When we search for the seat of humanity, are we looking at the wrong part of the brain? Most neuroscientists assume that the neocortex, the brain鈥檚 distinctive folded outer layer, is the thing that makes us uniquely human. But a new study suggests that another part of the brain, the cerebellum, grew much faster in our ape ancestors.
鈥淐ontrary to traditional wisdom, in the human lineage the cerebellum was the part of the brain that accelerated its expansion most rapidly, rather than the neocortex,鈥� says of Durham University in the UK.
With of the University of Reading in the UK, Barton examined how the relative sizes of different parts of the brain changed as primates evolved.
Advertisement
During the evolution of monkeys, the neocortex and cerebellum grew in tandem, a change in one being swiftly followed by a change in the other. But starting with the first apes around 25 million years ago through to chimpanzees and humans, the cerebellum grew much faster.
Learning to swing
As a result, the cerebellums of apes and humans contain far more neurons than the cerebellum of a monkey, even if that monkey were scaled up to the size of an ape. 鈥淭he difference in ape cerebellar volume, relative to a scaled monkey brain, is equal to 16 billion extra neurons,鈥� says Barton. 鈥淭hat鈥檚 the number of neurons in the entire human neocortex.鈥�
鈥淭hat鈥檚 not to say the neocortex is boring,鈥� says Barton. But such rapid growth in the cerebellum must have happened for a reason. Since the cerebellum is heavily involved in the control of muscles, particularly in coordination, he suggests the trigger may have been the first apes learning to swing from branch to branch, as modern gibbons do.
The extra coordination skills could then have unleashed other 鈥渢echnical鈥� skills like making tools and fine finger movements. Some researchers, like of the University of St Andrews in the UK, think that such technical intelligence is a defining feature of apes.
Barton鈥檚 data is solid, but he overstates the importance of the cerebellum, says of the University of Manchester, UK. 鈥淭he cerebellum is important in coordination and synthesis,鈥� she says. 鈥淏ut I don鈥檛 think it takes away from the fundamental importance of the neocortex.鈥�
Person skills
Shultz points out that over a century of neuroscience has demonstrated the importance of the neocortex for distinctively human traits like social skills and the ability to plan many years ahead. The famous case of Phineas Gage, a 19th-century railway worker who had a rod driven through the front of his brain in an accident, illustrates this. Before his injury he was clean-living and meticulous, but afterwards he became unable to control his impulses. 鈥淎ll you have to do is knock out the prefrontal area and you start to see all these problems in 鈥榟uman鈥� areas,鈥� says Shultz.
Since the cerebellum pulls together disparate sources of information from all over the brain and uses it to control motor functions such as hand gestures and walking, it may simply have had to grow once the rest of the ape brain ballooned. 鈥淎s your brain gets increasingly large, it becomes increasingly important to synthesise and coordinate all the information you鈥檙e holding,鈥� says Schultz.
Where Shultz and Barton agree is that the neocortex and cerebellum are densely interlinked, so we might mislead ourselves by focusing on one to the exclusion of the other. 鈥淲e should think about integration in the brain as a whole,鈥� says Shultz.
鈥淭he broader part of the story is the way the cortex and cerebellum work together,鈥� says Barton. 鈥淚t鈥檚 hard to damage one without affecting the other.鈥�
So although the rise of the neocortex is probably still the source of our mental prowess, it may be that it would never have worked without the cerebellum outpacing it.
Journal reference:
