Scientists find surprising link between thumb length and brain size

Next time you’re trying to size someone up, take a look at their thumbs. 

If they have particularly long thumbs, they might be especially clever, a new study suggests. 

Scientists at the universities of Reading and Durham have found a link between brain size and thumb size across 95 primate species, including humans. 

Across all species, they found that the longer the thumb, the bigger the brain. 

Thumbs and brains likely grew in tandem as primates evolved over millions of years, they claim.

So, as primates developed better manual skills for handling objects, their brains had to grow to process and use these new abilities effectively. 

‘As our ancestors got better at picking up and manipulating objects, their brains had to grow to handle these new skills,’ said lead author Dr Joanna Baker. 

‘These abilities have been fine-tuned through millions of years of brain evolution.’ 

The researchers conducted an analysis of 95 primate species, including modern and extinct humans, as well as other modern and extinct primates. 

Modern-day primates that were part of the study included chimps, gorillas, monkeys, lemurs, marmosets, baboons and orangutans. 

‘We found the link across all species,’ Dr Baker told the Daily Mail. 

‘That is to say that the relationship is consistent across all species including all the most well-known primates, including our own species.’ 

As expected, thumbs of hominins – the group consisting of modern humans, extinct human species and all our immediate ancestors – were found to be significantly longer than those of other primates. 

Human brains are much larger than non-human primate brains. 

The scientists also made a surprising discovery about which part of the brain grows alongside longer thumbs in primates. 

They expected longer thumbs to be linked to the cerebellum, the portion of the brain in the back of the head heavily involved with movement and coordination. 

But actually, longer thumbs were connected to the neocortex, a complex layered region comprising approximately half the volume of the human brain, also involved in movement and coordination. 

The neocortex processes sensory information and handles cognition and consciousness, but further research is needed to find out how exactly it supports manipulative abilities, the team say. 

The research, published in Communications Biology, provides the first direct evidence that manual dexterity and brain evolution are connected across the entire primate lineage, from lemurs to humans. 

However, the team stress that longer thumbs does not fully capture the ‘complexity of primate variation in dextrous behaviour and ability’. 

‘Primate dexterity is clearly facilitated by more than just thumb length alone; thumb length can provide us with some general insights,’ they add, 

Also, brain size doesn’t necessarily make people more intelligent, and research has suggested that there is only a slight relationship between the two. 

‘It is not quite so simple as so say that big brain equals more intelligence,’ Dr Baker told the Daily Mail. 

‘Rather, a larger brain can involve increases in different brain regions responsible for various elements of behaviour, cognition, motor control, etc.’  

Interestingly, dexterity comes at a cost – even for species with large brains like humans. 

Although humans have the biggest brains of the primates and are very skilled with their hands, it takes us a long time to master even the simplest hand and finger movements, when compared to other primates. 

It takes human babies generally around five months before they can purposely grip an object, while learning more complicated skills, such as eating with fork and knife or tying shoelaces, can take another five to six years.

By that age, many other primate species are at the peak of their development and have even produced offspring of their own.