I have to think evolution isn’t the real story, at least based on the concept of survival of the fittest. Every toddler I’ve ever seen has been prone to rip-roaring temper tantrums, mine now among them. (I think we’ve had three today so far. Unknown as the reason – Cora’s language skills aren’t yet advanced enough for her to tell me “Mother, I’m more than mildly peeved that you’ve…”) I just can’t see a cave-dweller armed with a club who’d put up with that for very long. For one, that sound would quickly alert predators that there’s a mini-meal in the vicinity. Similarly, there’d be no potential food for the cave dwellers within a ten mile area of that piercing cry. The healthily wailing tyke would be promptly bopped over the head by their mom or dad, and that would be the end of that kid. Unless there were a whole contingent of toddlers on hand, and they were remarkably quick on the uptake that loud tantrums meant a longer night-night than the usual nap time, kids just wouldn’t survive past the toddler stage. The species would have been wiped out, one wail at a time.
You’ve got it the wrong way around. We ended up with whiny toddlers not because of the children’s whiny genes but because of the parent’s don’t-bash-junior-with-a-club genes. Only parents who could refrain from bopping their children with clubs managed to get their genes expressed in the next generation. The baby-bopper genes died with the babies that got bopped. This is perhaps part of the reason why the meek will inherit the Earth.
Now, as far as preditors entering the cave in search of crying two year olds, it was probably the pent up frustration of listening to their children whine that gave these meek, unassuming parents the ability to kill saber-toothed tigers with pointy sticks.