To many of us, the bilateral symmetry that characterizes the human face (Fig. 1) and the animal kingdom may be one of the first impressions we get of the world. From babies to butterflies, cats to crocodiles, if you reflect the left half of the face or body in a mirror, you obtain something that closely resembles the right half. Bilateral symmetry is so prevalent that it could hardly be due to chance. If you think of the human body as a collection of some ten thousand trillion trillion molecules, then there are infinitely more ways to combine them into non-symmetrical shapes than into symmetrical ones. Why is it, then, that bilateral symmetry is so common? Given that all life on Earth slowly evolved through natural selection, the fact that we and many animals possess left-right symmetry, not, say, up-down or front-back symmetry, must have somehow conferred certain advantages. But how?
Figure 1. The human face is nearly bilaterally symmetrical. Accessories that are not symmetrical tend to disturb us. (Credit: J. Bedke)
We can understand at least some parts of this natural "engineering" puzzle by considering some simple physical principles. First, on the surface of the Earth, not all the directions are the same -- the Earth's gravity introduces a marked difference between up and down. Animals had to be able to remain stable and to cope with their weight under Earth's relentless downward pull. The ability to move further enhanced the difference between top and bottom (the dorsal and ventral surfaces). On one hand, birds had to handle the aerodynamics of flying, and on the other, be able to handle landing. Land animals had to develop some "mechanical" appendages (in other words, legs) to allow them to transport themselves efficiently.
Animal locomotion was probably also responsible for the front-back (anterior-posterior) asymmetry. Whether it was a bird, a fish, or a lion, having the sensory organs and the mouth in the front could mean the difference between finding lunch or missing it. The digestive and reproductive systems were pushed to the rear. Having eyes, ears, and noses in the front was also crucial for the avoidance of predators, and for the ability to explore an area before actually entering it. Life forms that are anchored in one place or that move very slowly, such as trees or jellyfish, tend to have a different kind of symmetry, similar to that of a cone -- they produce similar reflections in any mirror passing through their central axis (Fig. 2).
Figure 2. Catostylus species. Stingless jellyfish from Palau, Micronesia. (Photo by Lee R. Berger)
So far I have only explained what might have led to the asymmetries, but what caused the left-right symmetry? We don't actually know the precise answer to this question, but we can identify a number of elements that could have made bilateral symmetry advantageous. First, any departure from bilateral symmetry would have created an imbalance that would have been an impediment to straight motion, be it on the ground, in water, or in the air. Second, bilateral symmetry is both economical -- you essentially get two organs for the "price" of one -- and important for such things as depth perception and peripheral orientation. Mirror symmetry also favors the formation of a central nerve center, and the collection of sensory nerve cells at the anterior end eventually became the forerunner of the brain.
Note that in contrast to the symmetry in the external bodies and faces of humans and many animals, neither the internal anatomy nor the functions of the brain possess bilateral symmetry. This may reflect an economical evolution that, in this case, avoided unnecessary duplication. Amusingly, telescopes exhibit a similar dichotomy; the mirrors of telescopes such as Hubble and Webb are symmetric, while the internal instrumentation is not. Finally, an interesting question is whether we would expect complex extraterrestrial life to also be bilaterally symmetric. Given the universality of the laws of gravity and motion, life forms on extrasolar planets would have to face the same environmental challenges that life on Earth had to. Therefore, my guess is that E.T. is most likely ambidextrous and symmetric (as indeed depicted in the movie by Steven Spielberg, E.T.: The Extra-Terrestrial).