Steve Stewart-Williams in Psychology Today:
This post is the first of three looking at the implications of Darwin’s theory of evolution for some of the most important topics in applied ethics, including suicide and euthanasia, and the proper treatment of nonhuman animals. These are controversial topics and you may well disagree with some of the ideas I’m going to float. I hope, though, that you’ll at least find it interesting and perhaps that you’ll find some food for thought in here.
First things first, a lot of people claim that evolutionary theory simply has no implications for any moral question. I suspect this view has been motivated in part by some of the unpopular and unpleasant conclusions that were drawn from the theory in the past. For instance, the Social Darwinists of the late 19th century and first half of the 20th century argued that giving aid to the weak, sick, and poor goes against nature – it undermines natural selection – and thus that it should be stopped. The vast majority of people reject this view now, and that includes the vast majority of evolutionists. But there have been some recent suggestions about other possible implications of evolutionary theory for ethics. One of the most important of these stems from the work of moral philosophers such as James Rachels and Peter Singer. According to these philosophers, in the wake of evolutionary theory, we must rethink or recalibrate our ethical commitments. Specifically, we must rethink the value we place on the lives of human beings vs. other animals.
Rachels identified an important trend in traditional Western moral thinking, which he dubbed the doctrine of human dignity. (Peter Singer uses the phrase sanctity of human life to refer to essentially the same thing.) Although the doctrine of human dignity is often not explicitly expressed, it is the heart and soul of the Western moral system, and provides the moorings for traditional morality.
The doctrine has two parts; the first pertains to humans, the second to nonhuman animals. The part pertaining to humans is the idea that human life has supreme worth – according to some, it has literally infinite value. A corollary of this view is that any activity that involves taking a human life (or at least an innocent human life) is utterly forbidden. This includes suicide, euthanasia, and abortion.