Why aren鈥檛 humans covered in hair like other primates are?
David Muir
Edinburgh, UK
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Under specific environmental conditions, natural selection picks the winners and losers. The winners鈥� genes are passed to future generations and the losers鈥� genes are lost to time. If the environment changes, what was previously a genetic disadvantage may quickly become a distinct advantage, and vice versa.
Other primates have hair that helps their thermoregulation, keeping them warm when temperatures drop. When their environment heats up, they don鈥檛 need to move around much, as they mainly eat plant foods that don鈥檛 run away.
When our hominin ancestors came out of the woods and chased animal prey on the savannah, they would have overheated when running long distances if they had been covered in fur. The savannah hypothesis, also called the body-cooling hypothesis, is one view as to why our ancestors lost their thick, hairy coats. This thermoregulatory driver for fur loss, plus an increase in our cooling sweat glands compared with other primates, allowed our hominin antecedents to hunt on hot grasslands without overheating. For them, on an evolutionary timescale, it was a case of hair today, gone tomorrow.
Trevor Campbell
Berrima, Australia
Hair is metabolically expensive to produce and, like any expensive attribute that isn鈥檛 needed, will gradually be lost as a function of normal evolution. Think of animals in caves, which are often lacking in pigment and/or blind.
Humans don鈥檛 need much hair because, somewhere in our past, we learned to steal it from other animals and wear their pelts as clothes when required. Later, we learned to use other materials, such as plant fibres. These options are cheaper, from an energy point of view, than growing your own.
Of course, all ideas about how evolution progressed are just speculation, as rerunning the experiment isn鈥檛 possible, and so should be taken with a large pinch of salt.
Marc Verhaegen
Putte, Belgium
One possible answer could be the hypothesis of our semi-aquatic evolution, which I have . There is zero doubt that at least our early Pleistocene ancestors frequently waded bipedally in shallow water and regularly dived for shellfish.
Only a lifestyle including the collection of shallow-water foods 鈥� which also makes sense of why we lost our fur 鈥� can explain why our ancestor Homo erectus evolved pachyosteosclerotic skeletons (with very thick and dense bones, which are exclusively seen in shallow-diving tetrapods).
A semi-aquatic lifestyle also explains why some hominins had skulls with auditory exostoses, bony outgrowths in the ear canal that are typically caused by chronic colder-water irrigation. And it accounts for why they had descendants who colonised islands far overseas like Flores; evolved brains twice as large as those of great apes and australopiths, facilitated by DHA and other brain-specific nutrients found in seafood; and frequently used stone tools with their very dexterous hands, comparable to sea otters opening shellfish.
Bernard Harper
Liverpool, UK
This question was answered in New Scientist by marine biologist Alistair Hardy in 1960. Your magazine has covered many ideas about our 鈥渦nique鈥� naked skin. But only one has grown without scientific rebuttal and explains new findings. For five decades, Hardy鈥檚 insight was expanded by the late Elaine Morgan, author of books including The Aquatic Ape, and often featured here. She showed that nakedness and other human traits are most elegantly explained by aquatic adaptation. She predicted that many new aquatic-human commonalities would be found, as was eventually shown.
This Last Word question appeared in the same edition as your article on the menopause, which, while once thought to be 鈥渦niquely human鈥�, has been shown to be common in orcas and other whale relatives.
Morgan would have also been delighted to discover that vernix caseosa, the waxy biofilm found on newborn babies, is also on the naked skin of newborn seals. This covering seems to be an adaptation to being born near or in water. Combined with our subcutaneous fat layer (essentially blubber), relatively bare skin is necessary because our ancestors foraged in rivers, lakes and coastal waters that most primates regard as an existential threat.
This counterintuitive idea is like continental drift: an often ridiculed concept that, half a century later, became plate tectonics. But who鈥檚 laughing now?
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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