Perhaps we should blame it on the weather. When the rains failed and the trees the ape-men lived in started dying, brainier and perhaps more sociable mutants might well have gained an edge over their more apelike relatives. Instead of retreating ahead of the grasslands, the clever apes found ways to survive on them, and in the twinkling of an eye (on the timescale of evolution) a handful of mutants spread their genes through the whole pool and completely replaced the slower-witted, undersized, forest-loving Homo habilis.
