Wonderful Life: The Burgess Shale and the Nature of History (51 page)

I particularly welcome this demonstration that several general principles of large-scale evolution promote the importance of contingency. The generalization—on the bottom-heaviness of lineages and the properties of mass extinction—are the stuff of traditional nonhistorical science, the style that usually opposes, or at least downgrades, a historical principle like contingency. This reinforcement is a happy situation for scientific pluralism. I do not relish the idea of defending historical science by building a bunker and fighting for respect and self-determination. Better to move forward in partnership; general patterns of evolution imply the unpredictability of specific outcomes.

The collapse of the cone and the ladder opens the floodgates to alternative worlds that didn’t emerge, but might have arisen with slight and sensible changes in some early events. These unrealized universes would have been every bit as ordered and explainable as the world we know, but ever so different in ways that we can never specify in detail. The enumeration of unrealized worlds is a parlor game without end, for who can count the possibilities? The universe is not so tightly interconnected that the fall of a petal disrupts a distant star, whatever our poets sing. But most quirky changes of topography or environment, most appearances and disappearances of groups (if not of single species), can irrevocably alter the pathways of life in substantial ways. The playground of contingency is immeasurable. Let us consider just seven alternative scenarios, arranged in chronological order to home in on the biological object that most excites our parochial fancy—
Homo sapiens
.

Life arose at least 3.5 billion years ago, about as soon as the earth became cool enough for stability of the chief chemical components. (I do not, by the way, view the origin of life itself as a chancy or unpredictable event. I suspect that given the composition of early atmospheres and oceans, life’s origin was a chemical necessity. Contingency arises later, when historical complexity enters the picture of evolution.)

With respect to the old belief in steady progress, nothing could be stranger than the early evolution of life—for nothing much happened for ever so long. The oldest fossils are prokaryotic cells some 3.5 billion years old (see pages 57–58). The fossil record of this time also includes the highest form of macroscopic complexity evolved by these prokaryote—stromatolites. These are layers of sediment trapped and bound by prokaryotic cells. The layers may pile up one atop the other, as tides bury and re-form the mat—and the whole structure may come to resemble a cabbage in cross section (also in size).

Stromatolites and their prokaryotic builders dominated the fossil record throughout the world for more than 2 billion years. The first eukaryotic cells (the complex textbook variety, complete with nucleus and numerous structures of the cytoplasm) appeared some 1.4 billion years ago. The conventional argument holds that eukaryotic cells are a prerequisite for multicellular complexity, if only because sexual reproduction required paired chromosomes, and only sex can supply the variation that natural selection needs as raw material for further complexity.

But multicellular animals did not arise soon after the origin of eukaryotic cells; they first appeared just before the Cambrian explosion some 570 million years ago. Hence, a good deal more than half the history of life is a story of prokaryotic cells alone, and only the last one-sixth of life’s time on earth has included multicellular animals.

Such delays and long lead times strongly suggest contingency and a vast realm of unrealized possibilities. If prokaryotes had to advance toward eukaryotic complexity, they certainly took their time about it. Moreover, when we consider the favored hypothesis for the origin of the eukaryotic cell, we enter the realm of quirky and incidental side consequences as unpredictable sources of change. Our best theory identifies at least some major organelle—the mitochondria and chloroplasts almost surely, and others with less confidence—as descendants of entire prokaryotic cells that evolved to live symbiotically within other cells (Margulis, 1981). In this view, each eukaryotic cell is, by descent, a colony that later achieved tighter integration. Surely, the mitochondrion that first entered another cell was not thinking about the future benefits of cooperation and integration; it was merely trying to make its own living in a tough Darwinian world. Accordingly, this fundamental step in the evolution of multicellular life arose for an immediate reason quite unrelated to its eventual effect upon organic complexity. This scenario seems to portray fortunate contingency rather than predictable cause and effect. And if you wish nevertheless to view the origin of organelles and the transition from symbiosis to integration as predictable in some orderly fashion, then tell me why more than half the history of life passed before the process got started.

One final point that I find chilling with respect to the possibility of something like human evolution in an alternative world: Even though this first event took more than half the known history of life, I might be prepared to accept the probability of an eventual origin for higher intelligence if the earth were slated to endure for hundreds of billion of year—so that this initial step took but a tiny fraction of potential time. But cosmologists tell us that the sun is just about at the halfway point of existence in its current state; and that some five billion years from now, it will explode, expanding in diameter beyond the orbit of Jupiter and engulfing the earth. Life will end unless it can move elsewhere; and life on earth will terminate in any case.

Since human intelligence arose just a geological second ago, we face the stunning fact that the evolution of self-consciousness required about half of the earth’s potential time. Given the errors and uncertainties, the variations of rates and pathways in other runs of the tape, what possible confidence can we have in the eventual origin of our distinctive mental abilities? Run the tape again, and even if the same general pathways emerge, it might take twenty billion years to reach self-consciousness this time—except that the earth would be incinerated billions of years before. Run the tape again, and the first step from prokaryotic to eukaryotic cell might take twelve billion instead of two billion year—and stromatolites, never awarded the time needed to move on, might be the highest mute witnesses to Armageddon.

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 them, under traditional concepts of the cone, as primitive representatives of modern group—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 group—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 1980s, 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 Ediacara 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 living group, despite some superficial similarity of outward form. For example, most Ediacara animals have been allied with the soft corals, a group including the 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 the 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 be 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 (figure 5.5). Since this design matches no modern anatomical plan, Seilacher concludes that the Ediacara creatures represent an 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 simpler form—and evolution must be channeled up 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 Precambrian animals fall within the taxonomic boundaries of this alternative and independent experiment in multicellular life. By studying the varied and abundant 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 Ediacara lost entirely, and we don’t know why.

5.5. Seilacher’s classification of the Ediacara organisms according to their variations on a single flattened, quiltlike anatomical plan. These organisms are conventionally placed in several different modern phyla.

Suppose that we could replay life’s tape from late Precambrian times, and that the flat quilts of Ediacara won on their second attempt, while metazoans were eliminated. Could life have ever moved to consciousness along this alternate 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 (length
²
) increase so much more slowly than volumes (length
³
), 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 of Ediacara creatures might have foreclosed the evolution of internal organs, and animal life would then have remained permanently in the rut of sheets and pancake—a most unpropitious shape for self-conscious complexity as we know it. If, on the other hand, Ediacara survivors had been able to evolve internal complexity later on, then the pathways from this radically different starting point would have produced a world worthy of science fiction at its best.