Evolution of the human brain – YourGenome

Evolution of the human brain
Evolution of the human brain
Evolution of the human brain
The human brain, in all its staggering complexity, is the product of millions of years of evolution.
Evolution of the human brain
The brain has undergone some remarkable changes through its evolution. The most primitive brains are little more than clusters of cells bunched together at the front of an organism. These cells process information received from sense organs also located at the head.
Humans have the largest brain in proportion to their body size of any living creatures.
Over time, brains have evolved. The brains of vertebrate animals have developed in both size and sophistication. Humans have the largest brain in proportion to their body size of any living creatures, but also the most complex. Different regions of the brain have become specialised with distinctive structures and functions. For example, the cerebellum is involved in movement and coordination, whereas the cerebral cortex is involved in memory, language and consciousness.
Behaviour can influence the success of a species, so have been shaped by evolution.
By understanding how the human brain evolved, researchers hope to identify the biological basis of the behaviours that set humans apart from other animals. Behaviour can influence the success of a species, so it is reasonable to assume that human behaviours have been shaped by evolution. Understanding the biology of the brain may also shed some light on many conditions linked to human behaviour, such as depression, autism and schizophrenia.
Brain size and intelligence
The human brain is around four times bigger than a chimp brain and around 15 times larger than a mouse brain.
If you were to put a mouse brain, a chimp brain and a human brain next to each other and compare them it might seem obvious why the species have different intellectual abilities. The human brain is around four times bigger than the chimp’s and around 15 times larger than the mouse’s. Even allowing for differences in body size, humans have unusually large brains.
Bigger isn’t always better
But size isn’t the whole story. Studies have shown that there is not a particularly strong relationship between brain size and intelligence in humans. This is further strengthened when we compare the human brain to the Neanderthal brain. Because no Neanderthal brains exist today scientists have to study the inside of fossil skulls to understand the brains that were inside. The Neanderthal brain was just as big as ours, in fact probably bigger.
The skulls of modern humans, while generally larger than those of our earlier ancestors, are also different in shape. This suggests that the modern brain is less of a fixed shape than that of earlier humans and can be influenced over its lifetime by environmental or genetic factors (this is called plasticity).
There are some interesting differences when we compare the pattern of brain growth in humans to chimpanzees, our closest living relatives. Both brains grow steadily in the first few years, but the shape of the human brain changes significantly during the first year of life. During this period, the developing brain will be picking up information from its environment providing an opportunity for the outside world to shape the growing neural circuits.
Prehistoric skulls.
Image credit: Grant Museum, Wellcome Images
An analysis of a Neanderthal child’s skull has shown that their growth patterns were more similar to the chimpanzee than to modern humans. This suggests that although the brains of modern humans and Neanderthals reached a similar size by adulthood, this was achieved through different patterns of growth in different regions of the brain.
A major constraint on human brain size is the pelvic girdle, which (in females) has to contend with the demands of delivering a large-headed baby. Humans have evolved to extend the period when the brain grows to include the period after birth. This subtle difference in early development might have had big implications for our survival.
Language and brain development
Language is probably the key characteristic that distinguishes us from other animals. Thanks to our sophisticated language skills, we can convey information rapidly and efficiently to other members of our species. We can coordinate what we do and plan actions, things that would have provided a great advantage early on in our evolution.
To understand what someone is saying we need to detect their speech and transmit this information to the brain.
Language is complex and we are only just beginning to understand its various components. For example, we have to consider the sensory aspects of language. To understand what someone is saying we need to detect their speech and transmit this information to the brain. The brain then has to process these signals to make sense of them. Parts of our brain have to deal with syntax (how the order of words affects meaning) and semantics (what the words actually mean).
Memory is also very important as we need to remember what words mean. Then there is the entire vocalisation system which is involved in working out what we want to say and making sure we say it clearly by coordinating muscles to make the right noises.
Some birds are talented mimics but you couldn’t have a conversation with a Mynah bird!
Studying language by comparing different species is difficult because no other animals come close to our language abilities. Some birds are talented mimics but you couldn’t have a conversation with a Mynah bird! Even when our closest relatives, chimpanzees, are raised in human families they never gain verbal language skills. Although chimpanzees can learn to understand our language and use ‘graphical’ symbols, they show little inclination to communicate anything other than basic information, such as requests for food. Humans, by contrast, seem to be compulsive communicators.
A master gene for language?
Perhaps the greatest insight into the evolution of language has come from work on the FOXP2 gene. This gene plays a key role in language and vocalization and allows us to explore the changes underpinning the evolution of complex language.
The FOXP2 gene was first discovered by Simon Fisher, Anthony Monaco and colleagues at the University of Oxford in 2001. They came across the gene through their studies of DNA samples from a family with distinctive speech and language difficulties. Around 15 members of the family, across three generations, were able to understand spoken words perfectly, but struggled to string words together in order to form a response. The pattern in which this condition was inherited, suggested that it was a dominant single-gene condition (one copy of the altered gene was enough to disrupt their overall language abilities). The researchers identified the area of the genome likely to contain the affected gene but were unable to identify the specific gene mutation within this region.
They then had a stroke of luck, in the form of another unrelated child with very similar symptoms. Looking at this child’s DNA they identified a chromosome rearrangement that sliced through a gene in the region of DNA where they suspected the mutated gene was. This gene was FOXP2. After sequencing the FOXP2 gene in the family they found a specific mutation in the gene that was shared by all the affected family members. This confirmed the importance of FOXP2 in human language.
Mutations in the FOXP2 gene interfere with the part of the brain responsible for language development.
Simon and his colleagues went on to characterise FOXP2 as a ‘master controller’, regulating the activity of many different genes in several areas of the brain. One key role is in the growth of nerve cells and the connections they make with other nerve cells during learning and development. Mutations in the FOXP2 gene interfere with the part of the brain responsible for language development, leading to the language problems seen in this family.
The evolution of FOXP2
The FOXP2 gene is highly conserved between species. This means that the gene has a very similar DNA sequence in different species, suggesting it has not evolved much over time. The FOXP2 protein in the mouse only differs from the human version by three amino acids. The chimpanzee version only differs from the human version by two amino acids. These two changes in amino acids may be key steps in the evolution of language in humans.
What difference do these small changes in sequence make to the functionality of the FOXP2 protein? Studies with mice show that changing the mouse version of the FOXP2 gene to be the same sequence as the human version only has subtle effects. Remarkably, the resulting mouse pups are essentially normal but show subtle changes in the frequency of their high-pitched vocalisations. They also show distinctive changes to wiring in certain parts of their brain.
From these studies scientists have concluded that FOXP2 is involved in the brain’s ability to learn sequences of movements. In humans this has translated into the complex muscle movements needed to produce the sounds for speech, whereas in other species it may have a different role, coordinating other movements.
FOXP2 regulates many other genes in the body and evolution seems to have favoured a subset of these as well, particularly in Europeans. FOXP2 regulated genes are important not only in brain development, but they also play important roles in human reproduction and immunity.
FOXP2 and the Neanderthals
Neanderthals may have had some capacity for speech and communication.
Neanderthals have generally been characterised as a large, brutish species with little or no intellectual, social or cultural development. However, the fact that they had the same FOXP2 gene as modern humans suggests that Neanderthals may have had some capacity for speech and communication.
Various strands of evidence have helped to establish a picture of how Neanderthals might have lived and communicated. Archaeological records suggest that they probably lived in small groups and due to their high energy needs, spent most of their time hunting.
Neanderthals are unlikely to have developed social groups bound together by effective communication. This is probably because they lacked the key mental abilities needed to establish and maintain social groups. Recursive thinking (thinking about thinking), theory of mind (appreciating what is going on in someone else’s head) and inhibition of impulsive reactions (being able to control impulses) are all important elements to successful social interactions. Interestingly brain injury and developmental disorders, such as autism, can interfere with these abilities and social skills in humans.
This evidence suggests that the Neanderthal brain may not have been wired to support effective communication and diplomatic skills. They would have been extremely difficult to get along with! The Neanderthal brain was probably better adapted to maximise their visual abilities. They would have used their oversized eyes and large brains to survive and hunt in the lower-light levels in Europe. This would limit the space available in the brain to develop the systems needed for communication and social interactions. However, their smaller social brain regions could have enabled them to establish smaller social networks which may have improved their chances of survival in the harsh European environment.