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“Vestigial or not vestigial” that is the question? Sample Response Definition Vestigial structures are structures of organisms in a species whose original function is considered to have been lost, or reduced during evolution. These are either in a degenerate, atrophied, imperfect condition or form, or in a similar state. Vestigial organs - evolution or devolution? First, it is in principle not possible to prove that an organ is useless, because there is always the possibility that a use may be discovered in the future. This has happened with over a hundred alleged useless vestigial organs that are now known to be essential. Second, even if the alleged vestigial organ were no longer needed, would it prove devolution or evolution? The creation model allows for deterioration of a perfect creation. However the particles-to-people evolution model needs to find examples of nascent organs, i.e. those that are increasing in complexity. However, we now know that many, if not all, of the considered vestigial organs do have a function of their own. In the context of human evolution structures usually called ‘vestigial’ are largely entirely functionless but they may have retained lesser functions or developed new ones. But what about the organs that have never been considered to have a function in their own right, e.g. the male nipple? Can these be considered as vestigial? Why do men have nipples? - Men are women in the womb A well-trained embryologist cannot visually tell the difference between a male and a female embryo if the embryo is less than seven weeks old. At that early stage of prenatal life, there is a fork in the road of development. If there are no hormonal changes in the prenatal environment, the embryo develops into a female. If, however, the embryo is destined to become a male, its Ychromosomes trigger the production of testosterone (the male hormone), which masculinises the brain and genitalia (internal as well as external) of the embryo. But it takes no female hormone to produce a female, as this is the default state of the developing embryo (only at age ten or later does the female body begin to produce the hormones that transform the girl into a woman). It seems obvious that for females, these structures have a function, both to mother and child. But wouldn’t it be just as useful for the child to have access to milk from both parents? In many species, food is provided for offspring by both male and female parents, to the obvious advantage of the young. This is a common behaviour in birds, for example. Why, then, are male mammals so selfish? There is absolutely no physiological reason why men couldn’t nurse babies - all the equipment is there. The development of mammary glands in embryonic development happens independently of sex. These glands remain indistinguishable between the sexes until puberty, during which there is exposure to hormones from the ovaries, adrenal glands, and pituitary gland in the female body. Pregnancy further enhances the development of these glands, especially the hormone prolactin, which is produced in the pituitary gland at the base of the brain. But exposure to the same kinds of hormones at the same times would give the same results for men, or, more specifically, removing (or reducing) the effect of the masculinising hormones like testosterone. Mutations of the testosterone pathway can cause males to be born without recognizable masculine genitalia, and they develop usually into women carrying the Y chromosome, simply because they’ve had their masculinisation progress impaired. Therefore, genetically and genitally they’re still male - they have internal testes, not ovaries. However, men can lactate - there is no physiological barrier. So why don’t they? The reason is not physiological, but evolutionary. In the vast majority of mammalian species, offspring are born and raised solely by the female - the male plays no parental role at all. His evolutionary interests are best served by having children with as many females as possible, and so he doesn’t gain any advantage to sticking around to help raise one or two of them. According to the general consensus of evolutionary selection, the organisms that pass on the most copies of their DNA are the most successful. What about those species for whom males do play a role in the parenting of their offspring e.g. humans? We see that the male contributions in these species include things like bringing back food for the female, chasing off potential competitors within its species, and looking out for predators from other species. These alternatives to lactation are well-adapted traits for the species in which they’re found, and offset any benefit which male lactation would provide evolutionarily. But this doesn’t preclude the existence of any species for which male lactation would be an evolutionary benefit, and in fact one has been recently discovered, a fruit bat found in Malaysia. Males of this species were discovered with functioning mammary glands, both full and drained, indicating that they were providing milk to their young. This species hasn’t been studied enough to determine why male lactation was advantageous for development, but as you should know already, the physiological requirements would already have been in place. As bats are closely related to humans (just outside of the primates), this seems to suggest that male lactation in humans is only a few environmental variables away from becoming commonplace. More than two? All mammals can develop an extra nipple. It is not widely known, but about one in 50 women and one in 100 men have a third nipple. Polymastia refers to the appearance of the additional nipple along with the presence of mammary glands. The third nipple is merely one of those anomalies that arise in human populations. The third nipple usually forms along the two vertical “milk lines” which start from the armpit, run down through the breasts, and further down to the groin in both women and men. These milk lines are clearly seen in other mammals e.g. dogs who have larger ‘litters’ than humans and therefore require many ‘feeding stations’ for their brood.