I recently received the following question:

Why do we, and animals generally, have single or twin body parts, but not not triple or quadruple ones? Surely three eyes are better than one. (Four legged animals tend to have two front legs the same and two back legs the same) . And why is one side of the body a mirror image of the other anyway?. Why dont we just have two lungs or two eyes that look exactly the same?

This is an interesting question. First, however, I have to address the premise that animals in general display bilateral symmetry. This is mostly true for vertebrates, but not for many invertebrates. Sea stars, for example show radial symmetry, and have 5 or more radially symmetrical arms. Spiders have 8 legs and more than two eyes.

Most animals (the vast majority) do have some form of symmetry. Sponges are a notable exception.

Bilateral symmetry if found in several phyla of animals – Platyhelminthes, Arthropoda, Annelida, Mollusca and Chordata. Bilateral symmetry occurs in most phyla in just one plane, which is called the sagittal plane. If you imagine a human, there is only one plane that can bisect a person and create two halves that are essentially mirror images of each other.

Internal organs are more complex. Some come in bilateral pairs – lungs, kidneys, eyes, ears; while others are roughly midline and single, like the heart, brain, and bladder. Still others are one one side and single, like the liver or spleen.

Bilateral symmetry has mostly to do with developmental biology. As cells divide chemical gradients determine how cells specialize, migrate, further reproduce, and die. By necessity certain geometric and mathematical principles will apply.

Remember – development does not follow a blueprint. Cells are not placed like bricks in a building. Rather genetic instructions are a list of processes. Structure emerges from those processes. Repetitive processes, gradients, feedback loops, and geometrical relationships necessarily follow mathematical principles.

Bilateral symmetry is simply how our phylum evolved. It is so deeply embedded in our genetic instructions, in fact, that biologists find it very difficult to break this symmetry – to mess with the genes in order for one side of a bilaterally symmetrical creature to look different from the other side. Related to this is the fact that bilateral symmetry emerges very early on in development – matched pairs of cells called somites form in the very early embryo. This is the origin of much of the vertebrate bilateral symmetry.

Also keep in mind that the bilateral symmetry mainly applies to external morphology. Internal organs, as I noted above, show significant asymmetry. This is because organ development involves processes that break symmetry. This may involve, for example, the action of cilia in tissue migration.

In short, symmetry vs asymmetry derive from underlying developmental processes. These are, of course, complex and not fully understood, but developmental biologists have identified a number of developmental processes from which some form of symmetry spontaneously emerges.