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.
