Aaron Sloman pointed out in a talk I saw once (Sloman & Chappell, 2005) that certain species are "precocious," which means that they are quite capable of doing what they need to do in the world soon after being born. Certain birds start hopping around, looking for food seconds after hatching. Others are "altricious," meaning they take a long time to learn what they need to. Humans, of course, are altricious. Paradoxically, our utter inability to take care of ourselves as babies is a key to our success.
There is an evolutionary pressure to make brains bigger. Bigger brains means smarter brains, in general. Not always, of course, but even in human beings brain size accounts for 16% of the variance in intelligence (Hoppe & Stojanovic, 2008). Anyway, this can't go on forever. Know why? Because if human brains got any bigger, women's pelvises would be so wide they would not be able to run very effectively. So giving birth to a baby, particularly the baby's head, is very painful-- even life-threatening. If I might anthropomorphize evolution for a moment, it tries to get the biggest head it can through the biggest pelvis it can handle. I speculate that the result of this is that the human brain couldn't come with everything it needed. The strategy it adopted was to have the brain learn stuff after birth. Now, in industrialized societies, death in childbirth is not a big scare, but the fact that kids can learn so well helps us a whole lot, because we would not be able to change the world like we are and have the new kids be able to adjust if they'd come without an incredible ability to learn. Lucky us!
I just read something that supports the idea that even individual differences in the altriciousness might account for differences in ability. Turns out the smart kids younger than 8 years of age have an unusually thin cerebral cortex. But by the time they are in late childhood, they have a thicker cerebral cortex (Shaw et al., 2006). Perhaps these kids are smarter because they are more altricious than their peers.
If all this is true I would predict that gifted children are actually less competent in the world before age 8, since, being less precocious, they start with less, but learn more.
Pictured: A baby, perhaps angry that he's so precocious.
Hoppe, C. & Stojanovic, J. (2008). High-aptitude minds. Scientific American Mind August/September 2008, 60--67.
Shaw, P. et al. (2006). Intellectual ability and cortical development in children and adolescents. Nature, vol. 440, pp676--679.