Wonderful Life: The Burgess Shale and the Nature of History (38 page)

Now we ask, How often does a large jump yield a successful outcome (a new body plan)? Kauffman proves that the probability of success is quite high at first, but drops precipitously and soon reaches an effective zero—just like the history of life. This pattern matches our intuitions. The first few species are placed on the landscape at random. This means that, on average, half the peaks are higher, half lower, than the initial homes. Therefore, the first long jump has a roughly 50 percent chance of success. But now the triumphant species stands on a higher peak—and the percentage of still loftier peaks has decreased. After a few successful jumps, not many higher peaks remain unoccupied, and the probability of being able to move at all drops precipitously. In fact, if long jumps occur fairly often, all the high peaks will be occupied pretty early in the game, and no one has anyplace to go. So the victors dig in and evolve developmental systems so tied to their peaks that they couldn’t change even if the opportunity arose later. Thereafter, all they can do is hang tough on their peak or die. It’s a difficult world, and many meet the latter fate, not because ecology is a Darwinian log packed tight with wedges, but because even random extinctions leave spaces now inaccessible to everyone.

Kauffman could even quantify the precipitous decline of possibilities for successful jumps. The waiting time to the next higher peak doubles after each successful jump. (Stu told me that a mountain of athletic data shows that when a record is fractured, the average time to the next break doubles.) If your first success needed only two tries on average, your tenth will require more than a thousand. Soon you have effectively no chance of ever getting anywhere better, for geological time may be long, but it is not infinite.

We need no more than the descriptive pattern of Burgess disparity and later decimation to impose a major reform upon our traditional view of life. For the new iconography (see figure 3.72) not only alters but thoroughly inverts the conventional cone of increasing diversity. Instead of a narrow beginning and a constantly expanding upward range, multicellular life reaches its maximal scope at the start, while later decimation leaves only a few surviving designs.

But the inverted iconography, however notable, does not have revolutionary impact by itself because it does not exclude the possibility of a fallback to conventionality. Remember what is at stake! Our most precious hope for the history of life, a hope that we would relinquish with greatest reluctance, involves the concepts of progress and predictability. Since the human mind arose so late, and therefore threatens to demand interpretation as an accidental afterthought in a quirky evolutionary play, we are incited to dig in our heels all the harder and to postulate that all previous life followed a sensible order implying the eventual rise of consciousness. The greatest threat lies in a history of numerous possibilities, each sensible in itself after the fact, but each utterly unpredictable at the outset—and with only one (or a very few) roads leading to anything like our exalted state.

Burgess disparity and later decimation is a worst-case nightmare for this hope of inevitable order. If life started with a handful of simple models and then moved upward, any replay from the initial handful would follow the same basic course, however different the details. But if life started with all its models present, and constructed a later history from just a few survivors, then we face a disturbing possibility. Suppose that only a few will prevail, but all have an equal chance. The history of any surviving set is sensible, but each leads to a world thoroughly different from any other. If the human mind is a product of only one such set, then we may not be randomly evolved in the sense of coin flipping, but our origin is the product of massive historical contingency, and we would probably never arise again even if life’s tape could be replayed a thousand times.

But we can wake up from this nightmare—with a simple and obvious conventional argument. Granted, massive extinction occurred and only a few original designs survived. But we need not assume that the extinction was a crap shoot. Suppose that survivors prevailed for cause. The early Cambrian was an era of experimentation. Let a bunch of engineers tinker, and most results don’t work worth a damn: the Burgess losers were destined for extinction by faulty anatomical construction. The winners were best adapted and assured of survival by their Darwinian edge. What does it matter if the early Cambrian threw up a hundred possibilities, or a thousand? If only half a dozen worked well enough to prevail in a tough world, then these six would form the rootstocks for all later life no matter how many times we replayed the tape.

This idea of survival for cause based on anatomical deftness or complexity—“superior competitive ability” in the jargon—has been the favored explanation, virtually unchallenged, for the reduction of Burgess disparity, and indeed for all episodes of extinction in the history of life. This traditional interpretation is tightly linked with the conventional view for the origin of Burgess disparity as a filling of the empty ecological barrel. An empty barrel is a forgiving place. It contains so much space that even a clap-trap disaster of anatomical design can hunker down in a cranny and hang on without facing competition from the big boys of superior anatomy. But the party is soon over. The barrel fills, and everyone is thrown into the maelstrom of Darwinian competition. In this “war of all against all,” the inefficient survivors from gentler times soon make their permanent exit. Only the powerful gladiators win. Thumbs up for good anatomy!

You will read this interpretation in textbooks, in articles of science magazines, even in the
Yoho National Park Highline
, the official newsletter for the home of the Burgess Shale (1987 edition). Under the headline “Yoho’s Fossils Have World Significance,” we are told: “The first animals moved into the environment devoid of competition. Later, more efficient life forms held sway only to be supplanted again and again as changing conditions and evolution took its course.” And when, in 1988, Parks Canada put out the first tourist brochure for its nation’s most famous fossils (“Animals of the Burgess Shale”), they wrote that all creatures outside the bounds of modern phyla (the weird wonders of my text) “appear to have been evolutionary dead ends, destined to be replaced by better-adapted or more efficient organisms.”

Whittington and colleagues did not, until recently, challenge this comforting view. It makes too much sense. For example, in the summary comments of his monograph on
Wiwaxia
, Conway Morris explicitly linked the two traditional scenarios—barrel filling as a cause of disparity followed by stringent competition as the source of later extinction:

Briggs made the same point for a French popular audience:

Whittington also made the almost automatic equation between survival and adaptive superiority:

Conway Morris and Whittington put the matter most directly in an article for
Scientific American
—probably the best-read source on the Burgess Shale:

Words have subtle power. Phrases that we intend as descriptions betray our notions of cause and ultimate meaning. I suspect that Simon and Harry thought they were only delineating a pattern in this passage, but consider the weight of such phrases as “destined to be supplanted” and “at the expense of.” Yes, most died and some proliferated. Our earth has always worked on the old principle that many are called and few chosen. But the mere pattern of life and death offers no evidence that survivors directly vanquished the losers. The sources of victory are as varied and mysterious as the four phenomena proclaimed so wonderful that we know them not (Proverbs 30:19)—the way of an eagle in the air, the way of a serpent upon a rock, the way of a ship in the midst of the sea, and the way of a man with a maid.

Arguments that propose adaptive superiority as the basis for survival risk the classic error of circular reasoning. Survival is the phenomenon to be explained, not the proof,
ipso facto
, that those who survived were “better adapted” than those who died. This issue has been kicking around the courtyards of Darwinian theory for more than a century. It even has a name—the “tautology argument.” Critics claim that our motto “survival of the fittest” is a meaningless tautology because fitness is defined by survival, and the definition of natural selection reduces to an empty “survival of those who survive.”

Creationists have even been known to trot out this argument as a supposed disproof of evolution (Bethell, 1976; see my response in Gould, 1977)—as if more than a century of data could come crashing down through a schoolboy error in syllogistic logic. In fact, the supposed problem has an easy resolution, one that Darwin himself recognized and presented. Fitness—in this context, superior adaptation—cannot be defined after the fact by survival, but must be predictable before the challenge by an analysis of form, physiology, or behavior. As Darwin argued, the deer that should run faster and longer (as indicated by an analysis of bones, joints, and muscles) ought to survive better in a world of dangerous predators. Better survival is a prediction to be tested, not a definition of adaptation.

This requirement applies in exactly the same way to the Burgess fauna. If we wish to assert that Burgess extinctions preserved the best designs and eliminated predictable losers, then we cannot use mere survival as evidence for superiority. We must, in principle, be able to identify winners by recognizing their anatomical excellence, or their competitive edge. Ideally, we should be able to “visit” the Burgess fauna in its heyday, while all its elements flourished, and pick out the species destined for success by some definable, structural advantage.

But if we face the Burgess fauna honestly, we must admit that we have no evidence whatsoever—not a shred—that losers in the great decimation were systematically inferior in adaptive design to those that survived. Anyone can invent a plausible story after the fact. For example,
Anomalocaris
, though the largest of Cambrian predators, did not come up a winner. So I could argue that its unique nutcracker jaw, incapable of closing entirely, and probably working by constriction rather than tearing apart of prey, really wasn’t as adaptive as a more conventional jaw made of two pieces clamping together. Perhaps. But I must honestly face the counterfactual situation. Suppose that
Anomalocaris
had lived and flourished. Would I not then have been tempted to say, without any additional evidence, that
Anomalocaris
had survived because its unique jaw worked so well? If so, then I have no reason to identify
Anomalocaris
as destined for failure. I only know that this creature died—and so, eventually, do we all.

As the monographic revisions of Burgess genera continued, and as Harry, Derek, and Simon became more adept at reconstructing such unconventional creatures as functioning organisms, their respect grew for the anatomical integrity and efficient feeding and locomotion of the Burgess oddballs. They talked less and less about “primitive” designs, and labored more and more to identify the functional specializations of Burgess animals—see Briggs (1981a) on the tail of
Odaraia
, Conway Morris (1985) on the protective spines of
Wiwaxia
, Whittington and Briggs (1985) on the inferred mode of swimming for
Anomalocaris
. They wrote less about predictable, ill-adapted losers, and began to acknowledge that we do not know why
Sanctacaris
is cousin to a major living group, while
Opabinia
is a memory frozen into stone. The later articles talk more and more about good fortune. Briggs tacked a proviso onto his claim, quoted earlier, about survival due to superior adaptation: “… and also, without doubt, because certain species were luckier than others” (1985, p. 348).

All three scientists also begin to emphasize—as a positive note of interest, not an admission of defeat in the struggle to rank Burgess organisms by adaptive worth—the theme that a contemporary observer could not have selected the organisms destined for success. Whittington wrote of
Aysheaia
as a potential cousin to insects, the greatest of all multicellular success stories:

Conway Morris wrote that “a hypothetical observer in the Cambrian would presumably have had no means of predicting which of the early metazoans were destined for phylogenetic success as established body plans and which were doomed to extinction” (1985, p. 572). He then commented explicitly on the dangers of circular reasoning. Suppose that the jaw of
Wiwaxia
is homologous with the molluscan radula and that the two groups, as closest cousins, represent alternative Burgess possibilities. Since wiwaxiids died and mollusks lived to diversify, one might be tempted to argue that the wiwaxiid molting cycle was less efficient than the continuous accretionary growth of mollusks. But Conway Morris acknowledged that if wiwaxiids had lived and mollusks died, we could have ginned up just as good an argument about the benefits of molting: