Big hands might mean big feet, but it seems long thumbs are linked to large brains – at least in primates.
Researchers say the results suggest the brain co-evolved with manual dexterity in such mammals.
“We imagine an evolutionary scenario in which a primate or human has become more intelligent, and with that comes the ability to think about action planning, think about what you are doing with your hands, and realise that actually you are more efficient at doing it one way or another,” said Dr Joanna Baker, lead author of the research from the University of Reading.
“And those that have longer thumbs or more ability to manipulate the objects in the way that the mind can see were likely to be more successful.”
Large brains and manual dexterity are both thought to have played an important role in human evolution, with opposable thumbs a key feature that enabled a greater ability to grip and manipulate items – including tools.
However, with some other primates having partly opposable thumbs, questions have remained over whether other changes in the hand – such as thumb length – could also be important in the evolution of tool use.
“In general terms, you can say that the longer the thumb you have, the more motion you have to pick up and control small objects,” said Baker.
To explore the issue Baker and colleagues studied the estimated brain mass and thumb length of 94 primate species, from five of our ancient hominin relatives to lemurs.
The results, published in the journal Communications Biology, reveal humans and most other hominins have thumbs that are significantly longer than would be predicted based on the hand proportions of primates as a while. However, further analysis revealed an intriguing pattern.
“When you have longer thumbs relative to your overall hand, that tends to come in conjunction with overall increased brain size,” said Baker.
Indeed, once the size of our brains are considered, humans and their close relatives are no longer outliers among primates.
“We’re not saying we don’t have exceptionally long thumbs. We do. And we’re not saying we don’t have exceptionally large brains. We do. But given the relationship between the two, that’s happening across all primates,” said Baker. “So if you have a longer thumb, you have a larger brain, regardless of what species you are.”
Only the early hominin A sediba broke the trend, with a thumb that was longer than expected, even after accounting for brain size – something Baker said is probably related to a life lived in the trees and on the ground.
Further analysis revealed that it is the neocortex, a brain region involved in cognition, sensation and planning of actions, that is larger in primates with longer thumbs.
“The fact that it isn’t one of the other very important parts of the brain associated with motor control [such as the cerebellum] was really surprising,” said Baker.
However, the study does not support the idea that thumb lengths alone can be used to identify tool use, with the relationship to brain size constant across all primates, regardless of whether they used tools.
“Whilst not completely surprising, we did expect that there might be some marked shift in hominins that we just didn’t see,” said Baker.
Dr Fotios Alexandros Karakostis, a group leader and senior researcher in biological anthropology at the University of Tübingen who was not involved in the work, said the study suggested that hand and brain adaptations probably co-evolved.
But Karakostis said the study also noted that thumb length and brain size alone could not fully explain or represent human-like manual dexterity or the evolution of our brains.
“A fuller understanding will require integrating other key hand anatomical traits, biomechanical model simulations, and further experimental research on the specific neural mechanisms linked to manual dexterity and human-like tool use,” he said.