BECOMING HUMAN: BRAIN EXPANSION

We already established that the evolution of bipedalism was the first step towards what we perceive as being human. However, there was another process that was absolutely crucial to making the human species: encephalization, or the evolutionary brain expansion relative to body size (Hofman: 2014). This development resulted in one of the most complex and efficient structures in the animal kingdom: the human brain.

During the first 4 million years of hominin evolution, the brain growth was rather slow. More pronounced enlargement started 2.5 million years ago, most likely from a bipedal Australopithecine whose brain size was similar to that of a modern chimpanzee. The dramatic increase in the hominin brain size happened only in the last 800,000 years. Over the course of approximately 7 million years, the human brain tripled in size (Robson and Wood: 2008).  In this blog post I will try to explain what pressures could have led to brain growth, and what effects it had on the survival of our species.

When I talked about bipedalism, I said that for a very long time the leading theory of hominin evolution went more or less like that: walking upright freed our hands, free hands led to tool use and tool use resulted in a rapid growth in brain size. I also explained how we learned that this theory is unlikely. Nowadays the increase in hominin brain size is explained in two ways: through environmental or social factors.

Pressures of the physical environment

Some of the theories explaining the increase in brain size focus on the environmental selection pressures, especially climate. The rapid encephalization in hominins coincided with the period of particularly large climatic fluctuations, as shown in the figure below.

Source: http://humanorigins.si.edu/research/climate-research/effects

In order to deal with such unpredictability, our ancestors had to think ahead and develop much more sophisticated cognitive abilities in order to adapt. Some scientists say that the climate variability led to a diversification of our diet – in order to survive our ancestors had to become omnivores and come up with innovative ways of obtaining food (Willemet: 2013). Moreover, the addition of meat to their diet was a very important source of energy, much needed for the growth of brain (right now our brain is approx. 2% of our body weight, but requires at least 20% of our calorie intake to function).

Pressures of the social environment

There are also hypotheses about social factors leading to brain expansion in hominins. Population growth could have been an important factor, and it could have affected brain size in various ways. There could have been a competition for resources and consequently smarter, more innovative individuals would have better chances of survival (Falk: 1990). Alternatively, due to an increase in population our ancestors could have started to form larger, more complex social groups which involved cooperation, coalition formation or reciprocal altruism – all requiring intelligence.

The debate on the factors leading to hominin brain expansion is still ongoing and the lack of sufficient fossil evidence means we might never be certain about what actually triggered encephalization. However, there are things we know for sure: our human brains can collect, process and store unimaginable amount of information; they can find innovative solutions to problems; they can create abstract ideas. They made us the dominant species. However, there is a price to pay: our brains have enormous energy requirements, and – due to the large size – childbirth is painful and difficult for human mothers. There is no doubt, though, that it is only a small price for all the possibilities our brains give us.