As a preliminary, keep in mind that in placental mammals most neural connections are made before birth.

• In a small group of our hominid ancestors a mutation that affected developmental timing occurred and spread by genetic drift through the group. The effect of this mutation was to prolong the period, characteristic of mammalian fetuses, in which relative growth rate of the brain is greater than the growth rate of the body. The immediate effect of this was to increase the brain size at birth making births more difficult than before. Individuals who were born early and thus smaller (and their mothers) tended to survive better. The premature individuals, however, needed care -- they weren't able to hang on for themselves, move around, and so on for some months. Their caregivers tended to remain more on the ground and more sedentary during this time, enhancing both the level of face-to-face interaction (the parents’ hands being free from holding on to branches) and the level of social interaction involving and surrounding the babies. The babies having been born premature, however, meant that this was just the period when neural connections were being set down. The neural connections then reflected this high level of social interaction, whereas other placental fetuses experience mostly the physical environment of the womb while their connections are being set down. The sophistication and flexibility of these post-birth neural connections accounts for human intelligence.