One of the ongoing arguments within the field of Human Evolution is how and why our big brains evolved. Brains are really expensive. Our brains are much larger than they ought to be. There has to be a reason for a brain to both evolve, and be sustained, that is larger than necessary for the basic physical metabolic processes. Big brains need an increased and more efficient diet in order to be maintained. Evolving a large brain must mean that brain size offered an important fitness advantage; otherwise it’s hard to see why it would happen.
There are two main competing sets of hypotheses about why or how the larger brain of primates evolved: through ecological demands or social complexity and competition. The currently favored model is the Social Brain Hypothesis (SBH). SBH is based on increased demands of living in complex social systems. Dunbar argues that the Social Brain Hypothesis is more strongly supported since most of primate brain expansion is in the neocortex which is where social computational processes would be taking place.
Conversely, more ecological processing such as cognitive mapping would take place within the sub-cortical structures. Byrne and Whiten, 1988 first brought up the idea that complex social environments differentiate primates from other mammals in regards to their brain size. There is an increased brain size, usually associated with an increased size in the neo-cortex, within primates. This allows primates the ability to operate effectively in complicated social groups and may allow them greater flexibility and ability to both plan for the future, and strategize how best to utilize the environment.
Sociality helps solve both social and ecological problems that innately arise within groups; bigger brains increase sociality. Understanding and being able to adhere to or manipulate social relationships or behaviors has a significant effect on survival and fitness in complex social groups. Social skills are very important in primates, so they definitely could be important factor. Sociality is very important to humans and primates in general. Our ability to survive in, understand, and manipulate a complex social environment directly affects our ability to reproduce.
Unfortunately, there is the issue of testability. The extremely limited amounts of samples that represent our evolutionary past do not even begin to hint at the amount of variation that occurred both within and between populations and even species. While Dunbar and Shultz (2007) were able to find that there is a positive relationship between social complexity and cognitive complexity, it does not answer why the brains were selected to be larger in the first place.
Simply taking this simple relationship, the implication could mean that a big brain was selected simply to keep track of increasingly complex social interactions as a response to the inherent social and ecological conflicts that occur with group living and the unique situations that are found living within a group (e. g. group cohesion; hierarchy; social roles). Living in large, complex societies selected for bigger brains – perhaps specifically because of the demands of intense pair bonding. This seems to be circular reasoning.
If a group grew too large and complex for smaller brains to socially maneuver, why did the groups not split off, or simply stop developing socially? Why, and how, did these groups develop increasingly complex social roles? Was it simply because they could since their brains had become larger? Dunbar and Shultz (2007) acknowledge that “… in final analysis, all of these hypotheses (social and ecological alike) are at root ecological: they allow animals to survive and reproduce more effectively”.
They still maintain, however, that the ecological model is simply not as effective as the Social Brain Hypothesis when solving the same ecological problem: the SBH would solve it socially, while the ecological hypothesis solves it individually. The authors have a point. Solving the issues of foraging or learning how to scavenge efficiently is accomplished much easier through a group. Knowledge can be learned and then passed on. For all that, there is still a major flaw. The SBH explains what can be done with these big brains, but it still does not quite address why they exist.
The opposing academics put forth that ecological pressures, and not social pressures, were the driving force in brain evolution. With a larger brain, a better diet is necessary, but when was there a change in diet? Did an improved diet allow for brain expansion, or did it arise simply from necessity? Brain size is also linked to ecological challenges, such as the increased demands of hunting meat or selecting ripe fruit. There are some variations of this theory. For example, the idea that ecological problem solving drove brain evolution is a very early hypothesis.
Primates fed on certain plants (e. g. fruit) that require evaluation (ripeness, nutrition, poison, etc) and a good spatial awareness and memory. They had to be able to remember which foods were good or bad, where they were located, and the seasonality. Brain expansion could not have occurred without a shift in diet. Nutrient-rich food that that was high in calories, such as fruit or meat, was imperative in supporting a larger brain. These types of foods required more complex processing and extraction involving more complicated techniques.
One of the original tenants of the ecological hypotheses is that with subsistence diet and long ranges, a significant amount of mental capacity is necessary to form and utilize detailed mental maps; however Shultz and Dunbar (2007) dismiss this. They argue that the ecological approach to the evolution of a bigger brain relegate the larger neo-cortex to a simple by-product of a prolonged development and more efficient metabolism. They argue that ecological problem solving happens in more species than primates; insomuch ecological theory alone does not explain why primates, let alone humans, are a special case in brain size evolution.
Additionally, the ecological hypothesis explores the idea that natural selection favored flexibility in primates. This is very useful to increase the ability to learn and plan and would help deal with both ecological and social changes. We can possess, use, and thrive in nearly any environment in the world. Humans are inherently adaptable. Keep in mind that our diet is not singular; instead it is broad, adaptable, and variable. As the environment changed, either through migration or climate change, they were faced with new difficulties of finding food and other resources.
Being able to figure out a new environment favored those that were cleverer. Leonard reasons that this big brain evolution followed a linear sequence. There was a preliminary period of brain growth that then stimulated a reciprocal relationship between diet and brain expansion. Continuing in this vein, it can be argued that these bigger brains would have produced more complex social behavior, which in turn would lead to shifts in diet, behavior, and social structure. One of the problems with proving any, or all, of these hypotheses is that we cannot be completely sure of what ecological problems our ancestors faced.
The ecological problems we face today are variable and complicated; they are too complex for any individual to overcome in order to survive, let alone reproduce. We rely on a social and shared intelligence. We rely on being taught how to survive. Our social system is inherently complex. It relies on the teaching and sharing of knowledge to all individuals within the group. It is expected that every individual will perform some defined role in society, and those that do not are ostracized.
No matter how much evidence is accumulated or how many hypotheses are posited, it is nearly impossible to prove that primates evolved their oversized brains in response to any single factor. It is truly possible that either the social challenges of group living spurred an increase of the neocortex, but it could also be possible that a more intense environment required higher processing in order to extract the necessary resources. In all likelihood, the answer is probably that many, many factors contributed to the selection and continued evolution of large brains.