If you like watching nature shows on TV, you've probably seen films of animals being born. The newborn tiger/wildebeest/whatever shakes itself off, stumbles to its feet, and trots along after Mom. If you've had children of your own, you probably noticed that your newborn didn't do that, even though the newborns of all our closest relatives — chimpanzees, gorillas, and so on — are reasonably mobile from day one. Humans are the branch of the primate family tree in which something changed.
To make a long story short, human babies are born before their heads are finished developing. (If you've had a baby pass through your birth canal, you might realize that a malleable skull made delivery easier; if it still didn't seem like a stroll through the park, remember that evolution often involves compromises!) The human child's head and brain continue to develop and grow after birth. This process of juvenile traits persisting later in development is called neoteny.
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