Well, according to the theory of evolution, man is the last surviving member of the homo sapiens group. The main reason for this would be that man developed the best qualifying set of characteristics that enabled him to survive. That is the basic point of natural selection, or "survival of the fittest". The other subgroups of humans, such as cro-magnon man and neanderthal man, did not develop that same set of characteristics, and thus were eventually weeded out of the great race we call evolutiion.
The first characteristic worthy of mention was "the development of a bipedal locomotor adaptation from an arboreal or semiarboreal setting". This means man ceased to walk on all fours and became a creature that used bipedal walking (upright, on two legs). Man also became less of a tree dweller and more of a ground dweller. Other characteristics worthy of mention are increased brain capacity, giving man the ability to think and reason, and the descent of the larynx (voice box), givig him the ability to speak and develop means of oral communication.