OUR intelligence, more than any particular behaviour or anatomical feature, is what distinguishes humans from the myriad other species with which we share our planet. It is a key factor in everything from our anatomy to our technology. To ask why we are intelligent is to ask why we are human; it admits no discrete answer. But let's ask it here anyway. Why are we, alone in nature, so smart?
Perhaps we are not. Maybe our anthropocentric conceit prevents us from fully appreciating the intelligence of other animals, be they ants, cephalopods or cetaceans. As Douglas Adams put it: "Man had always assumed that he was more intelligent than dolphins because he had achieved so much - the wheel, New York, wars and so on - whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man - for precisely the same reasons."
So let's rephrase the question. There is a cluster of abilities that seems unique to humans: language, tool use, culture and empathy. Other animals may have rudimentary forms of these abilities, but they do not approach humans' sophistication and flexibility. Why not?
Some come closer than others. German psychologists say they have identified a chimp whose mental abilities far surpass those of its peers (see "Chimp prodigy shows signs of human-like intelligence"). Intriguingly, they go on to suggest that this might be because Natasha, the simian prodigy, exhibits strong social-reasoning skills, such as learning from others. These are the same skills to which the explosive development of human intelligence is increasingly attributed.
At least some chimps, then, may have the potential to evolve human-scale intelligence. But they haven't - at least not so far. Let's rephrase the question again. Why haven't all chimps evolved to be as smart as Natasha?
Some did, but a long time ago: our own ancestors. Somewhere in our evolutionary history, there were presumably similarly prodigious protohumans, produced by some accident of genetics or environment, whose greater intelligence gave them the edge over their less gifted peers. Today's chimp prodigies do not seem to profit from their intelligence in the same way. Their society and environment do not reward it as ours did.
So our ancestors may have been fortuitously placed to embark on the runaway cycle of biological and cultural development that led to modern, multitasking humans (see "Puzzles of evolution: Why aren't we more like chimps?") - and to a level of adaptability that allows us to adjust readily to changes in our environment, and even modify it to suit ourselves.
Yet we should beware of hubris. In the not-too-distant past, we shared the planet with distinctly different branches of the human family: the Neanderthals and Denisovans, perhaps the Flores and recently discovered Red Deer Cave peoples, and possibly many more. These hominins probably shared many of our mental abilities - and yet still found themselves unable to overcome their environmental challenges and so ultimately died out.
So a final question: does our intelligence really make us the rulers of the world? It's too early to say.