Brain evolution
The principles that govern the evolution of brain structure are not well understood.
Brain to body size does not scale isometrically (in a linear fashion) but rather allometrically. The brains and bodies of mammals do not scale linearly. Small bodied mammals have relatively large brains compared to their bodies and large mammals (such as whales) have small brains; similar to growth.
If brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs for example fall below this line which means that for a primate of equivalent size, we would expect a larger brain size. Humans lie well above the line indicating that humans are more encephalized than lemurs. In fact, humans are more encephalized than all other primates.
Encephalization quotients (EQs) may indicate how much brain power a species has in comparison to other mammals. Primates lie at the top of this range with humans having the highest EQ score. EQ has a high degree of correlation with the ecological conditions of an animal such as its feeding behaviours and food it consumes. Leaf eating monkeys have lower EQ than frugivorous or omnivorous monkeys since they have to work harder to forage than monkeys which eat abundant, easy to find leaves.
The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to Homo sapiens. Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine brains were little larger than chimpanzee brains. The increase has been seen as larger human brain volume as we progressed along the human timeline of evolution (see Homininae), starting from about 600 cm3 in Homo habilis up to 1736 cm3 in Homo neanderthalensis which is the hominid with the biggest brain size. The increase in brain size topped with neanderthals; since then the average brain size has been shrinking over the past 28,000 years. The male brain has decreased from 1,500 cm3 to 1,350 cm3 while the female brain has shrunk by the same relative proportion.
However it is argued that another essential element of brain evolution in humans is rearrangement (Hoffman et al. 2004). Larger brains require more wiring, but more wiring can become inefficient (Hofman 2001). The brain has therefore become reorganized for efficiency. Furthermore, the average body size of neanderthals was larger which led to bigger brain size (see Brain-to-body mass ratio).
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