The average human brain weighs about 2.8 pounds, has 100 billion neurons, and uses up 20% of the oxygen we breathe. Compared to the size of the human body, our brain is much bigger than it needs to be. A recent study by PhD student Luke McNally, and colleagues at Trinity College in Dublin, Ireland, suggests a possible reason why this ratio exists. With the use of a computer model, McNally was able to recreate a small scale evolution of the human brain over 50,000 generations and found that the more cooperative and social the brains were, the more neurons they grew.
In order to run the experiment, McNally and colleagues used a computer program that simulated evolution for 50 virtual brains. Each brain started with three to six neurons and had to challenge another brain to a game of either “the prisoner’s dilemma,” a moral dilemma situation meant to test cooperation and trust or “the snowdrift game,” a hypothetical situation meant to test willingness to help others and work together.
After the brains had played one of the games they reproduced asexually. The program then adjusted the outcome of reproduction so that the successful brains were more likely to have offspring. Then all brains in the new generation underwent a random mutation that could change the brain’s structure, neuron number, or connection strength between neurons. The researchers ran the simulation for 50,000 generations with 10 runs of the simulation for each game. As the simulation ran, McNally and colleagues compared how often the brains cooperated with each other, to how many neurons the brain had. McNally found that “As you transition towards a more cooperative society, that’s where you get the maximum selection for big brains … as cooperation is increasing, there is more selection for intelligence.”
Even though the virtual brains used in McNally’s experiment did not function at the capacity of a primate brain, the researchers were still able to conclude that just the existence of cooperation is enough to make brains evolve and become more complex. Although the test was a long way from a real life scenario, the results give a boost to the social intelligence hypothesis, which were published online April 10, 2012 in the proceedings of the Royal Society B