Why five?

A few days ago I climbed into bed to find one of Miró’s hind feet nearly in my face. I eased down carefully to avoid waking him. If he gave one of his kangaroo kicks, I’d wind up with a red welt across my cheek—not my preferred method of putting color in my complexion. For no reason at all, I started wondering why dogs have five pads and people have five fingers. Why five and not four or six?


I had to get up to Google the question. The short answer is that we have five because we can’t make six. Our arms and legs, like those of all other four-limbed creatures (tetrapods) evolved from the pectoral and pelvic fins of our fish ancestors. There are genetic constraints on how creative we can get about how we’re formed. As far as I can understand one of the articles I found, the genes that governed fin and, later, limb formation allow for no more than five individual digits. In the rare cases where you find, for instance, six toes, two of the toes are actually divisions of a single digit.

The most readable essay on the topic is still Stephen Jay Gould’s "The Panda's Peculiar Thumb." He notes that we’re made up of odd parts that were adapted from previous uses: “they are familiar bits of anatomy remodeled for a new function.” My kitchen is the same. It has been remodeled and updated over the years but still retains its original configuration when it would be a much more efficient kitchen if the whole thing were torn out and built from scratch to meet its current purpose. But who can afford that?