Monday, September 22, 2008

Why aren't other mammals furless?

I'm one of those kids that never grew up. Always been a wonderer.

So today isn't the first time that I've wondered about why humans don't have fur, and, more interestingly, why so few other mammals are furless (no other primates). I guessed it was related to sweating. Being hairless we can use evaporative cooling, being a running primate (the only one) with a big hot brain we need lots of cooling, having a big hot brain we're good at finding water.

Oh, and Vitamin D is easier to make.

There are lots of downsides to losing fur of course. Mosquitoes love us, even if lice don't. Fur is nice in cold weather. Skin must be black to prevent burning - that requires new proteins and evolutionary costs. (Primate skin is pale beneath the fur, so making skin black means expensive evolution is needed.)

The downsides must be severe, because most every mammal has stuck with fur. So it has to be something weird about us. Hence the brain, running, sweating thought.

These days, of course, I can research my wonderings -- esp since the NYT has liberated their archives.

I started with an article stuck in draft since last year. The "aquatic ape" hypothesis is that we lost our fur during an aquatic lifestyle phase. The theory has suffered from "new age" enthusiasms, but it got a boost a year ago (BBC Science) ...

The waste from shellfish dinners discarded in a South African cave is said to be the earliest evidence of humans living and thriving by the sea.

The material was found by scientists working in a sandstone opening at Pinnacle Point on the Cape.

Researchers tell the journal Nature the remains were buried in sediments that are 164,000 years old.

The exploitation of coastal resources is thought to have been key in allowing early humans to move across the globe....

... The team excavated from the cave the cooked remains of some 15 types of marine invertebrate, mainly brown mussels, as well as other animal bones.

The very earliest human species would have been restricted to a diet of plants, such as berries and tubers, and the meat of animals they could catch.

The expansion to shellfish is one of the last additions of a new class of food to the human diet before the introduction of domesticated livestock meat just a few thousand years ago, the researchers tell Nature...

Interesting, but 164,000 years ago is not that long. From another article I learned we've been furless for much longer than that ...

Why Humans and Their Fur Parted Ways - New York Times 2003

One of the most distinctive evolutionary changes as humans parted company from their fellow apes was their loss of body hair. But why and when human body hair disappeared, together with the matter of when people first started to wear clothes, are questions that have long lain beyond the reach of archaeology and paleontology.

Ingenious solutions to both issues have now been proposed, independently, by two research groups analyzing changes in DNA. The result [imply] ... we were naked for more than a million years before we started wearing clothes.

Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, has figured out when humans lost their hair by an indirect method depending on the gene that determines skin color. Dr. Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, believes he has established when humans first wore clothes. His method too is indirect: it involves dating the evolution of the human body louse, which infests only clothes.

Meanwhile a third group of researchers, resurrecting a suggestion of Darwin, has come up with a novel explanation of why humans lost their body hair in the first place.

Mammals need body hair to keep warm, and lose it only for special evolutionary reasons. Whales and walruses shed their hair to improve speed in their new medium, the sea. Elephants and rhinoceroses have specially thick skins and are too bulky to lose much heat on cold nights. But why did humans, the only hairless primates, lose their body hair?

One theory holds that the hominid line went through a semi-aquatic phase -- witness the slight webbing on our hands. A better suggestion is that loss of body hair helped our distant ancestors keep cool when they first ventured beyond the forest's shade and across the hot African savannah. But loss of hair is not an unmixed blessing in regulating body temperature because the naked skin absorbs more energy in the heat of the day and loses more in the cold of the night.

Dr. Mark Pagel of the University of Reading in England and Dr. Walter Bodmer of the John Radcliffe Hospital in Oxford have proposed a different solution to the mystery and their idea, if true, goes far toward explaining contemporary attitudes about hirsuteness. Humans lost their body hair, they say, to free themselves of external parasites that infest fur -- blood-sucking lice, fleas and ticks and the diseases they spread...

... others could take more convincing. ''There are all kinds of notions as to the advantage of hair loss, but they are all just-so stories,'' said Dr. Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York.

Causes aside, when did humans first lose their body hair? Dr. Rogers, of the University of Utah, saw a way to get a fix on the date after reading an article about a gene that helps determine skin color. The gene, called MC1R, specifies a protein that serves as a switch between the two kinds of pigment made by human cells. Eumelanin, which protects against the ultraviolet rays of the sun, is brown-black; pheomelanin, which is not protective, is a red-yellow color.

Three years ago Dr. Rosalind Harding of Oxford University and others made a worldwide study of the MC1R gene by extracting it from blood samples and analyzing the sequence of DNA units in the gene. They found that the protein made by the gene is invariant in African populations, but outside of Africa the gene, and its protein, tended to vary a lot.

Dr. Harding concluded that the gene was kept under tight constraint in Africa, presumably because any change in its protein increased vulnerability to the sun's ultraviolet light, and was fatal to its owner. But outside Africa, in northern Asia and Europe, the gene was free to accept mutations, the constant natural changes in DNA, and produced skin colors that were not dark.

Reading Dr. Harding's article recently as part of a different project, Dr. Rogers wondered why all Africans had acquired the same version of the gene. Chimpanzees, Dr. Harding had noted, have many different forms of the gene, as presumably did the common ancestor of chimps and people.

As soon as the ancestral human population in Africa started losing its fur, Dr. Rogers surmised, people would have needed dark skin as a protection against sunlight. Anyone who had a version of the MC1R gene that produced darker skin would have had a survival advantage, and in a few generations this version of the gene would have made a clean sweep through the population.

... Dr. Rogers and two colleagues, Dr. David Iltis and Dr. Stephen Wooding, calculate that the last sweep probably occurred 1.2 million years ago, when the human population consisted of a mere 14,000 breeding individuals. In other words, humans have been hairless at least since this time, and maybe for much longer...

... From 1.6 million years ago the world was in the grip of the Pleistocene ice age, which ended only 10,000 years ago. Even in Africa, nights could have been cold for fur-less primates. But Dr. Ropers noted that people lived without clothes until recently in chilly places like Tasmania and Tierra del Fuego.

Chimpanzees have pale skin and are born with pale faces that tan as they grow older. So the prototype hominid too probably had fair skin under dark hair, said Dr. Nina Jablonski, an expert on the evolution of skin color at the California Academy of Sciences. ''It was only later that we lost our hair and at the same time evolved an evenly dark pigmentation,'' she said.

... Humans have the distinction of being host to three different kinds: the head louse, the body louse and the pubic louse. The body louse, unlike all other kinds that infect mammals, clings to clothing, not hair. It presumably evolved from the head louse after humans lost their body hair and started wearing clothes.

Dr. Stoneking, together with Dr. Ralf Kittler and Dr. Manfred Kayser, report in today's issue of Current Biology that they compared the DNA of human head and body lice from around the world, as well as chimpanzee lice as a point of evolutionary comparison. From study of the DNA differences, they find that the human body louse indeed evolved from the louse, as expected, but that this event took place surprisingly recently, sometime between 42,000 and 72,000 years ago. Humans must have been wearing clothes at least since this time.

Modern humans left Africa about 50,000 years ago. Dr. Stoneking and his colleagues say the invention of clothing may have been a factor in the successful spread of humans around the world, especially in the cooler climates of the north...

So it ends up that the state of the art isn't that different from my uninformed musings. We really don't know why other mammals stuck with fur, and why that wasn't an option for us. I don't buy the tick explanation, if that were it other mammals would be nekkid.

I'll stick with the running, sweating, brainy bit.

1 comment:

DDeden said...


Pagel is wrong, since hairy orangutans have no lice while humans have 3 species. All human hair except the eyelashes appears to have been affected by a daily regimen of daily diving/wading foraging for shore foods about a million years ago. Elephants, rhinos and hippos lost their fur coat and got big while living amphibious foraging habitats, like semi-aquatic tapirs and babirusas.