From the latest issue of Inference: International Review of Science:
It is easy to invent a selectionist explanation for almost any specific observation; proving it is another story.
HUMAN NEWBORNS are helpless. Without adult care, human infants would not stand a chance. They are unable to locomote on their own, or to eat anything beyond the most limited of diets. There are species whose young exhibit adult behavioral characteristics virtually from birth. The horse is an example. If the horse is not helpless at birth, neither is he particularly smart—nor does he get very much smarter, Clever Hans notwithstanding. With human infants, it is the other way around, a fact that requires an evolutionary explanation.
The emergence of human intelligence is in part attributable to an increase in their brain volume, which is thought to have roughly tripled over the last 2.5 million years. At 1,300cc, human brains are enormous in comparison to those of chimpanzees and gorillas. Brain size relative to total body size has also increased. The human brain constitutes 2% of total adult body weight, and consumes 20–25% of basal metabolism. It is also three times larger than would otherwise be expected for a primate of human body size.
For at least thirty years, the standard solution to the riddle of human neonate helplessness has been the obstetrical dilemma. Because our ancestors from the time of Australopithecus walked on two legs, their pelvises had to remain narrow to preserve mechanically efficient movement. But their larger brains required an increasingly wide birth canal. These competing selective pressures resulted in an evolutionary trade-off; the human pelvis became as wide as bipedalism permitted. Since intelligence was under selective pressure, the most straightforward trade-off required human beings to give birth when newborn crania were still relatively small, and their brains relatively underdeveloped.
Hence the helpless, or altricial, human newborn…
Read the entire essay at Inference.