Eight Little Piggies (34 page)

A hypothetical genealogy of the early history of multicellular life, illustrating the likelihood that many of these early forms had anatomies that would be judged outside the range of modern survivors.
Iromie Weeramontry. Courtesy
Natural History.

Thus, on my first argument, we cannot exclude
Microdictyon
and
Halkieria
from fascination just because we recognize that any genealogical system—like our diagram, like modern life, like the Romance languages—must include a great majority of early lineages deemed unique by modern standards. For
Microdictyon
and
Halkieria
may belong to the special group of
outsiders
(a-d and m-p), truly resident in the world of science fiction, and not to the more comfortable
insiders
(e-l) that only mix and match the cardinal features of later groups.

But suppose that
Microdictyon
and
Halkieria
do turn out to rank among the insiders? Do we then lose interest, shrug our shoulders, put down this essay, and move on to the horoscopes and gossip columns? We now come to the second, and I think more important, argument—an aesthetic or moral claim really, not an empirical proposition. What is fascination? Do we invest our interest only in unknown things beyond the boundaries of current categories? Do we not yearn for more beauty, more diversity, more examples, more wrinkles of novelty, more cases for inspiration, in the things we love and partially know? Do we not grieve for one hundred lost cantatas of Bach even though we may listen to more than two hundred? Would we not give our eye tooth (what’s a canine more or less) for the unknown works of Aristotle? Would we not trade half our GNP for tapes of Socrates in conversation with his students?

Why do intellectuals feel such special pain in the destruction of the library of Alexandria—the greatest repository of ancient texts, begun by Alexander the Great, maintained by the Ptolemaic monarchs of Egypt, and finally destroyed, according to the legend you choose to follow, by the Romans, the early Christians, or the conquering Moslems? In part, we lament the loss of the utterly unknown. But we miss just as much the opportunity to relish a greatly expanded diversity among people and ideas that we already know and love. We miss the joy of making concrete, the pleasure of holding what has disappeared forever. What is history all about if not the exquisite delight of knowing the details, and not only the abstract patterns. Even if
Microdictyon
and
Halkieria
are only “inside” animals between surviving phyla, they are still prominent creatures of our earliest multicellular world. They have unique forms and peculiar features—shells on both ends, or lateral dabs in pairs. We want to know as many of these creatures as we can, for they are papyrus rolls in the great and largely lost library of our own past.

One legend of Alexandria, almost surely false, states that the library was still intact when Moslems captured the city in the seventh century. The emir Amrou Ibn el-Ass, having conquered Alexandria in 640, wrote to the caliph Omar asking (in part) what should be done with the library (and hoping against hope that the caliph would spare this great treasure). But Omar replied with the most stunning statement of “heads I win, tails you lose” in all human history. The books, he proclaimed, are either contrary to the Koran, in which case they are heretical and must be destroyed—or they are consonant with the Koran, in which case they are superfluous and must also be destroyed. The contents of the library were therefore burned to heat water in the public baths of Alexandria. The books and scrolls kept the fires going for six months.

The Omar of this legend will never win any praise from intellectuals, but I do grasp his point in an entirely reversed way.
Microdictyon
and
Halkieria
are, in a sense, either heretical (if lying outside the range of modern forms) or superfluous (if lying inside). But they are equally wonderful, and worthy of our most cherished interest and protection, in either case—and in this judgment lies the difference between most of us and the enemies of the light. In this lies the turf that we must defend at all costs.

YOU CAN GENERATE
a lot of mischief just by strolling. When God asked Satan what he’d been doing, the foremost of the fallen angels responded: “…going to and fro in the earth and…walking up and down in it” (Job 1:7). But you can also do a lot of good. Aristotle preferred to teach while ambling along the covered walk, or
peripatos
, of his Lyceum in Athens. His followers were therefore called peripatetics. In Greek, a
patos
is a path, and
peri
means “about.” The name for Aristotle’s philosophical school therefore reflects the master’s favorite activity.

The same etymology lies behind my all-time favorite technical name for an animal—the genus
Peripatus
. I just love the sound, especially when pronounced by my Scottish friends who really know how to roll their
r
’s. I can hardly ever bring myself to write about this animal without expressing delight in its name. The only reference in my book
Wonderful Life
speaks of the “genus with the lovely name
Peripatus
.”

Peripatus
is an elongated invertebrate with many pairs of stout, fleshy legs—hence the chosen name for this obligate walker. The Reverend Lansdown Guilding—quite a name itself, especially given the old stereotype of English clergymen as amateur natural historians—discovered and designated
Peripatus
in 1826. He falsely placed his new creature into the mollusk phylum (with clams, snails, and squids) because he mistook the antennae of
Peripatus
for the tentacles of a slug. Since true mollusks don’t have legs, Guilding named his new beast for a supposed peculiarity.

Peripatus
is the most prominent member of a small group known as Onychophora. Modern onychophorans are terrestrial invertebrates of the Southern Hemisphere (with limited extension into a few regions of the Northern Hemisphere tropics)—hence little known and never observed in natural settings by residents of northern temperate zones.

About eighty species of living onychophorans have been described. They live exclusively in moist habitats, usually amid wet leaves or rotting wood. Most species are one to three inches in length, although the size champion from Trinidad, appropriately named
Macroperipatus
, reaches half a foot. They resemble caterpillars in outward appearance (although not in close evolutionary relationship). They are elongated, soft bodied, and unsegmented (the ringlike “annulations” on antennae, legs, and sometimes on the trunk are superficial and do not indicate the presence of segments, or true divisions of the body). The onychophoran head bears three paired appendages: antennae, jaws, and just adjacent to the jaws, the so-called slime papillae. Onychophorans are carnivores and can shoot a sticky substance from these papillae, thus ensnaring their prey or their enemies. Behind the head, and all along the body, onychophorans carry fourteen to forty-three pairs (depending on the species) of simple walking legs, called lobopods. The legs terminate in a claw with several spines—the source of their name, for Onychophora means “talon bearer.”

The Onychophora present the primary case for a classical dilemma in taxonomy: How do we classify small groups of odd anatomy? (Oddness, remember, is largely a function of rarity. If the world contained a million species of onychophorans and only fifty of beetles, we would consider the insects as bizarre.) The chief fault and foible of classical taxonomy lies in its passion for clean order—an imposition bound to distort a messy world of continuity and complexity. A small group of distinctive anatomy sticks out like the proverbial sore thumb, and taxonomists yearn to heal the conceptual challenge by enforcing an alliance with something more familiar. Two related traditions have generally been followed in this attempt, both misleading and restrictive: the shoehorn (“cram ’em in”) and the straightening rod (“push ’em between”).

The shoehorn works by cramming odd groups into large and well-established categories, usually by forced and fanciful comparison of one or two features with characteristic forms of the larger group. For example, the Onychophora have sometimes been allied with the Uniramia, the dominant arthropod group that includes insects and myriapods (millipedes and centipedes), because both have single-branched legs (never mind that arthropod legs are truly segmented and that onychophoran lobopods are constructed on an entirely different pattern).

The straightening rod tries to push a jutting thumb of oddness back into a linear array by designating the small and peculiar group as intermediary between two large and conventional categories. The Onychophora owe whatever small recognition they possess to this strategy—for they have most commonly been interpreted as living relicts of the evolutionary transition between two great phyla: the Annelida (segmented worms, including leeches and the common garden earthworm) and the Arthropoda (about 80 percent of animal species, including insects, spiders, and crustaceans). In this argument,
Peripatus
is a superworm for its legs and a diddly fly for building these legs without true segments.

A third possibility obviously exists and clearly bears interesting implications. This third way has been supported, often by well-respected taxonomists, but our general preference for shoehorns and straightening rods has given it short shrift. The Onychophora, under this view, might represent a separate group, endowed with sufficient anatomical uniqueness to constitute its own major division of the animal kingdom, despite the low diversity of living representatives. After all, the criterion for separate status should be degree of genealogical distinctness, not current success as measured by number of species. A lineage may need a certain minimal membership just to provide enough raw material so that evolution can craft sufficient difference for high taxonomic rank. But current diversity is no measure of available raw material through geological history. Evolution is ebb and flow, waxing and waning; once-great groups can be reduced to a fraction of their past glory. A great man once told us that the last shall be first, but just by the geometry of evolution, and not by moral law, the first can also become last. Perhaps the Onychophora were once a much more diverse group, standing wide and tall in their distinctness, while
Peripatus
and its allies now form a pitifully reduced remnant.

(By speaking of potential distinctness, I am not making an untenable claim for total separation without any relationship to other phyla. Very few taxonomists doubt that onychophorans, along with other potentially distinct groups known as tardigrades and pentastomes, have their evolutionary linkages close to annelids and arthropods. But this third view places onychophorans as a separate branch of life’s tree—splitting off near the limbs of annelids and arthropods and eventually joining them to form a major trunk—whereas the shoehorn would stuff onychophorans into the Arthropoda, and the straightening rod would change life’s geometry from a tree to a line and place onychophorans between primitive worms and more advanced insects.)

We can only test this third possibility by searching for onychophorans in the fossil record—a daunting task because they have no preservable hard parts and therefore do not usually fossilize. I write this essay because several striking new discoveries and interpretations, all made in the past year or two, now point to a markedly greater diversity for onychophorans right at the beginning of modern multicellular life, following the Cambrian explosion some 550 million years ago. These discoveries arise from two fortunate circumstances: First, onychophorans have been found in the rare soft-bodied faunas occasionally preserved by happy geological accidents in the fossil record; second, some ancient onychophorans possessed hard parts and can therefore appear in ordinary fossil deposits.

I fully realize that this expansion in onychophoran diversity at the beginning of multicellular animal life can scarcely rank as the hottest news item of the year. Most readers of these essays, after all, have probably never heard the word
onychophoran
and, lamentably, have no acquaintance with poor, lovely
Peripatus
. So why get excited about old onychophorans if you never knew that modern ones existed in the first place? Do hear me out if you harbor these doubts. Much more than
Peripatus
lies at stake, for validation of the third position—that onychophorans represent a separate branch of life’s tree—has broad and interesting implications for our entire concept of evolution and organic order. I also think that you will marvel at the details of these early onychophorans for their own sake—and their weirdness.

We have actually known a bit about ancient onychophorans for most of this century, thanks once again to that greatest of treasure troves for soft-bodied fossils, the Burgess Shale. In 1911, two years after discovering the Burgess Shale, C. D. Walcott gave the unpronounceable name
Aysheaia
(we generally call it “a-shy-a” in the trade) to an animal that he described as an annelid worm. Many taxonomists, just viewing Walcott’s illustrations, immediately saw that the creature looked much more like an onychophoran. In 1931, G. Evelyn Hutchinson, who became the world’s greatest ecologist and was, perhaps, the finest person I have ever had the privilege of knowing, published a definitive account on the onychophoran affinities of
Aysheaia
. Hutchinson had studied
Peripatus
in South Africa and he knew onychophoran anatomy intimately. As an ecologist, he was powerfully intrigued by the issue of how an ordinary marine invertebrate like
Aysheaia
could evolve into a terrestrial creature like
Peripatus
with such minimal change in outward anatomy. (
Aysheaia
had fewer pairs of legs and fewer claws per leg than do modern onychophorans. It also bore a terminal mouth at the body’s end, while living onychophorans have a ventral mouth on the underside. In addition,
Aysheaia
had no slime papillae and could not use such a device to shoot sticky stuff through ocean waters in any case. But, all told, the differences are astonishingly slight for more than 500 million years of time and a maximal ecological shift from ocean to land.)

One other ancient onychophoran was recognized before last year—a European form named
Xenusion
, found during the 1920s. But
Aysheaia
and
Xenusion
did not shake the shoehorn or the straightening rod. Only two fossils, both so similar to modern forms, do not make an impressive show of diversity. Onychophorans remained a tiny and uniform group, ripe for stuffing in or between larger phyla and not meriting a status of its own.

In the last essay, I described the beginning of the onychophoran coming of age (I was going to say “renaissance,” but a renaissance is a rebirth, and onychophorans never had an earlier period of flowering in our consciousness). I described the discovery in China of the animal that bore the small, circular, meshwork plates known for many years from lowermost Cambrian rocks as
Microdictyon
. This fossil comes from the remarkable Chengjiang fauna of south-central China, a Burgess Shale equivalent (although slightly older), with beautiful soft-bodied preservation of many animals already known from the more famous Canadian site (and several novelties as well, including the
Microdictyon
animal). The plates called
Microdictyon
are attached in pairs to the side of the animal just above the junction of paired lobopods with the trunk of the body. The animal itself looks like an onychophoran. If this interpretation holds, then some ancient onychophorans had hard parts. The Chengjiang fauna also contains a second probable onychophoran with plates, named
Luolishania
.

Thus, the early fossil record of onychophorans had begun to expand in numbers and anatomical variety, including fully soft-bodies forms like
Aysheaia
and creatures with small pairs of plates like
Microdictyon
and
Luolishania
. But the big boost, the event that might finally push onychophorans over the border of distinct respectability, finally occurred on May 16, 1991, when the Swedish paleontologist L. Ramsköld and his Chinese colleague Hou Xianguang published an article in the British journal
Nature
(science, at its best, is truly international—see bibliography).

Ramsköld and Hou dropped a bombshell that makes elegant sense of a major paleontological puzzle of recent years. In 1977, Simon Conway Morris described the weirdest of all Burgess Shale organisms with the oddest of all monikers:
Hallucigenia
, named, as Simon wrote, for “the bizarre and dream-like appearance of the animal.” He described
Hallucigenia
(see figure) as a tubular body supported by seven pairs of long, pointed spines—not jointed arthropod appendages or fleshy lobopods, but rigid spikes. In Conway Morris’s reconstruction, a single row of seven fleshy tubes, each ending in a pair of little pincers, runs along the back, with a tuft of six smaller tubes, perhaps in three pairs, behind the larger seven. The head, not well preserved on any specimen, was depicted as a bulbous extension and the tail as a straight, upward-curving tube.

Hallucigenia
was bizarre enough in appearance, but even more puzzlement attended the issue of how such a creature could function. In particular, how could a tiny animal, no more than an inch in length, be stable on seven pairs of rigid spikes for “legs”? And if stable, how could it possibly move? Some of our best functional morphologists, including Mike Labarbera of the University of Chicago, struggled with this issue and found no resolution.

The unlikely morphology, and the even more troublesome question of function, led many paleontologists to dispute Conway Morris’s reconstruction (and Simon himself also began to doubt his original conclusions). In my book
Wonderful Life
, I presented Conway Morris’s original version and then opted for a different interpretation proposed by several colleagues before me—that
Hallucigenia
is a part broken off from a larger (and still unknown) animal. I wrote: