It is a good question(s) and I don't know all the answers. I do know that not all but many mammals do eat the afterbirth. Most animals can't afford to miss any meals and I suspect that is the primary reason. Humans have hands so I think we would tend to clean something with our hands as a first resort.

Cats lick their newborns to stimulate both eating to promote the movement of waste and of course to clean them and the kittens purr and enjoy it so it may have a social function as well. I think Moose probably have a primary interest of getting the smell off the fawn (if that is right term for a baby moose). I don't recall ever seeing an ape lick their babies but it wouldn't surprise me.

Even in modern times, there are some humans that will eat the afterbirth; to me, this seems like cannibalism, since the afterbirth (placenta) was part of the baby in the womb.

A lot of animals eat the afterbirth, and for them it serves 2 or 3 functions. It is food for the mother; it cleans up the nest; and its scent will not attract predators. If they were to "throw it away," they would still probably feed a predator. This is also why animals often eat any of their young that die: so that a predator will not get them. Animals with helpless babies in a nest will also eat the babies' poop, to clean the nest.

nutrients that helps give the mother energy that she used up having gone through labor and giving birth. And someone told me that it also helps stimulate her milk to start coming in so her young can start to nurse shortly afterwards. Does anyone know if that part is true, about the milk? I know for humans, our milk starts coming in during the 3rd trimester.

But I was wondering if all mother mammals eat the afterbirth? I would suppose that whales and dolphins probably don't since they need to get their young to the surface to breathe. Also, did primitive females, homo-sapiens, eat their afterbirth? I guess there's probably not proof as to if they did or did not. But I'm wondering as through evolution, when did the primates that we stem from, at what point, did human beings stop eating after birth? Also, do primates today lick their newborns like other mammals do? Cats, dogs, moose, cows, etc. all lick their newborns to stimulate their bodies. Do apes and chimps do that also? Then when did we, during evolution, stop doing that? I can't picture cave women licking them, just rubbing their bodies with their hands.

And please don't tell me about how some women have their placenta made into a smoothie. That's not at all what I'm talking about.