For no light matter is at stake; the question concerns the very manner in which human lfe is to be lived.
"Brains exist because the distribution of resources necessary for survival - and the hazards that threaten survival - vary in space and time"
John M Allman
Why we have a brain
Co-creating meaning since 2001
"The moment an idea has meaning for you - that is, it suddenly 'makes sense' to you - it becomes yours, a part of what you are"
Why do we have a brain?
Quick answers might be "to survive", or "we were born with one", or "we evolved with one"; but such statements aren't reasons and don't answer why?
At its simplest we have a brain for two reasons - to make sense our environment and to respond appropriately
The most fundamental sensing - in evolutionary terms (history of life) and developmental terms (individual embryo) - is chemosensory, the perception of chemicals in the environment.
In humans we recognise this as taste and smell. To early organisms and to a developing human embryo - both in liquid environments - this distinction into two different senses would not exist.
The most fundamental response to chemicals sensed in the environment is movement. This will be directed by the concentration gradient of the chemical - up the gradient (increasing concentration) towards the source, or down the gradient (decreasing concentration) away from the source.
So the most fundamental movement is either towards an opportunity, or away from a threat.
Early chordates combined the neural coordination of these functions through a primitive brain - the antecedent of what you are using now to make "head or tail" of this statement.
In other words we have a brain because we need to smell and we need to move!
All else - everything we are - follows from that.
You may not be interested in neuroscience, but neuroscience is interested in you