Humans are very instinctual. Infants rely entirely on instinct to survive, even with nurturing.
For instance, they have a suckling instinct. Without that, they'd starve. Babies who won't take the nipple WILL die in nature. That is instinct.
They have a facial recognition instinct, peoples deepest memories are often their mothers faces. This is why sometimes kids won't "take" to a babysitter. It's an instinctual throwback to danger, and not taking nourishment from potentially harmful people. Their cries are an inarticulate way of saying "rescue me from this foreign tribe".
They have a "falling" instinct, which can be seen by playing "whoopsie" with a baby. They'll fling their arms out in a steadying motion, even before they have the ability to walk or crawl. Sometimes it'll be observed during dreams, when they fling their arms out like "WOAH!" and then wake up crying. In nature a simple injury can lead to death.
They'll cry when they shit because it can lead to infection if left uncleaned.
A lot of the instincts you'd normally recognize as survival instincts are worthless when a child is essentially a human worm. You can't expect them to get up and walk like an infant Jackie Chan, defending themselves from complicated dangers.
The higher primates develop slower because of longevity of the species, and ultimately they aren't viable for reproduction until sexual maturity, which in the physical sense doesn't occur until their early teens. If they physically can't give birth, everything before that is learning time.
And thus, the instinct mechanism is identifying danger and satiating nourishment to allow them to reach that purpose.