seems all the more enigmatic. The embryological apparatus
remains capable of producing more than five (at least in many species),
as mutational and experimental data show. But these polydactylous mutations
remain as anomalies of individuals or of small and evanescent family lines.
They never stabilize within a larger group, and no vertebrate species has
more than five digits generated from the back-to-front axis of the Shubin
and Alberch model.
The best proof of this assertion lies in apparent (but not
actual) exceptions of several tetrapod species with six functional digits. Yes,
Virginia, several species do grow six fingers as a rule, not as an exceptional
state of mutant individuals. Yet this sixth finger is always generated in a different
manner, and not by the obvious (and apparently easy) mechanics of simple extension
past digit one on the Shubin-Alberch series. Frogs, for example, often have six
digits on their hind feet (or five on their normally four-fingered front feet).
But this extra digit forms in a unique manner by extension of the unbranched sequence
of bones leading out from the radius or tibia--the limb bones that never serve
as foci for branches and therefore do not (in any other tetrapod species) participate
in the production of digits. Anatomists have long recognized the anomalous character
of these unique digits by naming them prepollex (for the forelimb) or prehallux
(for the hind limb). (Pollex and hallux are technical names for digit number one--our
thumb and big toe. Prepollex and prehallux therefore designate an anomalous digit,
located in front of the usual front and formed differently.)A few
mammals also process a functional sixth digit--the panda, whose false "thumb"
has been a staple of these essays, and several species of moles. But these false
thumbs are formed from extended wrist bones and are not true digits at all. These
facts seem to heighten the oddity (and rigidity) of stabilization at five in a
sequence that was once extendable, remains so now for mutations and experimental
manipulations, but seems recalcitrant in setting a maximum of five as a normal
state in all tetrapod species. When six functional digits form, the extra item
must be built in another way.
So why five? Of two major approaches to this question, the conventional
Darwinian, or adaptationist, strategy tries to discern a marked advantage,
or even an inevitability, for five in terms of utility for an organism's
mode of life (a benefit that might promote this configuration by natural
selection). A plausible case can be made in terms of advantages for terrestrial
life. Creatures that evolve from water to land face many novel challenges,
none more severe than the new force of gravity and the consequent need
for support in the absence of buoyancy previously supplied by water. The
transition from fins to limbs provides the basis for this support, and
an old argument holds that five might be an optimal configuration for
weight-bearing--a central axis centered on digit three, with adequate
and symmetrical buttressing on each side (one or three toes might not
provide enough lateral support against wobbling, while seven toes might
be superfluous and interfere with locomotion). On this argument, tetrapods
have five toes because support and locomotion demand (or at least strongly
encourage) this configuration as optimal.
The argument is not implausible and
surely gains credence from the probability that five digits evolved twice--separately,
that is, in the two great divisions of tetrapods. The most obvious counterargument
may also be support in disguise: why, if five is best on land, do the earliest
tetrapods bear six, seven, and eight toes, respectively? A paradoxical retort
holds that these first tetrapods evolved their limbs for locomotion in water and
remained predominantly, if not entirely, aquatic. Ichthyostega, as long
recognized, maintained a small tail fin and lateral-line canals on the skull.
(Lateral-line organs "hear" sound by sensing vibrations propagated through
water, a method that does not work in thin air). Coates and Clack's restoration
of Ichthyostega and Acanthostega limbs adds support to this interpretation
in a streamlined shape and a limit to rotation that might keep the limb horizontal,
in fin position, rather than rotated downward to support a body on land (at least
for Acanthostega, although the Ichthyostega forelimb seems fully
load bearing).But strong elements of doubt also plague this adaptationist
view. First, as stated above, members of one tetrapod lineage, the amphibians,
grow but four toes on their front legs, and we have no evidence for an initial
five--so pentadactyly may not be a universal stage in terrestrial vertebrates.
Second, if five (with symmetry about a strong central toe) is the source of advantage,
then why does our favorite species, the traditional measure of all things--namely
Homo sapiens itself--retain five, require great strength in using but
two limbs against gravity, but construct the end-member first toe as the main
weight bearer? And why do the most successful of all large mammals, the "cloven-hoofed"
artiodactyls, or even-toed ungulates--including cows, deer, giraffes, camels,
sheep, pigs, and their numerous allies--bear an even number of toes, with
the central axis running through a space between the digits (the misnamed "cleft")?The
second major approach--historical contingency in my favored terminology--argues
that five was not meant to be, but just happens to be. Other configurations would
have worked and might have evolved, but they didn't--and five works well enough.
The obvious supports for this alternative view lie scattered throughout this essay.
If five is so good, why do so many species devise such curious and devious means
to produce six (prepollex or converted wrist bone)? If five is so predictable,
why does one of two lineages grow but four? (I should say right up front that
neither of these two positions--adaptation or contingency--really address
the greatest puzzle of all: the recalcitrant stability of five once it evolves.
I suspect that this is a question for embryologists and geneticists; phylogenetic
history may offer little in the way of clues. Why should five, once attained by
whatever route and for whatever reason, be so stubbornly intractable as an upper
limit thereafter--so that any lineage again evolving six or more must do so
by a different path? The inquiry could not be more important, for this issue of
digits is a microcosm for the grandest question of all about the history of animal
life: why, following a burst of anatomical exploration in the Cambrian explosion
some 550 million years ago, have anatomies so stabilized that not a single new
phylum [major new body plan] has evolved since?)But the greatest boost
to contingency lies in the discovery that prompted this essay in the first place--seven
digits in Ichthyostega and eight in Acanthostega. If tetrapods had
five at the beginning, and always retained five thereafter, then some predictability
or inevitability could legitimately be maintained. (At the very least, no fuel
would exist for an alternative proposal.) But if the first members of the lineage
had six, seven, or eight toes, then alternative possibilities are legion, and
an eventual five may be a happenstance, not a necessity.Embryologist
Jonathan Cooke, in a commentary accompanying Coates and Clack's paper, agrees
with me that possible contingency of pentadactyly is the most interesting implication
of the new discovery. But he makes a very curious statement in his advocacy. Cooke
writes:
But for most of us, philistine enough to accept the historically contingent
nature of evolution, there is nothing specially deep about the number
five. Pianists should ponder the challenge that our motor cortexes
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