My reading (caveat: not a biologist, other sort of scientist, nor philosopher) is that a brain is required to translate the environment and its collection of pr

My reading (caveat: not a biologist, other sort of scientist, nor philosopher) is that a brain is required to translate the environment and its collection of problems into something (or some things) that its simpler structures can “solve” (where “solve” could just mean “act usefully in response to” and that act/response could be to ignore), and then to translate any responses but out to that more complex environment. Cells can solve problems in their limited context, though that context can be less limited than you might first think (consider single celled life can have relatively complex interactions). Groups of cells can solve more complex problems, by working directly together or by some acting as support structures while others do the solving. Complex bodies and brains build up in parts from there over time.