Evolution

Babies have a really strong reflex to grab on to things with their powerful little hands. If I were a-swingin' through the trees with Rhys instead of putting him in the sling, he'd be grabbing my fur so as not to fall off.

They don't have any associated innate ability to let go, though. This means that when I helped the baby get his tiny fingers around a rattle the right way, he held it tight, shaking, at first in a surprised way, then a pleased one, and finally a disturbed one as he couldn't figure out how to let go of it.

Primates are funny.