Now that we are in the middle of summer here in the northern hemisphere, all thoughts of our seasonally affective disorder syndrome, SADS, are gone for the moment. Presumably we are all feeling relatively buoyant, or at least as buoyant as the world wide financial shenanigans will allow. The effect of light levels on our sense of well-being and mood is shared with most species, whether diurnal or nocturnal. Birds and primates that spend a lot of time in low light levels, have larger eyes than those addicted to the bright lights.
Of course, the amount of light we get depends on our latitude. Pearce and Dunbar of U of Oxford have been hard at it measuring eye-socket size and cranial volumes of collections of skulls from populations all over the world (1). Bigger eyes can collect more optical information and that greater amount of information requires more processing equipment.
It seems that in the last 10,000 years human eyes have increased in volume by about 15% for those living in the Scandinavian countries compared to those living close to the equator. The brain volume, required for that extra processing, has increased by almost 8%. The visual acuity is the same, so size is not an advantage. It just indicates our need to see what is coming down the pike in the dull, grey northern climes.
The surprising thing is the speed of this evolutionary change. Until the last ice age receded, humans weren’t whooping it up around the Arctic Circle. They only moved up after the ice, which is were the 10,000 years comes in.
We all rush to live in cities. Already more than half the world’s population is urbanized. The reason? As the size of a human agglomerate doubles in size, the wealth, health and benefits increase by 15% as, incidentally, so does crime. In the distant future when all citizens are in cities and city dwellers demand bright lights all the time, will we all end up with beady eyes and smaller heads like Henry Moore sculptures? Or maybe we should wear sunglasses all the time and keep our big heads.
