Evolution of the brain
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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 we plot the brain weight against body weight for primates, the regression line of the sample points can indicate the brain power of a primate species. Lemurs 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 i.e. humans are more encaphalized than lemurs. In fact, humans are more encephalized than all other primates.
Encaphalization quotients give us an indication of how much brain power a species has in comparison with other mammals. Primates (as expected) 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 fruit-eating or omnivorous monkeys since they have to work harder to forage than monkeys which eat abundant, easy to find leaves.
Human brain size in the fossil record 
Human brain size has been trending upwards since 2 million years ago, with a 3 factor increase. Early australopithecine's brains were little larger than chimpanzee's brains.
However it's 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.
See also 
Further reading 
- Falk, Dean (2011). The Fossil Chronicles: How Two Controversial Discoveries Changed Our View of Human Evolution. University of California Press. ISBN 978-0-520-26670-4.
- Striedter, G. F. (2005). Principles of Brain Evolution. Sinauer Associates.