I thought it was just my kids. We like to think of them as requiring "the path of greatest effort"

I think of it as a fitness measure: Are you active enough at moving towards food gathering sites, preparing and surveying the landscape, creating and fine tuning your tools? If it is an approval of sorts on behalf of the child, then it also points to some really basic learning that is beginning to flow between the parent and child

That's a mind-boggling theory actually, that some general survival skills are encoded in the parent-child relationship. Basically you have to become a parent to level-up your survival skills.

I wouldn't put it past evolution to encode useful information wherever possible, but it would be hard to prove that theory.

Not clear whether evolution would select for crying in the possible presence of predators (so fighting adults can be woken) or for silence (to avoid attracting attention as prey).

I think babies cry when they get tired to warn parents they are going to fall asleep soon, so that parents know they have to take them somewhere safe to sleep.

"You cannot sleep while enemies are nearby"

Crying garners attention from adults in either case. Any attention could be better than no attention I suppose.