Wonderful Life (Part II)

Excerpt from:

Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould



II. The First Fauna of Multicellular Animals

You might accept this last sobering scenario, but then claim, fine, I'll grant the unpredictability of getting beyond prokaryotic cells, but once you finally do get multicellular animals, then the basic pathways are surely set and further advance to consciousness must occur. But let's take a closer look.

The first multicellular animals, as discussed in chapter II, are members of a world-wide fauna named for the most famous outcrop at Ediacara, in Australia. Martin Glaessner, the paleontologist most responsible for describing the Ediacara animals, has always interpreted the, under traditional concepts of the cone, as primitive representatives of modern groups -- mostly members of the coelenterate phylum (soft corals and medusoids), but including annelid worms and arthropods (Glaessner, 1984). Glaessner's traditional reading evoked very little opposition (but see Pflug, 1972 and 1974), and the Ediacara fauna settled comfortably into textbooks as fitting ancestors for modern groups -- for their combination of maximal age with minimal complexity neatly matches expectations.

The Ediacara fauna has special importance as the only evidence for multicellular life before the great divide separating the Precambrian and Cambrian, a boundary marked by the celebrated Cambrian explosion of modern groups with hard parts. True, the Ediacara creatures are only barely Precambrian; they occur in strata just predating Cambrian and probably do not extend more than 100 million years into the uppermost Precambrian. In keeping with their position immediately below the boundary, the Ediacara animals are entirely soft-bodied. If taxonomic identity could be maintained right through this greatest of geological transitions, and without major disruption in design to accompany the evolution of hard parts, then the smooth continuity of the cone would be confirmed. This version of Ediacara begins to sound suspiciously like Walcott's shoehorn.

In the early 1980's, my friend Dolf Seilacher, professor of paleontology at Tubingen, Germany, and in my opinion the finest paleontological observer now active, proposed a radically different interpretation of the Edicara fauna (Seilacher, 1984). His twofold defense rests upon a negative and a positive argument. For his negative claim, he argues on functional grounds that the Ediacara creatures could not have operated as their supposed modern counterparts, and therefore may not be allied with any modern group, despite some superficial similarity of outward form. For example, most Ediacara animals have been allied with the soft corals, a group including modern sea fans. Coral skeletons represent colonies housing thousands of tiny individuals. In soft corals, the individual polyps line the branches of a tree or network structure, and the branches must be separated, so that water can bring food particles to polyps and sweep away waste products. But the apparent branches of the Ediacara forms are joined together, forming a flattened quiltlike mat with no spaces between the sections.

For his positive claim, Seilacher argues that most Ediacara animals may taxonomically united as variations on a single anatomical plan -- a flattened form divided into sections that are matted or quilted together, perhaps constituting a hydraulic skeleton much like an air mattress. Sine this design matches no modern anatomical plan, Seilacher concludes that the Ediacara creatures represent a entirely separate experiment in multicellular life -- one that ultimately failed in a previously unrecognized latest Precambrian extinction, for no Ediacara elements survived into the Cambrian.

For the Burgess fauna, the case against Walcott's shoehorn has been proven, I think, with as much confidence as science can muster. For the Ediacara fauna, Seilacher's hypothesis is a plausible and exciting, but as yet unproven, alternative to the traditional reading, which will one day be called either Glaessner's shoehorn or Glaessner's insight, as the case may be.

But consider the implications for unpredictability if Seilacher's view prevails, even partly. Under Glaessner's ranking in modern groups, the first animals share the anatomical designs of later organisms, but in a simpler form -- and evolution must be channelled upward and outward in the traditional cone of increasing diversity. Replay the tape, starting with simple coelenterates, worms, and arthropods, a hundred times, and I suppose that you will usually end up with more and better of the same.

But if Seilacher is right, other possibilities and other directions were once available. Seilacher does not believe that all late Cambrian animals fall within the taxonomic boundaries of this alternative and independent experiment in multicellular life. By studying the abundant and varied trace fossils (tracks, trails, and burrows) of the same strata, he is convinced that metazoan animals of modern design -- probably genuine worms in one form or another -- shared the earth with the Ediacara fauna. Thus, as with the Burgess, several different anatomical possibilities were present right at the beginning. Life might have taken either the Ediacara or the modern pathway, but the Ediacara lost entirely, and we don't know why.

Suppose that we could replay life's tape from the late Precambrian times, and that the fla quilts of Ediacara won on the second attempt, while metazoans were eliminated. Could life have ever moved to consciousness along this alternative pathway of Ediacara anatomy? Probably not. Ediacara design looks like an alternative solution to the problem of gaining enough surface area as size increases. Since surfaces (length2) increase so much more slowly than volumes (length3), and since animals perform most functions through surfaces, some way must be found to elaborate surface area in large creatures. Modern life followed the path of evolving internal organs (lungs, villi of the small intestine) to provide the requisite surfaces.. In a second solution -- proposed by Seilacher as the key to understanding Ediacara design -- organisms may not be able to evolve internal complexity and must rely instead on changes in overall form, taking the shape of threads, ribbons, sheets, or pancakes so that no internal space lies very far from the outer surface. (The complex quilting of Ediacara animals could then be viewed as a device for strengthening such a precarious form. A sheet one foot long and a fraction of an inch thick needs some extra support in a world of woe, tides, and storms.)

If Ediacara represents this second solution, and if Ediacara had won the replay, then I doubt that animal life would ever have gained much complexity, or attained anything close to self-consciousness. The developmental program or Ediacara creatures might have forclosed the evolution of internal organs, and animal life would then have remained permanently in the rut of sheets and pancakes -- a most unpropitous shape for self-consciousness as we know it. If, on the other hand, Ediacara survivors had been able to evolve internal complexity later on, then the pathways fro the radically different starting point would have produced a world worthy of science fiction at its best.

Wonderful Life (Part III)

Excerpt from:

Wonderful Life: The Burgess Shale and the Nature of History
by Stephen Jay Gould




III. The First Fauna of the Cambrian Explosion

Our hypothetical advocate of the cone and the ladder might be willing to give ground on the these first two incidents from the dim mists of time, but he might then be tempted to dig his entrenchment across the Cambrian boundary. Surely, once the great explosion occurs, and traditional fossils with hard parts enter the record, then the outlines must be set, and life must move upward and outward in predictable channels.

Not so. As noted in chapter II, the initial shelly fauna, called the Tommotian to honor a famous Russian locality, contains far more mysteries than precursors. Some modern groups make an undoubted first appearance in the Tommotian, but more of these fossils may represent anatomies beyond the current range. The story is becoming familiar -- a maximum of potential pathways at the beginning, followed by decimation to set the modern pattern.

The most characteristic and abundant of all Tommotian creatures, the archaeocyathids, represent a long-standing problem in classification. The familiar litany plays again. These first reef-forming creatures of the fossil record are simple in form, usually cone-shaped, with double walls -- cup within cup. In the traditional spirit of the shoehorn, they have been shunted from one modern group to another during more than a century of paleontological speculation. Corals and sponges have been their usual putative homes. But the more we learn about archaeocyathids, the stranger they appear, and most paleontologists now place them in a separate phylum destined to disappear before the Cambrian had run its course.

Even more impressive is the extensive disparity just now being recognized among organisms of the "small shelly fauna." Tommotian rocks house an enormous variety of tiny fossils (usually one to five millimeters in length) that cannot be allied with an modern group (Bengston, 1977; Bengston and Fletcher, 1983). We can arrange these fossils by outward appearance, as tubes, spines, cones, and plates, but we do not know their zoological affinities. Perhaps they are merely bits and pieces of an era of early, still imperfect skeletonization; perhaps the covered familiar organisms that later developed the more elaborate shells of their conventional fossils signatures. But perhaps -- and this interpretation has recently been gaining favor among aficionados of the small shelly fauna -- most of the Tommotian oddballs represent unique anatomies that arose early and disappeared quickly, For example, Rozanov, the leading Russian expert on this fauna, concludes his recent review by writing:

Early Cambrian rocks contain numerous remains of very peculiar organisms, both animals and plants, most of which are unknown after the Cambrian. I tend to think that numerous high-level taxa developed in the early Cambrian and rapidly became extinct (1986, p. 95).

Once again, we have a Christmas tree rather than a cone. Once again, the unpredictability of evolutionary pathways asserts itself against our hope for the inevitability of consciousness. The Tommotian contained many modern groups, but also a large range of alternative possibilities. Rewind the tape into the early Cambrian, and perhaps this time our modern reefs are built by archaeocyathids, not corals. Perhaps no Bikini, no Waikiki; perhaps, also, no people to sip rum swizzles and snorkle amidst great undersea gardens.