Hen’s Teeth and Horse’s Toes (30 page)

We can be sure that a removal of half the families requires the death of a much greater percentage of species. A family is not gone until all its species die, and many families contain tens or hundreds of species. The extinction of most individual species does not wipe out a family, just as, for example, the random excision of a single entry in a telephone directory rarely removes the name entirely—you’d have to bump off a lot of Smiths. How many species must die before 50 percent of families are gone?

David M. Raup of Chicago’s Field Museum has recently considered this question (see bibliography). The problem has no easy solution. If all families contained about the same number of species, then a simple formula would suffice. But variation is enormous. Many families contain only a single species. In this case, removal of the species also wipes out the family. Phone books contain their Zzyzzymanskis as well as their Wongs. Other families contain more than 100 species. We must know the empirical distribution of species per family before we can make a proper estimate. And we cannot construct an empirical distribution for Permian families because we cannot count the species directly.

Raup therefore made tabulations for a group that we do know well, the echinoids, or sea urchins. Echinoids include 894 species distributed into 222 genera and 40 families. How many species must, on average, be removed at random in order to eliminate 52 percent of the families? Raup considered this question both empirically and theoretically and came up with the astounding figure of 96 percent. If the rest of life maintains a distribution of species within families similar to the echinoids—and we have no evidence for major differences in this pattern—then the Permian debacle might have wiped out all but 4 percent of species.

Since estimates of living species in the late Permian range from 45,000 to 240,000, a removal of 96 percent would leave but 1,800 to 9,600 species as guardians of life’s continuity. Moreover, as Raup argues, we have no strong evidence, despite intensive and specific search, of selectivity in the Permian extinctions. The debacle did not seem to favor any particular kind of animal—bigger creatures, inhabitants of shallow water, more complex forms, for example.

I am not entirely persuaded by the 96 percent figure. Echinoids may not be a good model for all of life. More important, Raup is assuming that the 52 percent figure is not artificially inflated by biases in the fossil record. We know, for example, that late Permian marine sediments are relatively rare and we may be missing some successful families simply because so few late Permian fossils have been preserved. Nonetheless, even the most conservative figures indicate a removal of 80 to 85 percent of all species.

I think that we must therefore face an unpleasant fact. If anywhere near 96 percent of all species died, leaving as few as two thousand forms to propagate all subsequent life, then some groups probably died, and others survived, for no particular reason at all. Organisms can muster few defenses against a catastrophe of such magnitude, and survivors may simply be among the lucky 4 percent. Since the Permian extinction set the basic pattern of life’s subsequent diversity (no new phyla and few classes have originated since then), our current panoply of major designs may not represent a set of best adaptations but a group of fortunate survivors.

If we must admit randomness as an important agent of evolutionary
change
at all levels, what shall we make of it? Shall we surrender to despair and proclaim the history of life both chaotic and unknowable? Such a solution might embody Pope’s equation of chance with disorder, but it would represent a great misunderstanding of what randomness means, for two reasons.

First, chance may well describe a sequence of events without implying that each individual item has no cause. Take the classic random event, a coin flip or the throw of a die. I imagine that each flip has a determined outcome if we could (as we cannot) specify all the multifarious factors that enter into it—height above ground, force of the flip, initial side up, angle of first contact with the ground, for example. But the factors are too numerous and not under our control; an equal chance for each possible outcome is the best prediction we can make in the long run.

Perhaps the Permian extinction worked like a set of dice with few winning outcomes. Each species became extinct for a conventional local reason—this pond dried up, that estuary became too salty or suffered an invasion of a particularly efficient predator. But the reasons are so numerous and so beyond our ken that an equal chance for the removal of each species represents our best prediction for the overall outcome.

Only in this way could several inventors of probability theory stomach their own creation, for they were conventional believers in deterministic causation and convinced theists unwilling to debar purpose from the world. Charles Bell, author of a famous work on the human hand as a reflection of God’s wisdom through its intricate design, wrote in 1833:

This explanation may be comforting (and true), but I think we must face a second possibility. Perhaps randomness is not merely an adequate description for complex causes that we cannot specify. Perhaps the world really works this way, and many events are uncaused in any conventional sense of the word. Perhaps our gut feeling that it cannot be so reflects only our hopes and prejudices, our desperate striving to make sense of a complex and confusing world, and not the ways of nature.

What solace, then, can we have, if solace we need. An answer, I believe, lies in rejecting another traditional belief, Pope’s false equation of chance with a host of fearful cognates: disorder, chaos, lawlessness, unpredictability, and destruction. For contrary to popular belief, random means none of these. It may not, as in Bell’s dice, imply a lack of causation. And even if it does in many cases, a random process need not engender unpredictable disorder. Random processes can yield highly complex order. We have an elaborate theory for predicting the results of coin flipping, the archetypical random process. Suppose we flip six coins at once, over and over again. We can predict how many times the most common result of three heads and three tails will occur, and how many times the rare event of all heads or all tails will arise (one in sixty-four for each or one in thirty-two for either).

Admittedly, this is predictability of a different order. It works only for repeated trials in the long run. We can assign a probability to each individual flip, but we cannot determine a specific outcome. Nonetheless, the end result is ordered and predictable. Does not this kind of randomness offer sufficient comfort against the threat of chaos? Does it not even make for a more intriguing world? After all, it is chance, in this sense, that gives our own lives, and the course of human history, so much richness and interest. Call it by its older names of fortune or free will, if you like. Shall we deny a similar richness to the rest of nature?

BILL LEE
, certainly the most colorful if not the most skillful of baseball pitchers, once argued that his declining effectiveness on the mound could be traced to the “designated hitter” rule. (This rule, for you nonadepts, allows a manager in the American League to designate a permanent pinch hitter for any player in the regular lineup. Since most pitchers are hopeless at hitting, the designated hitter almost always substitutes for the pitcher, and pitchers, therefore, no longer come to bat.) “Every species that’s become extinct,” Lee proclaimed, “has done so because of overspecialization.”

In this statement, baseball’s self-styled philosopher repeats what may be the most common misconception about the history of life—that extinction is the ultimate sign of failure. No stigma seems to be greater than irrevocable disappearance. Dinosaurs dominated the land for 100 million years, yet a species that measures its own life in but tens of thousands has branded dinosaurs as a symbol of failure. Two years ago, for example, the good folks at Audi claimed (in unsubtle comparison with their large competitors) that
Brontosaurus
was “arguably the worst designed creature of all time.” “Evolution,” they continued, “has a sure way of correcting faulty design”—extinction, of course. Paleontologists rose in protest and Audi backed down. “You can be sure we will treat the
Brontosaurus
with more respect in the future,” they promised.

I believe that this equation of disappearance with incompetence reflects an outmoded approach based on the false metaphor of progress and an overly grim view of natural selection as a persistent and endless life-or-death struggle among competitors—the military version of such Darwinian phrases as “survival of the fittest” and “the struggle for life.” If life moves ever upward and onward by ruthless battle and elimination of losers, extinction must be the ultimate sign of inadequacy. But life is not a tale of progress; it is, rather, a story of intricate branching and wandering, with momentary survivors adapting to changing local environments, not approaching cosmic or engineering perfection. And success in natural selection is less the result of murder and mayhem than of producing more surviving offspring.

The equation of extinction with inadequacy makes no sense in the long view of paleontology. Extinction is the ultimate fate of all lineages, yet we surely cannot argue that all species are therefore badly designed or poorly adapted. Extinction is no shame. It is, in one sense, the enabling force of the biosphere. Since most species are extraordinarily resistant to major evolutionary change and since many habitats are fairly full of species, how could evolution proceed if extinction did not open space for novelty? Would I be writing, or you reading, if dinosaurs had survived and mammals remained, as they had for 100 million years, a minor group of small creatures living in ecological nooks and crannies that dinosaurs didn’t penetrate?

If most extinctions were the direct result of competition with superior species, or even if most represented an inevitable failure to meet the challenges of minor environmental change (as Bill Lee charged), then a stigma might be attached to disappearance. But many, if not most, extinctions are reactions to environmental challenges so severe and unpredictable that we have no right to expect a successful response and, therefore, no reason to “blame” a species for its disappearance. A freshwater fish might dart and swim so elegantly that an engineer would proclaim its anatomy optimal. But if the lakes and rivers dry up, what defense does it have? Will blue whales be any less exquisite in design if rapacious humanity does the last one in? Some insurance policies offer no protection against cataclysms so momentous and unexpected that legal language calls them “acts of God.” Species often die for reasons equally beyond control or calculation.

I may make these statements baldly, but you have no reason to accept them unless I can back them up with evidence and numbers. For the past decade, a group of researchers centered in Chicago (but admitting some outsiders like myself to marginal membership) has been working to quantify patterns of diversity in the history of life. These studies have provided our most extensive and consistent data on extinction. The findings support my central contention that extinction is no disgrace, but usually an inevitable result of circumstances beyond reasonable response. A pair of papers published in
Science
during March 1982 reach three conclusions central to this discussion.

1.
A quantification of mass extinction
.

We have known since the dawn of paleontology that extinctions are not spread evenly over time, but are concentrated in a few brief periods of markedly enhanced, often worldwide decimation—the so-called mass extinctions of the geological record. The boundaries of the geological time scale correspond with these epochs of extinction. (Each year, when my students groan at my request, or rather demand, that they memorize the geological time scale, I reply that those funny names—Cambrian, Ordovician, Silurian—are not capricious instruments of torture, but records of the outstanding events of life’s history.)

D. M. Raup and J. J. Sepkoski have gathered and summarized the data of geological longevity for all forms of marine life. Their plot of extinction rates (families of organisms disappearing per million years) versus geological time offers few surprises in general outline, but provides our best and most consistent account of the
quantities
involved. Four brief periods of mass extinction stand well above the ordinary, or “background,” rate of normal times, and a fifth nearly reaches the level of these major episodes. Two events mark the well-known era boundaries: the great Permian extinction, which may have extirpated more than ninety percent of shallow-water marine species some 225 million years ago (see essay 26), and the Cretaceous debacle, which wiped out remaining dinosaurs and a host of marine creatures some 65 million years ago (see essay 25). The three other events, although well enough known to paleontologists, are not emblazoned upon popular consciousness. Two occurred before the Permian (Ordovician and Devonian) and one between the Permian and Cretaceous (Triassic).

Raup and Sepkoski discovered that these brief mass extinctions are even more pronounced than previous data had suggested. The average background rate varies between 2.0 and 4.6 families per million years, while mass extinctions reach 19.3 families per million years. The authors conclude: “Our analysis shows that major mass extinctions are far more distinct from background extinction than has been indicated by previous analyses of other data sets.”

Proposals for causes of these mass extinctions (see other essays in this section) range from continental coalescence and its sequelae (for the Permian) to asteroidal impact (for the Cretaceous)—causes that lie in the category of fluctuations beyond control or reasonable response and thus surrounding their victims with no aura of shame. Since these mass extinctions are even more massive than previously recognized, the scope of “blameless” extinction has been greatly widened.

2.
The supposedly classic case of extinction due to competitive inferiority cannot be supported
.

During most of Tertiary time (the “age of mammals”) South America was an island continent—a sort of super Australia—with an indigenous fauna more than matching the marsupials down under for interest and peculiarity. The Australian region sports only one exclusive order of mammals, the egg-laying monotremes (echidna and duck-billed platypus). South America once housed several, with odd animals ranging from the rhinolike, but not rhino-related, toxodonts that Darwin discovered during his apprenticeship on the
Beagle
, to litopterns, which outhorsed horses by reducing toes from several to one and even losing the side splints (reduced vestiges of toes) that horses retain (see essay 14), to giant sloths and glyptodonts. Other oddities belonged to orders dwelling elsewhere but expressing a unique South American twist. All large mammalian carnivores, for example, were marsupials and included such outstanding creatures as the saber-toothed
Thylacosmilus
.

All these animals are gone today, victims of the greatest biological tragedy of the past five million years. For once, humans are absolved, and we must cite instead the rise of the Isthmus of Panama just a few million years ago. The isthmus connected South America with the more cosmopolitan fauna of northern continents. North American mammals came (wandering over the isthmus) and, in the traditional view, saw and conquered as well. What we usually regard as the modern “native” South American fauna—from Ilamas to alpacas to jaguars to tapirs to peccaries—are all relatively recent northern migrants.

The traditional view, with its odor of racist metaphor, pits a sleek, fine-tuned, and rigorously adapted northern fauna, tested in the crucible of harsh climates and relentless competition from previous waves of Asiatic and European migrants, against a lazy, stagnant, and unchallanged native South American fauna. What chance did the poor toxodonts and litopterns have? Superior northern forms came streaming over the isthmus and wiped them out. In return, only a few inferior South American forms managed to travel the other way and survive. We got porcupines, opossums, and the nine-banded armadillo. South America received an entire new regime.

If this tale be true, then perhaps battle is the law of life and extinction does connote defeat. But is it correct? Do the numbers support it? Did more northern forms go south than vice versa? Were extinction rates much higher for South American forms? L. G. Marshall and S. D. Webb joined Raup and Sepkoski in a second article that applies the same quantitative methods to the recent faunal history of South America. They conclude that several aspects of the traditional story are not true.

The interchange, first of all, was surprisingly symmetrical. Members of fourteen North American families now reside in South America, representing 40 percent of South American familial diversity. Twelve South American families now live in North America, forming 36 percent of North American families. At the finer scale of genera, reduction was also balanced on both sides of the isthmus. Native South American genera declined by 13 percent between pre- and post-isthmian faunas. Native North American genera declined by 11 percent during the same interval. Thus, about the same number of families moved successfully in both directions and about the same percentage of native forms became extinct on both sides following the initial interchange. Why then does the record carry its apparent message of North American victory and South American carnage?

I believe that three major reasons account for this impression, one social, one biological but largely spurious, and the third real and important. We must consider, first, the chauvinism of most Anglophones in the United States (whoops, I almost wrote Americans). Anything south of the Rio Grande is Spanish speaking and therefore culturally linked with South America. But a good part of North America lies between El Paso and Panama, and most South American migrants live there, not in the United States or Canada. After all, the equator runs through Quito (in a nation appropriately called Ecuador), and South America contains more tropical land than North America. Therefore, most migrating South American forms are tropical or subtropical in their climatic preferences, and their natural homes up north are Mexico or Central America. The paucity of South American migrants in my backyard (although I have seen an opossum) is no argument against their abundance or vigor.

Secondly, the taxonomic structure of South American forms dictated a greater effect upon overall diversity of design for an equal reduction in percentage of genera. When the isthmus rose, many of South America’s indigenous groups had already been reduced to such a low diversity that the removal of a genus or two extinguished the entire group. Few North American groups were tottering so near the brink. If one fauna has twenty groups with one genus each, and a second has two groups each with ten genera, then a removal of four genera from each fauna will wipe out four of the larger groups in one fauna and none in the other.

Finally, even though migrants moved with equal success in both directions and native forms declined in equal measure, North American migrants did fare “better” in one different and interesting way. When we count genera derived from migrants after they arrived in their new homes, we find an outstanding difference. In North America, genera originally from South America evolved very few new genera, while North American forms were remarkably prolific in South America. Twelve primary migrants from South America evolved but three secondary genera, while twenty-one North American migrants gave rise to forty-nine secondary genera in South America. Thus, North American forms radiated vigorously in South America and filled the continent with its modern fauna, while South American forms succeeded well enough in North America but did not radiate extensively.

Why this difference? The four authors suggest that a major phase in the rise of the Andes created a rain shadow over most of South America and led to the replacement of predominantly savanna-woodland habitats by drier forests and pampas and by deserts and semideserts in some areas. Perhaps North American forms radiated in a new habitat suited to their previous life styles, while South American forms continued to decline as their favored habitats shrank. Or perhaps the conventional explanation is true in part, and North American forms radiated because they are, in some unexplained way, competitively superior to South American natives—although most versions of “competitive superiority” will not explain higher rates of speciation but only success in battle (leading to longer duration of migrants coupled with higher rates of extinction among the vanquished, neither a component of this tale). In any case, the old story of “hail the conquering hero comes”—waves of differential migration and subsequent carnage—can no longer be maintained.

3.
A ray of comfort for the meliorists
.

The great eighteenth-century French naturalist George Buffon once expressed the fact of extinction in a fine literary image: “They must die out because time fights against them.” We may now say, with equal literary license, that organisms may be fighting back. When Raup and Sepkoski compiled their data on the quantitative effect of mass extinctions, they made an interesting and unexpected discovery about the “background” level of normal times. They found that the background level has been slowly but steadily decreasing for more than a half billion years. In early Cambrian times, at the beginning of our adequate fossil record some 600 million years ago, the average rate stood at 4.6 families extinct per million years. Since then, the rate has declined steadily to about 2.0 families per million years today. If the Cambrian rate had been maintained, 710 more family extinctions would have occurred. It is intriguing—although I don’t know what it means—that the total number of marine families has increased by almost the same number (680 families) since the Cambrian.