When we're born, the part of our brain that controls our most complex cognitive functions, such as self-awareness and creativity, isn't fully formed yet. It then starts developing very quickly...and much the same happens with chimpanzees.
Of course, chimp brains don't develop nearly as much as their human counterparts. But both species' forebrains see remarkable growth after birth. In humans, the prefrontal cortex ends up hugely enlarged, and that's crucial to the development of our unusual intelligence. So why do both humans and chimps have this built-in delay for one of the most crucial parts of their cognitive maturation?
The answer, it seems, is that this delay allows human and chimp brains to learn things they otherwise would not be able to. The delay gives their brains far greater plasticity, which, as Tetsuro Matsuzawa of Kyoto University explains, leaves "their neural network and brain function more susceptible to the influence of postnatal experience." Basically, the time spent waiting for the forebrain to enlarge is spent learning complex social interactions and establishing basic skills that will serve humans and chimps well over the course of their lives.
For humans, that development goes further, with the forebrain ultimately becoming complex enough to allow us to develop language and other unique abilities. But both humans and chimps - and, by extension, our common ancestor of roughly eight million years ago - share this ability to take advantage of an initially immature forebrain.
It's a nifty way for two of the planet's most intelligent species to have it both ways. First, we use the plasticity afforded by the still immature forebrains to soak up the basics of our society, and then our fully developed forebrains allow us to function in these complex groups. Of course, humans and chimps reach a pretty clear point of divergence in terms of their later development, but how we get to that point seems rather less unusual than we would have once imagined.