Wonderful Life: The Burgess Shale and the Nature of History (49 page)

The gigantic head and short, powerful neck identify
Diatryma
as a fierce carnivore, in sharp contrast with the small head and long, slender neck of the more peaceful ratites (ostriches, rheas, and their relatives). Like
Tyrannosaurus
, with its diminutive forelimbs but massive head and powerful hind limbs,
Diatryma
must have kicked, clawed, and bitten its prey into submission.

Diatrymids, distant relatives perhaps of cranes but no kin to ostriches and their ilk, ranged over Europe and North America for several million years. The plum of dominant carnivory could have fallen to the birds, but mammals finally prevailed, and we do not know why. We can invent stories about two legs, bird brains, and no teeth as necessarily inferior to all fours and sharp canines, but we know in our heart of hearts that if birds had won, we could tell just as good a tale about their inevitable success. A. S. Romer, leading vertebrate paleontologist of the generation just past, wrote in his textbook, the bible of the profession:

In all these speculations about replaying life’s tape, we lament our lack of any controlled experiment. We cannot instigate the actual replay, and our planet provided only one run-through. But the crucial Eocene pivot between birds and mammals provides more and different evidence. For once, our recalcitrant and complex planet actually performed a proper experiment for us. This particular tape did have a replay, in South America—and this time the birds won, or at least held the mammals to a respectable draw!

South America was an island continent, a kind of super-Australia, until the Isthmus of Panama arose just a few million years ago. Most animals usually considered as distinctively South American—jaguars, llamas, and tapirs, for example—are North American migrants of postisthmian arrival. The great native fauna of South America is largely gone (or surviving as a poor, if fascinating, remnant of armadillos, sloths, and the “Virginia” opossum, among others). No placental carnivores inhabited this giant ark. Most popular books tell us that the native South American carnivores were all marsupials, the so-called borhyaenids. They often neglect to say that another prominent group—the phororhacids, giant ground birds—fared just as well, if not better. Phororhacids also sported large heads and short, stout necks, but were not closely related to
Diatryma
. In South America, birds had a second and separate try as dominant carnivores, and this time they won, as suggested in Charles R. Knight’s famous reconstruction of a phororhacid standing in triumph over a mammalian victim (figure 5.3).

5.3. A phororhacid bird of South America stands in triumph over its mammalian prey in this depiction by Charles R. Knight.

In our smug, placental-centered parochialism, we may say that birds could triumph in South America only because marsupials are inferior to placentals and did not offer the kind of challenge that conquered predacious ground birds in Europe and North America. But can we be so sure? Borhyaenids could also be large and fierce, ranging to bear size and including such formidable creatures as
Thylacosmilus
, the marsupial sabertooth. We might also sneer and point out that, in any case, phororhacids quickly snuffed it (along with borhyaenids) as soon as superior placentals flooded over the rising isthmus. But this common saga of progress will not wash either. G. G. Simpson, our greatest expert on the evolution of South American mammals, wrote in one of his last books:

We must conclude, I think, that South America does represent a legitimate replay—round two for the birds.

This story of worms and birds—the first part graced with the sweep of history from Burgess times to now, the second with the virtues of repetition by natural experiment—moves contingency from a general statement about history into the realm of tangible things. A single story can establish plausibility by example, but it cannot make a complete case. The argument of this book needs two final supports: first, a statement about general properties of life’s history that reinforce the claims of contingency; and second, a chronology of examples illustrating the power of contingency not for selected and specific cases alone, but for the most general pathways and probabilities of life on our planet. This section and the next present these final supports for my argument; an epilogue on an arresting fact then completes the book.

If geological time had operated exactly as Darwin envisioned, contingency would still reign, with perhaps a bit more of life’s general pattern thrown into the realm of predictability under broad principles. Remember that Darwin viewed the history of life through his controlling metaphors of competition and the wedge (see page 229): the world is full of species, wedges crowded together on a log, and new forms can enter ecological communities only by displacing others (popping the wedges out). Displacement proceeds by competition under natural selection, and the better-adapted species win. Darwin felt that this process, operating in the micro-moment of the here and now, could be extrapolated into the countless millennia of geological time to yield the overall pattern of life’s history. For example, in chapter 10 of the
Origin of Species
, Darwin labored mightily (if incorrectly, in retrospect) to show that extinctions are not rapid and simultaneous across large differences of form and environment, but that each major group peters out slowly, its decline linked with the rise of a superior competitor.
*
But by “better adapted,” Darwin only meant “more suited to changing local environments,” not superior in any general anatomical sense. The pathways to local adaptation are as likely to restrict as to enhance the prospects for long-term success (simplification in parasites, overelaboration in peacocks). Moreover, nothing else is as quirky and unpredictable—both in our metaphors and on our planet—as trends in climate and geography. Continents fragment and disperse; oceanic circulation changes; rivers alter their course; mountains rise; estuaries dry up. If life works more by tracking environment than by climbing up a ladder of progress, then contingency should reign.

I assert the powerful role of contingency in Darwin’s system not as a logical corollary of his theory, but as an explicit theme central to his own life and work. Darwin invoked contingency in a fascinating way as his primary support for the fact of evolution itself. He embedded his defense in a paradox: One might think that the best evidence for evolution would reside in those exquisite examples of optimal adaptation presumably wrought by natural selection—the aerodynamic perfection of a feather or the flawless mimicry of insects that look like leaves or sticks. Such phenomena provide our standard textbook examples for the power of evolutionary modification—the mills of natural selection may operate slowly, but they grind exceedingly fine. Yet Darwin recognized that perfection cannot provide evidence for evolution because optimality covers the tracks of history.

If feathers are perfect, they may as well have been designed from scratch by an omnipotent God as from previous anatomy by a natural process. Darwin recognized that the primary evidence for evolution must be sought in quirks, oddities, and imperfections that lay bare the pathways of history. Whales, with their vestigial pelvic bones, must have descended from terrestrial ancestors with functional legs. Pandas, to eat bamboo, must build an imperfect “thumb” from a nubbin of a wrist bone, because carnivorous ancestors lost the requisite mobility of their first digit. Many animals of the Galapagos differ only slightly from neighbors in Ecuador, though the climate of these relatively cool volcanic islands diverges profoundly from conditions on the adjacent South American mainland. If whales retained no trace of their terrestrial heritage, if pandas bore perfect thumbs, if life on the Galápagos neatly matched the curious local environment—then history would not inhere in the productions of nature. But contingencies of “just history” do shape our world, and evolution lies exposed in the panoply of structures that have no other explanation than the shadow of their past.

Thus, contingency rules even in Darwin’s world of extrapolation from organic competition within local communities chock-full of species. However, an exciting intellectual movement of the last quarter century has led us to recognize that nature is not so smoothly and continuously ordered; the large does not emerge from the small simply by adding more time. Several large-scale pattern—based on the nature of macroevolution and the history of environment—impose their own signatures on nature’s pathways, and also disrupt, reset, and redirect whatever may be accumulating through time by the ticking of processes in the immediate here and now. Most of these patterns strongly reinforce the theme of contingency (see Gould, 1985a). Let us consider just two.

The major argument of this book holds that contingency is immeasurably enhanced by the primary insight won from the Burgess Shale—that current patterns were not slowly evolved by continuous proliferation and advance, but set by a pronounced decimation (after a rapid initial diversification of anatomical designs), probably accomplished with a strong, perhaps controlling, component of lottery.

But we must know if the Burgess represents an odd incident or a general theme in life’s history—for if most evolutionary bushes look like Christmas trees, with maximal breadth at their bottoms, then contingency wins its greatest possible boost as a predominant force in the history of organic disparity. My feeling about the importance of this question has led me to devote much of my technical research during the past fifteen years to the prevalence of “bottom-heaviness” in evolutionary trees (Raup
et al.
, 1973; Raup and Gould, 1974; Gould
et al.
, 1977; Gould, Gilinsky, and German, 1987).

Paleontologists have long recognized the Burgess pattern of maximal early disparity in conventional groups of fossils with hard parts. The echinoderms provide our premier example. All modern representatives of this exclusively marine phylum fall into five major group—the starfishes (Asteroidea), the brittle stars (Ophiuroidea), the sea urchins and sand dollars (Echinoidea), the sea lilies (Crinoidea), and the sea cucumbers (Holothuroidea). All share the basic pattern of fivefold radial symmetry. Yet Lower Paleozoic rocks, at the inception of the phylum, house some twenty to thirty basic groups of echinoderms, including some anatomies far outside the modern boundaries. The edrioasteroids built their globular skeletons in three-part symmetry. The bilateral symmetry of some “carpoids” is so pronounced that a few paleontologists view them as possible ancestors of fishes, and therefore of us as well (Jefferies, 1986). The bizarre helicoplacoids grew just a single food groove (not five), wound about the skeleton in a screwlike spiral. None of these groups survived the Paleozoic, and all modern echinoderms occupy the restricted realm of five-part symmetry. Yet none of these ancient groups shows any sign of anatomical insufficiency, or any hint of elimination by competition from surviving designs. Similar patterns may be found in the history of mollusks and vertebrates (where the early jawless and primitively jawed “fishes” show more variation in number and order of bones than all the later birds, reptiles, and mammals could muster; outward variety based on stereotypy of anatomical design has become a vertebrate hallmark).
*

In my recent studies I concluded that the pattern of maximal early breadth is a general characteristic of lineages at several scales and times, not only of major groups at the Cambrian explosion. In fact, we have proposed that this “bottom-heavy” asymmetry may rank among the few natural phenomena imparting a direction to time, thus serving as a rare example of “time’s arrow” (Gould, Gilinsky, and German, 1987; Morris, 1984). In our study, we portrayed evolutionary lineages and taxonomic groups as the traditional “spindle diagrams” of paleontology—read intuitively with the vertical dimension as time, and the width at any time proportional to the number of representatives in the group then living (figure 5.4). These diagrams may be bottom-heavy, top-heavy, or symmetrical (with maximum representation in the middle of the geological range). If bottom-heavy lineages characterize the history of life, then the Burgess pattern has generality across scales (for most of our spindle diagrams portray groups of low taxonomic rank, usually genera within families). If symmetrical lineages predominate, then the shape of diversification gives no direction to time.