Human Brain Evolution
Over time, the human brain has evolved in many different ways. There is a connection between species body size and the size of their brain, generally following a linear relationship. Although, some species do not fit this pattern having much larger brain sizes compared to their body mass, which is known as encephalization. Humans would fit into this later category. A larger brain has afforded humans with added benefits such as ability to make tools, adapt out behaviors to survive in different environments, allowed us to live in large groups (Benton, 2012).
An evolutionary increase in the complexity or relative size of the brain, involving a shift of function from noncortical parts of the brain to the cortex. (Oxford Dictionaries: https://www.oxforddictionaries.com/definition/american_english/encephalization)
Encephalization is also known as the brain-to-body mass ratio. This ratio compares the brain mass to the body mass. Species which yield a larger brain mass to body mass are hypothesized to be of higher intellegence. The formula used to determine the brain-to-body mass ratios is in Snell's equation of simple allometry, E = CS^r; E is the brain weight; C is the cephalization factor; S is the body weight; and r is the exponential constant. This formula allows scientists to compare the capacity of a brain to the body mass of the species. Below is a chart of different mammals and their relative "C" cephalization factor (Williams, 2002).
Image: (Serendip, 2012)
To date this data does not have a definitive link to intellegence.
Benton, A. (17, May 2012). Mutation helped human brain evolve. Retrieved from http://evoanth.wordpress.com/2012/05/17/mutation-brain-evolve/
Serendip. (2012, Sept 05). Thinking about brain size... Retrieved from http://serendip.brynmawr.edu/bb/kinser/Int3.html