Why do men have nipples?

Every day the home page of my internet service offers a selection of trivia including news of celebrities who are either sleeping with each other or splitting up. There is also usually an item of popular science, often packaged as ‘the latest research’. If one of these catches my eye I sometimes click on the link to read it in more detail. Two items I recently followed up in this way were headed ‘Why do women live longer?’ and ‘Why do men have nipples?’. The answers to these tantalising questions weren’t nearly as interesting as I hoped. In fact both were pretty old news. The reason women live longer, it said, is because their cells are better at repairing damage in later life. Men have nipples because embryos are always female in their early stages, and males only differentiate from the female prototype after a few weeks. Unfortunately the problem with these explanations wasn’t just their banality. They are merely accounts of how things happen, not actually why. They don’t address purpose, and hence don’t really answer the questions at all. In the language of biological science, they are proximate explanations but not distal ones. This confusion of mechanisms with purposedor of proximate causes with distal onesddoesn’t just occur in the media. It’s very common in medicine as well. If you ask most doctors for the cause of diabetes they may be able to give you an account of how insulin receptors work; they will look puzzled or offended if you tell them they haven’t answered your questiondbut of course they haven’t. All they have done is to trace a common pattern of human malfunction from the level of the whole organism back to the molecular level. That, of course, may be tremendously helpful knowledge and indeed lifesaving, but it leaves out whole realms of understanding. How many endocrinologists, I wonder, could tell you the proportion of mammalian species that are prone to diabetes, or at what stage in evolutionary history insulin resistance first appeared, and whether there are any credible hypotheses regarding the purpose this may originally have served?