Human Brain - Evolution

Evolution

In the course of evolution of the Homininae, the human brain has grown in volume from about 600 cc in Homo habilis to about 1500 cc in Homo sapiens neanderthalensis. Subsequently, there has been a shrinking over the past 28,000 years. The male brain has decreased from 1,500 cc to 1,350 cc while the female brain has shrunk by the same relative proportion. For comparison, Homo erectus, a relative of humans, had a brain size of 1,100 cc. However, the little Homo floresiensis, with a brain size of 380 cc, a third of that of their proposed ancestor H. erectus, used fire, hunted, and made stone tools at least as sophisticated as those of H. erectus. "As large as you need and as small as you can" has been said to summarize the opposite evolutionary constraints on human brain size.

Studies tend to indicate small to moderate correlations (averaging around 0.3 to 0.4) between brain volume and IQ. The most consistent associations are observed within the frontal, temporal, and parietal lobes, the hippocampi, and the cerebellum, but these only account for a relatively small amount of variance in IQ, which itself has only a partial relationship to general intelligence and real-world performance. Demographic studies have indicated that in humans, fertility and intelligence tend to be negatively correlated—that is to say, the more intelligent, as measured by IQ, exhibit a lower total fertility rate than the less intelligent. The present rate of decline is predicted to be 1.34 IQ points per decade.