The history of life on Earth can be visualized as a massive, branching tree which germinated 3.6 billion years ago. Mediated by genes and governed by natural selection, each branch of the tree represents a new species; some of these branches never advance beyond a twig while others become thick trunks, giving rise to many more species over the eons.
Mammals branched from reptiles about 200 million years ago (MYA) and primates branched from the mammalian line some 60 MYA. Apes split from other primates at least 15 MYA and hominids diverged from the chimp line about 7 MYA. Finally, modern man evolved from ancestral hominids just 125,000 years ago. Our species would thus be represented by a short twig on the periphery of this massive tree of evolution; whether that twig will die out, persist in isolation or eventually branch into other species is a matter of speculation.
Only natural species, which include man, belong on this tree. Those domestic animals and cultivated plants that have been engineered by man are not natural; they represent some of the many ways in which humans have manipulated nature and most could not survive without man's nurturing and assistance.
