The great Swedish paleontologist
Erik Jarvik closed his two-volume magnum opus on vertebrate 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 lose practically
the whole hind limb. Jarvik noted that an essential coupling of a multidigited
hand, fit for wing 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 newfangled, but ultimately more limiting, configuration.
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?
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.
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 forelimbs and hind limbs of terrestrial
vertebrates. In any case, fish do not display the pentadactyl pattern,
and this central feature of canonical design arises only with the evolution
of the Tetrapoda.)
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 research 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 reconstructed the earliest tetrapods with
the canonical number of five digits per limb.
Our confidence in this evidence-free assumption of an initial five
began to crumble in 1984, when the Soviet palcontologist O. A. Lebedev
reported 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 represent 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. Just
last September, 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 ("Polydactyly
in the Earliest Known Tetrapod Limbs," Nature, September
6, 1990, pp. 66-69). Some remarkable new specimens--a complete hind
limb 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. 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
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 naïve "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"; why 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
toward a human pinnacle--the silliest and most self-centered view of evolution
that I can imagine.)
The first reptile fossils are just about as old as the first amphibians
in the group that eventually yielded our modern frogs and salamanders.
Thus, rather than a lad-