more intelligent, and with it comes the ability to think about planning the action, to think about what you do with your hands and to realize that in fact, you are more effective in doing it in one way or another,” said Dr. Joanna Baker, principal author of the Reading University.

“And those who have longer thumbs or more ability to manipulate objects like the mind can probably see more success.”

It is believed that large brains and manual dexterity both played an important role in human evolution, with opposable thumbs up a key characteristic which has enabled greater capacity to grasp and manipulate elements – including tools.

However, with some other primates with partially opposable thumbs, the questions have remained on the question of whether other changes in hand – such as the length of the thumb – could also be important in the evolution of the use of tools.

“In general, you can say that the more the thumb you have, the more you have a movement to pick up and control small objects,” said Baker.

To explore the problem, Baker and his colleagues studied the estimated brain mass and the length of the thumb of 94 species of primates, five of our former hominine parents to lemurs.

The results, published in the journal Communications Biology, reveal that humans and most other hominins have thumbs that are significantly longer than what would be planned on the basis of the proportions of the primates as a time. However, a more in -depth analysis revealed an intriguing model.

“When you have longer thumbs compared to your overall hand, it tends to come in conjunction with a global increase in brain size,” said Baker.

Indeed, once the size of our brain is considered, humans and their close relatives are no longer aberrant values ​​among primates.

“We do not say that we do not have exceptionally long inches. We do it. And we do not say that we do not have an exceptionally important brain. We do it. But given the relationship between the two, this happens to all primates,” said Baker. “So if you have a longer thumb, you have a larger brain, whatever the species you are.”

Only the first hominine A sediba Broken the trend, with an inch that was longer than expected, even after taking into account the size of the brain – something Baker said is probably linked to a life lived in trees and on the ground.

A more in -depth analysis revealed that it is the neocortex, a brain region involved in cognition, sensation and planning of actions, which is greater in primates with longer thumbs.

“The fact that this is not one of the other very important parts of the brain associated with the engine control [such as the cerebellum] was really surprising, “said Baker.

However, the study does not have the idea that the length of the thumb can be used alone to identify the use of tools, with the relationship to the size of the constant brain of all primates, that they have used tools.

“Although it is not completely surprising, we expected that there was a marked change in hominins that we simply have not seen,” said Baker.

Dr. Fotios Alexandros Karakostis, group leader and principal researcher in biological anthropology at the University of Tübingen, who was not involved in work, said that the study suggested that manual and brain adaptations probably co-evolved.

But Karakostis said that the study also noted that the length of the thumb and the brain size could not explain or represent a manual dexterity of the human type or the evolution of our brain.

“A more complete understanding will need to integrate other key anatomical features, simulations of biomechanical models and other experimental research on specific neural mechanisms linked to manual dexterity and the use of the human type tool,” he said.