Why is two-hemisphere brain favored in evolution?

There are two parts to this answer. All animals with a central nervous system (thus a brain) are bilaterians: they're characterised by having bilateral symmetry. read more

So that answers why there are two-hemisphered brains: it's simply a consequence of bilateral symmetry. But it's wrong to assume that a two-hemisphere brain is "favoured by" evolution, because it isn't especially favoured by it. read more

The left hemisphere of the human brain controls language, arguably our greatest mental attribute. It also controls the remarkable dexterity of the human right hand. The right hemisphere is dominant in the control of, among other things, our sense of how objects interrelate in space. read more