If conditions are ideal on earth for the development of one intelligent species, why haven't two species developed?It may have. Primitive warfare is ubiquitous and, relative to population, far more deadly than modern war. A slightly less intelligent species would have been wiped out.
Such as the Neanderthals? But they were something of a subspecies.
Besides, there was plenty of opportunity for a completely different intelligent species to develop in the Americas before contact with Homo-Sapiens.