Friday, October 26, 2007

Colour Riddles

There's an old riddle that runs like this:
A hunter sets off from his base camp. He travels one mile south, sees bear tracks, follows them for one mile east, finds and kills the bear. He skins it and carries the skin back to his camp, which is one mile to the north.

What is the colour of the bear's fur?
You are supposed to reason that if a trip of one mile south, one mile east, and then one mile north brings the hunter back to his base camp then he must have started from the North Pole, and so the bear must be a Polar bear, and hence its fur white. Of course Polar bears don't range that far north , but global warming is rapidly depleting the Arctic ice pack, and perhaps one day there will be Polar bears near the North Pole, searching for enough ice to stand on.

Nit-pickers have pointed out that you can also choose a starting point near the South Pole such that after you have travelled one mile further south, a trip of one mile east will take you in a complete circle round the South Pole (or two or more whole circuits if you started closer) back to where you started circling, and the one mile leg back north would then take you back to where you started. But there are no bears at all in the Antarctic, so that doesn't fit the riddle's premises. Perhaps, if the Arctic ice pack thaws out completely, we will transport some Polar bears to the Antarctic; it's solid ground near the pole, with a high enough altitude that it will stay cold far longer than the North Pole will.

I have devised another "colour" riddle for readers to ponder:
The scene is a valley in what is now western Germany, 10,000 years ago. An ice age has the northern hemisphere in its grip; although the season is late spring, snow still blankets the land. A curl of smoke issues from the mouth of a cave, which is largely closed off by hanging hides. A weak sun shines through thin cloud. A group of men straggle out of the cave. They wear thick furs; only their faces are exposed to the cold weather. They carry bows, arrows, and spears. They cast about, looking for animal tracks in the snow. Soon they find some deer tracks, and set off in pursuit of their prey. Almost all the men walk with some difficulty; their legs are badly bowed, they walk with bandy gaits. But the leader of the group has straight legs, and walks easily and quickly. After a while they close in on their prey carefully; they unleash a volley of arrows, and the deer is fatally wounded. It swiftly bounds away, but quite soon weakens and slows. The group leader runs fast and closes in on the deer. He uses his spear to kill it. The rest of the group join him. They exult in their success. They quickly butcher the deer into sections with their sharp flint knives so that they can transport the meat back to their cave. The leader has done more than the others to secure their quarry, and so he takes a larger share than the others. He and his family will eat well.

What is the colour of the leader's eyes?
Rickets is a human disease that causes softening of the bones in children, resulting in bowed legs in severe cases. It is caused primarily by a vitamin D deficiency. Our bodies will create all the vitamin D that they need if exposed to sunlight. Early humans who had to live through ice ages spent much of their time in the shelter of caves, and they wore comprehensive fur clothing to keep warm. Archaeological remains tell us that many of them suffered from rickets, often severely.

What has this got to do with the colour of the leader's eyes, you may ask? I am blessed (or cursed) with a fair complexion – a pale pink skin, blue eyes, blond hair when young that darkened somewhat with age, before it fell out. There are hundreds of millions of other humans with my general colouration; we are called Caucasians, which tells you something of our origin, the Caucasus in Russia. While we are many, we are greatly outnumbered by other varieties of human, almost all of which have darker skins, dark hair, and brown eyes.

As a fair person living in Africa, I am vividly aware of how poorly equipped I am to cope with the hot sun. The melanin in my skin, which should protect the deeper layers of skin from the sun's ultraviolet radiation, is dysfunctional; it is clumped into small circular disks called "freckles", and all of the skin between is unprotected. I have often felt the sharp sting of sunburn, and had to deal with the outer layers of my skin peeling off. My blue irises do a poor job of keeping strong sunlight out of my eyes. When the light is bright, much gets through my blue irises, suffusing the scene with glare that washes out the details that I need to see. Hair on heads may benefit us in many different ways; the way that I miss most is to protect my pate from sunburn. Dark hair does a better job of this than does blond hair.

Given my dysfunctional complexion, I have often wondered why it ever persisted when by chance it first arose amongst my forebears. They didn't have the benefit of sun tan lotions, lip balms, dark glasses, and fedora hats. How did they manage to out-compete their darker kin? The riddle that I posed above may give us an explanation. Pale people arose only in cold climes; there were ice ages that made these climes colder still; rickets was a major problem under these conditions; pale people absorb what little sunlight there is more than do darker people, and hence would suffer less from rickets.

Interestingly, it was recently determined that red hair was common amongst the Neanderthal people (see here). They too developed a form of albinism, as did the Caucasians. They too lived in Europe through a number of ice ages. They too suffered from rickets. So far as we can tell, Homo Sapiens did not descend from the Neanderthals, so this is probably an example of parallel evolution; a common solution to a common problem.