Free Eight Little Piggies by Stephen Jay Gould Page B

that the newly discovered early tetrapod Tulerpeton , also of latest Devonian age, bore six digits on its limbs. This find led anatomist and embryologist J. R. Hinchliffe to suggest in 1989, prophetically as we have just learned, that five digits represents a secondary stabilization, not an original state. Hinchliffe entitled his article “Reconstructing the archetype: Evolution of the pentadactyl limb,” and ended with these words: “Restriction to the pentadactyl form may have followed an evolutionary experimental phase.”
    Hinchliffe’s suspicion has now been confirmed—in spades. In October 1990, M. I. Coates and J. A. Clack reported on new material of Ichthyostega and Acanthostega , collected by a joint Cambridge-Copenhagen expedition to East Greenland in 1987 (see bibliography). Some remarkable new specimens—a complete hindlimb of Ichthyostega and a forelimb of Acanthostega —permit direct counting of digits for the first time.
    In an admirable convention of scientific writing that maximizes praise for past work done well and minimizes the disturbing impact of novelty, Coates and Clack write: “The proximal region [closest to the body] of the hindlimb of Ichthyostega corresponds closely with the published description, but the tarsus [foot] and digits differ.” In fact, the back legs of Ichthyostega bear, count ’em, seven toes!—with three smallish and closely bound digits corresponding to the hallux (“big toe” in human terms) of ordinary five-toed tetrapods (see figure). Acanthostega departs even more strongly from a model supposedly common to all; its forelimb bears eight digits in a broad arch of increasing and then decreasing size (see figure).
    The conclusion seems inescapable, and an old “certainty” must be starkly reversed. Only three Devonian tetrapods are known. None has five toes. They bear, respectively, six, seven, and eight digits on their preserved limbs. Five is not a canonical, or archetypal, number of digits for tetrapods—at least not in the primary sense of “present from the beginning.” At best (for fans of pentadactyly) five is a later stabilization, not an initial condition.
    Moreover, in the light of this new information, an old fact may cast further doubt on the primacy of five. The naive “ladder of life” view depicts vertebrate evolution as a linearly ascending series of amphibian—reptile—mammal—human (with birds as the only acknowledged branch). But ladders are culturally comforting fictions, and copious branching is the true stuff of evolution. Tetrapods had a common ancestor to be sure, but modern amphibians (frogs and salamanders) represent the termini of a large branch, not the inception of a series. Moreover, no fossil amphibian seems clearly ancestral to the lineage of fully terrestrial vertebrates (reptiles, birds, and mammals), called Amniota to honor the “amniote” egg (with hard covering and “internal pond”), the evolutionary invention that allowed, in our usual metaphors, “complete conquest of the land” or “true liberation from water.” (The point is tangential to this essay, but do pause for a moment and consider the biases inherent in such common “descriptions.” Why is the ability to lay eggs on land a “liberation” is water tantamount to slavery? Why is exclusive dwelling a “conquest”? Who is fighting for what? Such language only makes sense if life is struggling upward towards a human pinnacle—the silliest and most self-centered view of evolution that I can imagine.)

    Left: forelimb of Acanthostega with 8 digits. Right: hindlimb of Ichthyostega with seven digits. Reprinted by permission from Nature vol. 347, p. 67; Copyright © 1990 Macmillan Magazines Limited .
    The first fossil reptiles are just about as old as the first amphibians clearly in the group that eventually yielded our modern frogs and salamanders. Thus, rather than a ladder from amphibian to reptile, both the fossil record and the study of modern