An evolutionary argument against exclusively human dignity

Traditional morality assumes that there is something morally special about being human. The fact that someone is a human being, rather than, say, a dog or a cow, makes a big difference in how he or she may be treated. Humans have full and equal moral worth or dignity and thus may not be killed, even if doing so would promote the greater good, whereas non-human animals have a lesser moral status and can be sacrificed for even the most trivial human pleasures.

This moral worldview fits well with the Aristotelian idea of a hierarchy of being, according to which each species is a static group of organisms with a distinct essence. The philosophical line that morally distinguishes humans from other animals corresponds to the empirical line that Aristotle thought distinguishes the human species from other animal species. Since the publication of Charles Darwin’s The Origin of Species in 1859, however, we know that there is no such line on the empirical side of things. We now understand that all life is interrelated, and that biological characteristics come in degrees and continually evolve as a result of natural selection. As the principle of evolutionary continuity informs us, any differences between species are differences in degree, and not in kind. The real picture looks something like this:

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