There is no question about it, we have big heads. Relative to our body size we have the largest brains on earth, and even non-relative to body size, our brains are still some of the largest.
If we consider that we are the well-honed product of mother nature’s machinations then it follows logically that large head and brain size has been selected for; and that big-headed people were more likely to have and raise children than smaller headed ones. This was in spite of the extra risk in childbirth, giving birth to a larger (albeit squashable) skull.
So why are big headed people more likely to have children? To source an answer to this question, evolutionary biologists can go back and consider what our ancestors used their big heads for. Was it literally a case of big headedness = ego (“I am”); self-awareness, cognition, intelligence, or is there more than meets the I (eye).
The issue is that almost all traits we consider human are found to a greater or lesser extent in most apes. Furthermore, some of the largest portions of our brain are given over to visual perception; it is hard to see why human ancestors should suddenly have needed better acuity than their cousins. Other large regions in the human brain are devoted to memory, manual dexterity, hearing, face recognition and self-awareness. At one point in time the great gulf between humans and animals was considered that humans had learning; animals had instincts. Humans had culture, animals had natural behaviours. However this was a falsifiable theory; snails can learn, dolphins have language, orang-utans recognise themselves in the mirror, dogs are conscious, finches use tools, macaques and orca whales pass down cultural behaviours (hunting) and elephants will mourn their dead.
On the flipside, humans are also excessively instinctive creatures. The psychologist William James argued that we have more instincts than any other species, and profoundly, that these instincts formed the root of many of our so called higher traits. Language is a striking example.
We would argue that language is a learned trait. We cannot, after all, wake up and be fluent in Swahili (though there are cases of people waking up with a foreign accent). However most linguists now agree that while vocabulary, inflections and syntax are specific to their native language and must be acquired – a child reared in isolation will not grow up to speak Hebrew – all languages share a common deep grammatical structure, such as inflections indicating whether things are subjects or objects, and it is thought that this deep structure is indeed instinctive. A child, when taught the word ‘cup’, knows instinctively that it means a cup; not its contents, nor its handle, nor that specific cup – they can apply it to any cup. We also have an instinctive need to acquire vocabulary. Children will develop language and divine its grammar given little encouragement or instruction; something beyond the reach of our best computers. They can constantly generalise rules of spoken language and retain a remarkable plasticity for doing so between the ages of 18 months and puberty, until learning grammar stops being intuitive and we have to get out our verb tables and grammar books and apply discipline to get anywhere.
When we see a Christmas tree the brain is applying a complex set of innate, unteachable mathematical filters but we learn to recognise that a fir tree with lights on it is a Christmas tree; in the same way we can innately put edges and corners and contrast together to form recognisable but untaught shapes, we know that the name for a tree with lights on it is likely to behave like any other noun , and not like a verb, without having to think about it.
While learning is a fantastic survival tool that nature has propelled into the arsenal of the most dominant species on earth, it is in some ways silly to consider that it is a superior trait. Instincts are something we consider prehistoric and crude, but actually, if we acquired everything by learning, we would have to learn the world afresh each time. We could take on our forebear’s wisdom through their culture and training, but why learn to fear snakes culturally when it can be hardwired? A 3 year old, untaught in the ways of the world, will then leave that viper alone. Considering premature death is one of the preeminent natural selectors, then if learning is so beneficial, why have we not taught ourselves to be less afraid of snakes than we are of cars? There is a reason we are instinctive. But instincts do not require a bigger brain.
So again, what is all that space for?
One theory was that greater manual dexterity expanded with toolmaking with consequent changes in the motor cortex and cognitive/creativity centres of the brain. Since the large explosion in brain size occurred prior to and not alongside the progression of tools, this theory holds little weight.
Another theory was the hunter-gatherer lifestyle. To hunt one must know when and where and how to catch prey, how to avoid injury, planning how to eat, store and share food; knowing how to gather meant learning where seasonal crops of berries grew, which were poisonous, how to dig up tubers and access nuts. Relearning all this by trial and error in each generation would be laborious, so expansive memories and language were required to teach offspring. However this is not unique to humans. Any societies with overlapping generations will teach such habits by imitation; baboons dig up tubers, lions coordinate to bring down prey, the wolverine stores uneaten food under the ice and macaques teach how to crack nuts.
One of the most intriguing theories is that we are intelligent because other humans are intelligent. Like any arms race, you must constantly upgrade weaponry to outcompete and opponent. Bloom held that “Interacting with an opponent of approximately equal intelligence, whose motives at times are downright malevolent, creates demanding and ever-escalating pressures on the evolution of cognition”. To be a successful human ancestor one needed to be able to predict others’ feelings, behaviour and responses; for this they needed to understand or guess motives, and for this they were required to understand their own motives. This self-knowledge, analysis and prediction pushed for greater and greater cognition. This ‘theory of mind’ is considered one of the greatest reasons for our enlarged heads, and intriguingly is one often reported lessened in some social disorders like varieties of autism, though these people are arguably better at astonishing feats of memory and learning. Theory of mind encompasses the capacity to read others’ minds with the capacity to express our own. Nothing is more fascinating to us than gossip; discussion on the behaviours, feelings and frailties of others, present or absent, and is one reason theatres, soaps, plays and tabloids have dominated entertainment in human civilisations. We are obsessed with each other’s minds – our intuitive psychology far surpasses any achievements of scientific psychology.
Another theory, building on this, is that intelligence is a tool to deceive and therefore dominate. Calculated deception is common in mankind, occasional in chimpanzees, rare in baboons and almost unknown in other animals. Thus detecting and deflecting deception has driven our intelligence – it requires the ability to construct alternative possible worlds with a gallery of outcomes depending on the shifting variables of other uncontrollable individual minds.
There will never be a single factor driving evolution, but it is quite feasible than an obsession with our minds (still underway!) has driven the creation of our minds, and therefore our bigger brains.