Why don't I have two left feet?
November 28, 2012 4:50 PM   Subscribe

Why don't I have two left feet? or two left hands for that matter? I can see why the basic body plan makes sense, having legs and arms distributed as we have, but why exactly are the limbs mirror images of each other, instead of being exactly the same limb just repeated? What is the evolutionary logic behind having right and left hands?

I see that there are previous questions that ask why bilateral symmetry versus some other symmetry (like radial symmetry) but I am wondering more specifically why that body plan is mirror symmetric, and where that would come from.
posted by selenized to Science & Nature (12 answers total) 4 users marked this as a favorite
The evolutionary logic is that a foot doesn't work as well backwards, nor does a hand.

The more apt reason can be found in embryo development. Cell division doesn't work such that a second left limb can be formed by stem cells. (You're essentially a complicated tube, you know.) Or put another way-- bilateral symmetry is the repeated limb, since similarity is all relative to the central axis.

Taking your question to the extreme wouldn't just stop at limbs (why have two different forms of symmetry?); you'd just have an exact duplicate of the entire left side of your body, which couldn't function (or be formed from a zygote).
posted by supercres at 5:00 PM on November 28, 2012 [1 favorite]

More: Eclectic Reflections on Biological Asymmetry.
posted by supercres at 5:03 PM on November 28, 2012

As supercres says, this has to do with how bilateral symmetry is established in the developing embryo. This paper does a much better job of explaining it than I could:
In bilateral organisms there is a cell division that results in two daughter cells with identical control states, but with opposite orientations along the axis perpendicular to the future plane of division. That axis of orientation is perpendicular to the plane of symmetry. The same genome is then interpreted in exactly the same way, but since the orientation of the daughter cells is now in opposing directions along the axis of orientation that is perpendicular to the plane of division, the encoded developmental directives have the opposite eff ect. Since the orientation of the cell is epigenetically inherited, this results in each half of the organism mirroring the other half.
SATAN AT WORK: My 666th Ask Metafilter comment is about evolution and developmental biology

\m/ \m/
posted by pullayup at 5:06 PM on November 28, 2012 [13 favorites]

Feet and hands evolved in the forms they did because of the functions they serve.

For example: balance. A foot shaped in a certain way, with its toes in a certain configuration -- and when it's mirrored on the other side --provides optimum balance for the critter it supports. In many animal tasks, from grasping tree limbs to loping over the veldt to catching and eating seals, mirrored symmetry is a successful structure.

That's in no way counterposed to the embryonic development explained above. Rather, it's one of the evolutionary advantages that explain why that particular embryonic development, as opposed to some other, went forth and multiplied.
posted by LonnieK at 5:32 PM on November 28, 2012

Just from the standpoint of mechanics, the same "design" can be used if each side of the body is a mirror image of the other. On the other hand (heh) if both of your arms looked like your left arm, one would be joined to your body on its right side and one on its left side.

So on your actual left arm your axillary artery would be at the armpit where it's supposed to be whereas on the other side the axillary artery would be on the outside of the arm and would need a different arrangement to route it into your torso. The same with nerves, bones, etc.
posted by XMLicious at 6:05 PM on November 28, 2012

Because of the hox genes! Very readable and interesting book Your Inner Fish goes into it in a way reasonably accessible to the layman. Or at least reasonbly accessible to me!
posted by Eyebrows McGee at 6:27 PM on November 28, 2012 [1 favorite]

Well if you're thinking about it from an evolutionary perspective, you could start with primitive fish, which show maybe the first signs of body symmetry (primitive worms are just tubes so let's discount them and before that there were the spiral-shelled nautilus/ammonites which also have an interesting story).

With a backbone (which itself is symmetrical from left to right), used to propel the body forward, either you can move slowly like a worm, and create some directional stability by curving your body in one plane, or like a fish, evolve (through natural selection) left and right fins (and a symmetrical tail) to help with directional stability at higher speeds (no need to use the body itself as a slow, directional stabiliser. Fish which had one fin larger than the other, just one fin, or two left fins would have less directional stability at speed, and therefore be more likely to be caught by predators, and therefore less likely to procreate and therefore again, less likely pass on their genes (which are poorly adapted to their environment). So symmetrical fish fins evolved.

This is the basis of all symmetry in nature - survival of the fittest (best adapted to be able to pass on genes). Fish, then the mud skippers, then the land dwellers evolved from here on, with the advantage that again their symmetry either enabled them to be the best at out running the predators or the best at catching prey because they could swim, run or fly the fastest. This means that their offspring will probably have the same "best survivor" genes. A "two-left-footed" cheetah is not going to catch a zebra and will starve to death, and the animal kingdom's females are very choosey about their partners and you have to be a strong male to have access to the females. People are perhaps the pinnacle of evolution (OK, sharks are pretty damn amazing too), and in just the same way, symmetry has been an evolutionary advantage that we see today. Of course it's not just legs, it's arms, its binocular eyes, stereo ears that all help us survive. Interesting this symmetry is even a key factor in beauty as the most beautiful faces are usually the most symmetrical (if you fit the perfect facial mask (it's possible to be symmetrically ugly I suppose, but not so likely)).

Some animals' bodies have evolved asymmetrically to suit their environment. It might be worth looking into snakes' lungs (one big and one small to, fit in their body shape), Not sure of the evolutionary reasons behind internal organ symmetry - perhaps redundancy?). Check out those crabs with one big claw and one small one. The big one has evolved into a genetic health display object, while the small claw remains a functional one. The flat fish (turbot, sole) have evolved from a symmetrical form to an asymmetrical one over time as those that could better see predators maybe had some eye defect that actually became an evolutionary advantage when they lay on the sea floor of being able to see a predator with both eyes. That's why they have two eyes on one side of their body.

Sorry for the ramble, I just love evolution and the animal kingdom.
posted by guy72277 at 1:35 AM on November 29, 2012

Supercres and pullyaup have it-- you have bilateral symmetry because that's the way the mechanisms of embryonic development work. The key evolutionary moment is the original developmental burp that produced a mirrored bilateral organism instead of whatever body plan came immediately before (presumably, simple asymmetry); so the only fitness question worth considering is what adaptive advantages a bilateral organism might have over an asymmetric one.

Evolution isn't teleological: it doesn't have any long-term plan where it weighs all the imaginable options and selects the very best ones to create fit organisms, it just temporarily favors any advantageous trait that may accidentally arise. So unless we know that at some point mirror-symmetric organisms actually competed against repeat-unit bilateral organisms, and won, then it doesn't make much sense to ask "Why not two left hands?." The "why" is that it just didn't happen that way: a two-left-handed version of that ancestral organism simply never arose, or it arose well after the right-hand/left-hand version had already gained predominance, or the mechanisms of its production also turned out to result in other, disadvantageous traits. No rational path, just a lot of highly contingent local outcomes, which in this case didn't turn out to include two left hands.
posted by Bardolph at 10:42 AM on November 29, 2012 [2 favorites]

posted by stenseng at 12:24 PM on November 29, 2012

Note that your internal organs are less symmetric: your left and right lungs have two and three lobes, respectively, and your liver is on your right side, and so on. Your external symmetry is carefully selected for.
posted by fantabulous timewaster at 3:32 PM on November 29, 2012

An interesting aside is that there are, by virtue of the way that development of the vertebrate limb and hand is controlled, ways that "alternate" hand and finger configurations can arise when the normal pattern is disrupted. This usually involves the mirroring of one side of the extremity or some other failure of the hand's asymmetry, and some of the disorders are common enough to be immediately recognizable (ectrodactyly, "mirror hands" and the nightmare-inducing triphalangeal thumb, which leads to thumbs which closely resemble the other four fingers, are good examples). These hands can be more or less functional, and I can certainly wave my own and say that it's not impossible that they could be adaptive under certain circumstances--chameleons, for example, have a similarly bifurcated extremity which is good for hanging on to branches, though I honestly have no idea if it arose from the kind of developmental variation that leads to ectrodactyly in humans.

However, note that even though these variant hands and feet are often more similar to each other (I'm having trouble coming up with an excellent way to describe this, but imagine trying to distinguish the left and right hands of an individual with triphalangeal thumbs), they are still chiral, still mirrored along the organismal axis of bilateral symmetry, for the reasons noted above.
posted by pullayup at 5:19 PM on November 29, 2012

Wow, lots of good answers here.

And the triphalangeal thumbs are going to haunt my dreams forever.
posted by selenized at 5:20 PM on November 30, 2012

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