Eight (or Fewer) Little Piggies

Why do we and most other tetrapods have five digits on each climb?

by Stephen Jay Gould

Richard Owen, England's greatest vertebrate anatomist during Darwin's generation, developed the concept of an archetype to explicate the evident similarities that join us with frogs, flamingoes, and fishes. (An archetype is an abstract model constructed to generate the entire range of vertebrate design by simple transformation of the all-inclusive prototype.) Owen was so pleased with his conception that he even drew a picture of his archetype, engraved it upon a seal for his personal emblem, and in 1852, wrote a letter to his sister Maria, trying to explain this arcane concept in layperson's terms:

It represents the archetype, or primal pattern--what Plato would have called the "divine idea" on which the osseous frame of all vertebrate animals--i.e., all animals that have bones--has been constructed. The motto is "the one in the manifold," expressive of the unity of plan which may be traced through all the modifications of the pattern, by which it is adapted to the very habits and modes of life of fishes, reptiles, birds, beasts, and human kind.

Darwin took a much more worldly view of the concept, substituting a flesh and blood ancestor for a Platonic abstraction from the realm of ideas. Vertebrates had a unified architecture, Darwin argued, because they all evolved from a common ancestor. The similar shapes and positions ancestor. The similar shapes and positions of bones record the historical happenstance of ancestral form, retained by inheritance in all later species of the lineage, not the abstract perfection of an ideal shape in God's realm of ideas. Darwin burst Owen's bubble with a marginal note in his personal copy of Owen's major work, On the Nature of Limbs. Darwin wrote: "I look at Owen's archetype as more than idea, as a real representation as far as the most consummate skill and loftiest generalization can represent the parent form of the Vertebrata."

However we construe the concept of an organizing principle of design for major branches of the evolutionary tree--and Darwin's version gets the modern nod over Owen's--the idea remains central to biology. Consider the subset of terrestrial vertebrates, a group technically called Tetrapoda, or four legged (and including amphibians, reptiles, birds, and mammals in conventional classifications). Some fly, some swim, and others slither. In external appearance and functional role, a whale and a hummingbird seem sufficiently disparate to warrant ultimate separation. Yet we unite them by skeletal characters common to all tetrapods, features that set our modern concept of an archetype. Above all, the archetypal tetrapod has four limbs, each with five digits--the so-called pentadactyl (five-fingered) limb.

The archetypal concept does not require that each actual vertebrate display all canonical features, but only that uniqueness be recognized as extreme transformation of the primal form. Thus, a whale may retain but the tiniest vestige of a femur, only a few millimeters in length and entirely invisible on its streamlined exterior, to remind us of the ancestral hind limbs. And although a hummingbird grows only three toes on its feet, a study of embryological development marks them as digits two, three, and four of the full ancestral complement. The canonical elements are starting points and generating patterns, not universal presences.

In the tetrapod archetype, no feature has been more generally accepted than the pentadactyl limb, putative source of so many deep and transient human activities, from piano playing to touch typing, duck shooting, celebratory "high fives," and decimal counting (twice through the sequence of "this little piggy ... ". Yet this essay will challenge the usual view of such a canonical number, while not denying its sway in our lives.

A reconstruction of Ichthyostega shows the early tetrapod with five digits on each limb.