Why do we start growing copious amounts of ear, nose and eyebrow hair as we get older? How can this possibly help us survive in our twilight years?
David Muir
Edinburgh, UK
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A mutation, or random change, in a gene can result in an inheritable characteristic that can be beneficial, harmful or neutral. A beneficial mutation may be successful, or 鈥渟elected for鈥�, and increase in a population. Harmful mutations would be 鈥渟elected against鈥�, a euphemistic way of saying that the carrier could be more likely to die or, at the very least, have little reproductive success. Natural selection doesn鈥檛 operate on neutral mutations.
But a gene can affect more than one trait! A characteristic can become established in a population because natural selection favours another characteristic with which the first is genetically correlated. Let鈥檚 pretend that a mutation manifests itself by giving its possessor superior intelligence, but also leads to a hormonal effect in older age causing hairy ears and nostrils. If superior intelligence is positively selected for, then old hairy ears and nostrils would become more common by default, even though they offer no survival benefit and are unlikely to enhance reproductive success.
Clearly, this hair growth confers no survival benefit. On the other hand, there also seems to be no downside
Another idea is that old hair doesn鈥檛 have enough energy to push itself all the way through the scalp, so it finds alternative, lower exits via earholes and nostrils鈥�
Douglas Nichols
Hobart, Tasmania, Australia
For a desirable trait 鈥� such as an absence of ear or nose hair later in life 鈥� to spread in the population until it becomes the norm, those lucky people who possess that trait must, as a result, generally have more than the average number of children. The genes involved will then become increasingly common as the generations go by.
But there is a problem. How can genes that reduce unwanted hair later in life affect how many children you have earlier in life? If those genes happened to also provide some other advantage 鈥� one that manifests itself in youth 鈥� then we would be in luck. Failing that, we are saddled with the hair I鈥檓 afraid!
Peter Bursztyn
Barrie, Ontario, Canada
Clearly, this hair growth confers no survival benefit. On the other hand, there also seems to be no downside. Either way, this can鈥檛 have been due to natural selection pressure.
Our bodies appear to have a 鈥渄esign life鈥� of 50 to 60 years, meaning that things begin to go awry after then. For the most part, we stop reproducing around the age of 40. By age 55, our last child will have been reared to 鈥渁dulthood鈥� (as defined by most societies prior to 1900). Therefore, changes in our bodies after age 40 are unlikely to affect natural selection 鈥� although they will be preserved in the genetic record.
I needed corrective lenses for myopia (short-sightedness) by age 10. My myopia requires far more correction in my right eye than my left. As a youth, my father required the same unequal correction, as did my brother, clearly hereditary through the male line.
I often wondered what survival benefit that might have offered. My best explanation is linked to the tendency of Homo sapiens to live in cooperative groups. My inability to focus at distance would make me a poor hunter, but would have been a superpower when it came to creating stone tools or sewing skins into clothing and teepee covers. My clan would have fed me in exchange for my valuable skills in other endeavours.
Moreover, since stone knapping and sewing are low-risk occupations, I would have lived longer than most. Longevity would have given me time to develop wisdom 鈥� another superpower the clan might have valued.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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