Eight Little Piggies

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Authors: Stephen Jay Gould
structure and evolution with a telling point about pentadactyl limbs and human possibilities. He noted how many “advanced” mammals modify the original pattern by loss and specialization of digits—horses retain but one as a hoof; whales practically lose the whole hind limb. Jarvik noted that an essential coupling of a multidigited hand, fit for using tools, with an enlarging brain, well suited to devising new and better uses for such technology, established the basis and possibility of human evolution. If the ancestor of our lineage had lost the original flexibility of the “primitive” pentadactyl limb and evolved some modern and specialized reduction, human intelligence would never have developed. In this important sense, we are here because our ancestors retained the full archetypal complement of five and had not substituted some new-fangled, but ultimately more limiting, configuration. Jarvik writes:
    The most prominent feature of man is no doubt his large and elaborate brain. However, this big brain would certainly never have arisen—and what purpose would it have served—if our arm and hand had become specialized as strongly as has, for instance, the foreleg of a horse or the wing of a bird. It is the remarkable fact that it is the primitive condition, inherited from our osteolepiform ancestors [fishes immediately ancestral to tetrapods] and retained with relatively small changes in our arm and hand, that has paved the way for the emergence of man. We can say, with some justification, that it was when the basic pattern of our five-fingered hand for some unaccountable reason was laid down in the ancestors of the osteolepiforms that the prerequisite for the origin of man and the human culture arose.
    I don’t dispute Jarvik’s general point: The retention of “primitive” flexibility is often a key to evolutionary novelty and radiation. But is the five-fingered limb a constant and universal tetrapod archetype, interpreted in Darwin’s evolutionary way as an ancestral pattern retained in all descendant lineages?
    Erik Jarvik is maximally qualified to address this question (his rationale, of course, for raising it in the first place), for he has done by far the most extensive and important research on the earliest fossil tetrapods—the bearers and perpetrators of the five-fingered archetype in any evolutionary interpretation. (Fish fins are constructed on different principles, although the lobe-finned ancestors of tetrapods built a bony architecture easily transformable to the fore and hind limbs of terrestrial vertebrates. In any case, fish do not display the pentadactyl pattern, and this central feature of canonical design arose only with the evolution of the Tetrapoda.)
    The oldest tetrapods were discovered in eastern Greenland by a Danish expedition in 1929. They date from the very last phase of the Devonian period, a geological interval (some 390 to 340 million years ago) often dubbed the “age of fishes” in books and museum exhibits that follow the silly chauvinism of naming time for whatever vertebrate happened to be most prominent. The Swedish paleontologist Gunnar Säve-Söderbergh collected more extensive material in 1931 and directed the project until his untimely death in 1948. Erik Jarvik then took over the project and, during the 1950s, published his extensive anatomical studies of two genera that share the spotlight of greatest age for tetrapods— Ichthyostega and Acanthostega . Although no specimens preserved enough of the fingers or toes for an unambiguous count, Jarvik (see figure) reconstructed the earliest tetrapods with the canonical number of five digits per limb.

    The standard reconstruction of Ichthyostega from Jarvik’s 1980 book. Note the five digits on each limb. From Basic Structure and Evolution of Vertebrates, vol. 1, p. 235 .
    Our confidence in this evidence-free assumption of an initial five began to crumble in 1984, when the Soviet paleontologist O. A. Lebedev reported

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