The survival advantage gained by organisms with brains is the ability to exploit time. The ability to predict the future in terms of resources and dangers, and to act pre-emptively.
Clearly species that developed brains gained a significant survival advantage.
E.coli is a unicellular organism with the most simple of nervous systems. It is capable of sensing energy sources, nutrients and toxins, storing and evaluating the information and make a final decision keep swimming or roll (to alter direction).
Plants don’t need brains because they don’t move. Simple nervous systems connect sensory mechanisms directly to motor neurons. More complex brains are about exploiting time to take physical action. Brains are about using information from different times (seconds through years) to coordinate action. When brains compete in a Darwinian way it becomes about speed. Brains are about exploiting time, to take physical action, faster than the other brain.
If brains are about intelligence then this assumption leads to a natural definition.
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