. . .here's a question that has bothered me: How do we know our right from our left? No, really. The more I've learned (and internalized) about chirality, the more tricky this question gets. (Until you've thought about handedness and non-superimposibility for a while, these things just seem natural, of course. You have to train yourself to get this weird.)

But what we learn in chemistry is that chiral objects cannot be distinguished by an achiral probe. Fro example, you can't use plain silica gel columns to separate enantiomers; you have to pay through the nose for columns with chiral stuff on them. The thing is, we humans use chiral probes every day (our hands,) so we take the ability to discriminate chiral objects for granted. We shouldn't, though, because the next question is: what chiral probe do we use to tell which hand is which?

There, that's the original question restated. We're bilaterally symmetric, right? In stereochemistry terms, we're meso, with our own built-in reflection plane, and we shouldn't be able to distinguish chiral objects. Of course, we're not really that symmetric. We have identifying marks on each arm and hand, usually, that would give the game away. And faces often have the same sort of thing (sometimes a deliberately applied "beauty mark," which is interesting when you consider that research seems to say that the most beautiful perceived faces are the most symmetric ones.)

But I don't think that that's the real answer to my question. Where we really start to lose symmetry is in our internal organs. As everyone knows, the heart is on the left side (except in rare cases!), and the other organs follow suit in their own positions. The organ that I'm thinking of is the brain, which looks rather symmetric, true, but is about as full of handedness as an organ can get. If you're right-handed, as is well known, you do a lot of your verbal processing in your left hemisphere, and a lot of non-verbal work in your right. And your eyes each feed into the crossover hemisphere, (which has allowed some really alarming experiments with brain surgery patients that we'll have to talk about some time.)

And that's where I think the origin of our ability to perceive chirality lies. Our information-processing organ itself is chiral. But let's keep going: it's worth asking how the brain (and the rest of our internal arrangement) got that way, when you consider that we started out from a single cell (which then divided straight down the middle.) A lot of research has gone into answering that, and determining the earliest stage at which the blastula breaks symmetry.

I believe that the latest theory is that molecular signals and growth factors are believed to circulate around outside of the developing cells in a handed fashion, and that this may be the origin of the asymmetry. So where does this chiral flow come from? Well, it's driven by cilia on the cell surface, and it's well known that these always turn in one direction. (The mechanism (PDF file) is fascinating; it's a true molecular motor. You can just picture it as a Victorian-era machine, all polished brass and oiled fittings.) Correction: This is a bacterial flagellum, not a eukaryotic one. Our flagella and cilia work differently, so this picture (though still very interesting) isn't relevant.)

And the components of this machine are all different proteins. Which means that the direction of their motion relative to each other is determined by their three-dimensional shape, which is determined by the twists and turns of their constituent amino acids. . .which are chiral. And we're back to single molecules again.

So that's how you can tell your right from your left, as far as I can see. Simple, really.