I’m sure this topic has been covered by scientists before, but I haven’t researched it, so I’m raising it as a naive question.
First, it’s easy for you to tell up from down because down is where your feet are and up is what you see when you look away from your feet and toward the sky. Or you could drop something; the direction it falls is “down”.
It’s also easy for you to tell your front from your back. Your front is what you see when you look down, and the other side of your body is your back.
But how do you tell right from left at any given moment?
Now of course there are a number of cues that we could use to tell right from left. The side our heartbeat is most detectable by touch is on the left (unless you have situs inversus!), I wear my watch on my left wrist and my ring on my right hand, and so on. If you drive a car in the US, the steering wheel is on the left side.
But we don’t actually use these cues. When someone tells you “turn right” when you’re asking directions, you just know which way to go. But HOW?
Presumably we learn right from left when we’re kids: a parent presumably points out your right hand and says “that’s the right hand” and vice versa. But again, what cues do we use now? Surely not the hands! (I’m sure the answer is out there somewhere, but if a reader provides it, many of us will have learned something.)
That’s my question, but it’s related to a genetics question that I pondered for years before any answer was ever given. It’s about asymmetry in animals. There are basically two types of ways a bilaterally symmetrical animal can be asymmetrical in some ways. I’ve posted on this three times before (here ,here, and here), so have a look at those posts. Here’s just a brief summary.
1.) Fluctuating asymmetry. Individuals are asymmetrical for some features, but the direction of asymmetry varies from individual to individual. Handedness in humans is this way, though it has a genetic component, too, making right-handed people more common. Lobsters have asymmetrical claws: one is a “cutter” and the other a “crusher”, and it’s random whether the crusher claw is on the right or left. (We know, by the way, how this comes about. Young lobsters start their lives with identical claws, but the claw that is used most often provides more neurological activity, and that activity irrevocably creates the asymmetry, which lasts for life.The most-used one becomes the grinder.) Some species of flounders are randomly flat on the left or right sides, though all start off being vertically postured fish who develop into flat fish, with the eye on the bottom migrating to the top. Many human facial features are examples of fluctuating asymmetry: the right sides of our faces are not the same as the left, but the kind of differences differ in direction from person to person. Fluctuating asymmetry is also called “anti-symmetry” since the sides are different, but not in a consistent direction.
2.) Directional asymmetry. This is what always puzzled me. There are some basically bilaterally symmetric animals, like us, in which there are some asymmetries that are directional. That is, the right side always differs from the left in a consistent way. The narwhal tusk (a hyper-developed canine tooth) is always on the left side, some owls use directionally asymmetrical ears as a way to locate prey, I’ve mentioned the human heart before, and there are many examples. (In some flounder species, individuals are always right-flat, while individuals of other species are left-flat.)
The question I always had about this rests on the observation that because every individual is directionally asymmetrical the same way, that asymmetry must somehow rest on genes for those traits that are active in development. But how does a gene know it’s on the right or left side so it can turn on or off? Given a bilaterally symmetrical individual, it’s easy to genetically specify “front” and “back”, and “up and down”, but once those are specified, then the internal features of the organism should be identical on the right and left side. So how does a gene for say, hyper-development of the canine tooth “know” that it’s on the left side to become activated? There has to be some consistent physiological or metabolic difference between the right and left sides of an animal to provide the relevant developmental cues. But how could that occur?
We’re beginning to find out now, though we’re far from a complete understanding of the phenomenon. There are two suggestions I know of, based on either the asymmetry in the way embryonic cilia beat (causing an asymmetry in the flow of embryonic fluid) or in the “handedness” of our constituent amino acids. I describe these in the second post I wrote in the series.
Of course, once a single directional asymmetry has evolved in an animal or plant, then the evolution of further directional asymmetries can evolve using developmental cues provided by the first one.
But this is irrelevant to the question above, so I repeat it:
How do you know the difference between left and right?