• Developing a large brain requires a long, arduous gestation period followed by a laborious birthing, complicated by the incompatibility of an infant's large head and a woman's narrow pelvis region (which is necessary to stand on two feet). 

• Brain development continues years after birth, extending a human infant's vulnerability and dependency on their parents. 

• The brain demands a proportionately larger chunk of our energy, even when at rest, than other parts of our body. 

• The brain is maintained by more than half our genes, which are particularly vulnerable to errors, and can result in behavioral disorders. 

Negative Selection Pressures of a Large Brain

• A large brain aids in the managing of complex social relationships between conspecifics ("the social brain hypothesis"). 
• A large brain is fostered by the development of new skills and innovation that increase survivability (namely: innovations in behavior, use of tools, and social learning). 

Positive Selection Pressures of a Large Brain

Environmental demands that influence an organism's survivability. Can be positive (beneficial) or negative (detrimental). 

Villanova University

The evolution of a large cortex, the outer covering of the cerebral hemispheres,  occurred rapidly and recently due to a multitude of possible factors, including some that promoted certain behaviors/capabilities: selection pressures, sexual selection, and gene expression. 

Evolution of a Large Cortex in Hominins

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Selection Pressures 

One theory suggests human brain growth is related to the need for creativity in order to attract a mate. 

Sexual Selection Influence on Hominin Brain Evolution

Humans and chimpanzees, who share nearly 99% of DNA, still differ in a plethora of physical, behavioral and cognitive ways- this may be because we differ in the specific sequences of our DNA, as well as in how genes express themselves in us. 

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