If we accept this common argument of natural historians, and insist that the “essence” of humanity can be defined only by the overt variation among the more than 6 billion human beings on earth, then how can we characterize ourselves at all in a world of evolutionary continuity? If we descended smoothly from the apelike common ancestor of humans and chimps, then how can humanity achieve any clear definition? Doesn’t all life form a single glop of continuity, extending all the way back to primordial bacteria?

Darwin answered this false problem in a wonderfully simple way in his epochal book, On the Origin of Species, published in 1859. He admitted that if species never died, all life would form an unbroken continuum without natural boundaries to define categories and entities. But the vast majority of the species that have inhabited the earth have become extinct.

Chimps and humans evolved as separate lineages, each with unbroken continuity from a common ancestor that lived some 6 million to 8 million years ago. If all these intermediary forms still lived, the earth would house a complete continuum stretching back from these two terminal points to the common ancestor — and true distinctions would become impossible. But, in fact, all the intermediary forms died out long ago — and only chimps and humans remain as two unambiguously distinct species with no confusing intermediates living between our end points.