The Human Genre Project

Seeing Light

It starts small. But then, everything starts small. The universe itself started small, before the Big Bang, at least compared to any later size it had after.

It starts with perhaps a single cell — but then, everything that lived was single celled. Eukaryotes came later. This single cell has, for the first time ever, evolved a way of noticing differences in light levels. It has, for the first time, found a way out of the universal darkness, into a light both physical and metaphorical.

This is major. It's more than major, it's nigh impossible to overstate the importance of this one evolutionary leap. Almost all life that follows will be descended from this cell — both unicellular and multi-cellular organisms. And all because of a relatively simple piece of genetic code.

Oh, the code will be elaborated upon. Evolution is an iterative, if unguided, process, and its bug handling is unmatched in its ruthlessness. What works lives, what does not dies. It's that simple.

Before long, a simple photoreceptor is making ever finer distinctions between different light levels. No longer detecting merely a simple binary light or dark, but now a myriard of greys. At some point in this process, colour detection arises, merely a side effect at first, but increasingly important as time goes on.

Other evolutions proceed in other directions. There are cells that detect light levels, but use them as nothing more than triggers for chemical reactions. You've seen these everywhere: they make plants green.

Finer distinctions get made between light and dark, between stillness and movement, between reds, greens and blues. The simple piece of genetic code that began so many years earlier detecting only light or dark has elaborated itself, and split into many a specialisation, all descended from this same photoreceptive cell. But the difference between that cell and the millions of different kinds of eye that now exist is incalculable. Seeing colour is a thing we pay dearly for, in evolutionary terms — humanity's comparatively weak sense of smell is the price.

In humans, an entire mythology has grown up about the difference between light and dark, a mythology that endless separates itself as well, starting as simply light vs dark, but mirroring evolution's path in its elaborations and specialisations. Light vs Dark is a primary metaphor in every known culture, and none of it would be possible without the descendants of the original photoreceptive cell.

The basis of these cells resides in the human genome (among many, many other genomes). Like nearly half of the genes that control olfactory reception, they're to be found in the eleventh chromosone pair: the PAX6 (paired box gene 6) gene.

And without them, you couldn't read this.

Loki Carbis