Ever Since Darwin: Reflections in Natural History
Page 12
Animals evolving during the log phase should show different evolutionary patterns from those arising later in a regime of self-regulated equilibrium. Much of my own research in the past two years has been devoted to defining these differences. My colleagues (T. J. M. Schopf of the University of Chicago, D. M. Raup and J. J. Sepkoski of the University of Rochester, and D. S. Simberloff of Florida State University) and I have been modeling evolutionary trees as a random process. After “growing” a tree, we divide it into its major “limbs” and consider the history of each limb (technically called a “clade”) through time. We depict each clade as a so-called spindle diagram. Spindle diagrams are constructed in the following way: simply count the number of species living during each period of time and vary the width of the diagram according to this number.
We then measure several properties of these diagrams. One measure, called C.G., defines the position of the center of gravity (roughly, the place where the clade is widest, or most diverse). If this position of maximum diversity occurs at the midpoint of the clade’s duration, we give C.G. a value of 0.5 (halfway in the clade’s total existence). If a clade reaches its greatest diversity before its midpoint, it has a C.G. of less than 0.5.
In our random system, C. G. is always near 0.5—the ideal clade is a diamond widest at its center. But our random world is one of perfect equilibrium. No log phases of sigmoidal growth are permitted; a constant number of species is maintained through time, as rates of extinction match rates of origination.
I spent a good part of 1975 counting fossil genera and recording their longevity in order to construct spindle diagrams for actual clades. I now have more than 400 clades for groups that arose and died following the log phase of the Cambrian explosion. Their mean value is 0.4993—couldn’t ask for anything closer to the 0.5 of our idealized world at equilibrium. I also have as many spindle diagrams for clades that arose during the log phase and died out afterward. Their mean C.G. is significantly less than 0.5. They record an atypical world of increasing diversity, and their values can be used to assess both the timing and the strength of the Cambrian log phase. Their values are below 0.5 because they arose during times of rapid diversification, but died out during stable times of slower origin and extinction. Thus, they reached a maximum diversity early in their history since their first representatives participated in a log phase of unrestrained increase, but they petered out more slowly in the stabilized world that followed.
Spindle diagrams. The diagram on the left has a C.G. of 0.5 (widest at the midpoint of its duration); the diagram on the right has a C.G. lower than 0.5.
A quantitative approach has helped us to understand the Cambrian explosion in two ways. First, we can recognize its character of sigmoidal growth and identify its cause in an earlier event; the Cambrian problem, per se, disappears. Secondly, we can define the time and intensity of the Cambrian log phase by studying the statistics of spindle diagrams.
To my mind, the most remarkable result of this exercise is not the low C.G. of Cambrian clades, but the correspondence of C.G. for later clades to an idealized model for a world at equilibrium. Could it be that the diversity of marine life has remained at equilibrium through all the vicissitudes of an earth in motion, all the mass extinctions, the collision of continents, the swallowing up and creation of oceans? The log phase of the Cambrian filled up the earth’s oceans. Since then, evolution has produced endless variation on a limited set of basic designs. Marine life has been copious in its variety, ingenious in its adaptation, and (if I may be permitted an anthropocentric comment) wondrous in its beauty. Yet, in an important sense, evolution since the Cambrian has only recycled the basic products of its own explosive phase.
16 | The Great Dying
ABOUT 225 MILLION years ago, at the end of the Permian period, fully half the families of marine organisms died out during the short span of a few million years—a prodigious amount of time by most standards, but merely minutes to a geologist. The victims of this mass extinction included all surviving trilobites, all ancient corals, all but one lineage of ammonites, and most bryozoans, brachiopods, and crinoids.
This great dying was the most profound of several mass extinctions that have punctuated the evolution of life during the past 600 million years. The late Cretaceous extinction, some 70 million years ago, takes second place. It destroyed 25 percent of all families, and cleared the earth of its dominant terrestrial animals, the dinosaurs and their kin—thus setting a stage for the dominance of mammals and the eventual evolution of man.
No problem in paleontology has attracted more attention or led to more frustration than the search for causes of these extinctions. The catalog of proposals would fill a Manhattan telephone book and include almost all imaginable causes: mountain building of world wide extent, shifts in sea level, subtraction of salt from the oceans, supernovae, vast influxes of cosmic radiation, pandemics, restriction of habitat, abrupt changes in climate, and so on. Nor has the problem escaped public notice. I remember well my first exposure to it at age five: the dinosaurs of Disney’s Fantasia panting to their deaths across a desiccating landscape to the tune of Stravinsky’s Rite of Spring.
Since the Permian extinction dwarfs all the others, it has long been the major focus of inquiry. If we could explain this greatest of all dyings, we might hold the key to understanding mass extinctions in general.
During the past decade, important advances in both geology and evolutionary biology have combined to provide a probable answer. This solution has developed so gradually that some paleontologists scarcely realize that their oldest and deepest dilemma has been resolved.
The dinosaurs of Disney’s “Fantasia” pant their way to extinction across a desiccating landscape. (© 1940 Walt Disney Productions)
Ten years ago, geologists generally believed that the continents formed where they now stand. Large blocks of land might move up and down and continents might “grow” by accretion of uplifted mountain chains at their borders, but continents did not wander about the earth’s surface—their positions were fixed for all time. An alternative theory of continental drift had been proposed early in the century, but the absence of a mechanism for moving continents had assured its nearly universal rejection.
Now, studies of the ocean floor have yielded a mechanism in the theory of plate tectonics. The earth’s surface is divided into a small number of plates bordered by ridges and subduction zones. New ocean floor is formed at the ridges as older portions of the plates are pushed away. To balance this addition, old parts of plates are drawn into the earth’s interior at subduction zones.
Continents rest passively upon the plates and move with them; they do not “plow” through solid ocean floor as previous theories proposed. Continental drift, therefore, is but one consequence of plate tectonics. Other important consequences include earthquakes at plate boundaries (like the San Andreas Fault running past San Francisco) and mountain chains where two plates bearing continents collide (the Himalayas formed when the Indian “raft” hit Asia).
When we reconstruct the history of continental movements, we realize that a unique event occurred in the latest Permian: all the continents coalesced to form the single supercontinent of Pangaea. Quite simply, the consequences of this coalescence caused the great Permian extinction.
But which consequences and why? Such a fusion of fragments would produce a wide array of results, ranging from changes in weather and oceanic circulation to the interaction of previously isolated ecosystems. Here we must look to advances in evolutionary biology—to theoretical ecology and our new understanding of the diversity of living forms.
After several decades of highly descriptive and largely atheoretical work, the science of ecology has been enlivened by quantitative approaches that seek a general theory of organic diversity. We are gaining a better understanding of the influences of different environmental factors upon the abundance and distribution of life. Many studies now indicate that diversity—the numbers of different species present i
n a given area—is strongly influenced, if not largely controlled, by the amount of habitable area itself. If, for example, we count the number of ant species living on each of a group of islands differing only in size (and otherwise similar in such properties as climate, vegetation, and distance from the mainland), we find that, in general, the larger the island, the greater the number of species.
It is a long way from ants on tropical islands to the entire marine biota of the Permian period. Yet we have good reason to suspect that area might have played a major role in the great extinction. If we can estimate organic diversity and area for various times during the Permian (as the continents coalesced), then we can test the hypothesis of control by area.
We must first understand two things about the Permian extinction and the fossil record in general. First, the Permian extinction primarily affected marine organisms. The relatively few terrestrial plants and vertebrates then living were not so strongly disturbed. Second, the fossil record is very strongly biased toward the preservation of marine life in shallow water. We have almost no fossils of organisms inhabiting the ocean depths. Thus, if we want to test the theory that reduced area played a major role in the Permian extinction, we must look to the area occupied by shallow seas.
We can identify, in a qualitative way, two reasons why a coalescence of continents would drastically reduce the area of shallow seas. The first is basic geometry: If each separate land mass of pre-Permian times were completely surrounded by shallow seas, then their union would eliminate all area at the sutures. Make a single square out of four graham crackers and the total periphery is reduced by half. The second reason involves the mechanics of plate tectonics. When oceanic ridges are actively producing new sea floor to spread outward, then the ridges themselves stand high above the deepest parts of the ocean. This displaces water from the ocean basins, world sea level rises, and continents are partly flooded. Conversely, if spreading diminishes or stops, ridges begin to collapse and sea level falls.
When continents collided in the late Permian, the plates that carried them “locked” together. This set a brake upon new spreading. Ocean ridges sank and shallow seas withdrew from the continents. The drastic reduction in shallow seas was not caused by a drop in sea level per se, but rather by the configuration of sea floor over which the drop occurred. The ocean floor does not plunge uniformly from shoreline to ocean deep. Today’s continents are generally bordered by a very wide continental shelf of persistently shallow water. Seaward of the shelf lies the continental slope of much greater steepness. If sea level fell far enough to expose the entire continental shelf, then most of the world’s shallow seas would disappear. This may well have happened during the late Permian.
Thomas Schopf of the University of Chicago has recently tested this hypothesis of extinction by reduction in area. He studied the distribution of shallow water and terrestrial rocks to infer continental borders and extent of shallow seas for several times during the Permian as the continents coalesced. Then, by an exhaustive survey of the paleontological literature, he counted the numbers of different kinds of organisms living during each of these Permian times. Daniel Simberloff of Florida State University then showed that the standard mathematical equation relating numbers of species to area fits these data very well. Moreover, Schopf showed that the extinction did not affect certain groups differentially; its results were evenly spread over all shallow-water inhabitants. In other words, we do not need to seek a specific cause related to the peculiarities of a few animal groups. The effect was general. As shallow seas disappeared, the rich ecosystem of earlier Permian times simply lacked the space to support all its members. The bag became smaller and half the marbles had to be discarded.
Area alone is not the whole answer. Such a momentous event as the fusion of a single supercontinent must have entailed other consequences deterimental to the precariously balanced ecosystem of earlier Permian time. But Schopf and Simberloff have provided persuasive evidence for granting a major role to the basic factor of space.
It is gratifying that an answer to paleontology’s outstanding dilemma has arisen as a by-product of exciting advances in two related disciplines—ecology and geology. When a problem has proved intractable for more than one hundred years, it is not likely to yield to more data collected in the old way and under the old rubric. Theoretical ecology allowed us to ask the right questions and plate tectonics provided the right earth upon which to pose them.
5 | Theories of the Earth
17 | The Reverend Thomas’ Dirty Little Planet
“WE DO NOT seem to inhabit the same world that our first forefathers did.… To make one man easie, ten must work and do drudgery.… The earth doth not yield us food, but with much labor and industry.… The air is often impure or infectious.”
Modern eco-activism this is not. The sentiment is right, but the style is a giveaway. It is, instead, the lament of Rev. Thomas Burnet, author of the most popular geologic work of the seventeenth century—The Sacred Theory of the Earth. His words depict a planet fallen from the original grace of Eden, not a world depleted by too many greedy men.
Among the works of scriptural geology, Burnet’s Sacred Theory is surely the most famous, the most maligned, and the most misunderstood. In it, he tried to provide a geologic rationale for all biblical events, past and future. Now take a simplistic but common view of the relationship between science and religion—they are natural antagonists and the history of their interaction records the increasing advance of science into intellectual territory formerly occupied by religion. In this context, what could Burnet represent but a futile finger in a truly crumbling dike?
But the actual relationship between religion and science is far more complex and varied. Often, religion has actively encouraged science. If there is any consistent enemy of science, it is not religion, but irrationalism. Indeed, Burnet, the divine, fell prey to the same forces that persecuted Scopes, the science teacher, almost three centuries later in Tennessee. By examining Burnet’s case in a time and a world so different from our own, we may gain a broader understanding of the persistent forces arrayed against science.
I will begin by sketching Burnet’s theory. From our point of view, it will appear so silly and contrived that a role for Burnet among dogmatic antiscientists will seem almost inescapable. But I will then examine his methods of inquiry in order to place him among the scientific rationalists of his time. In noting his persecution by dogmatic theology, we merely watch the Huxley-Wilberforce debate or the creation controversy of California played again by the same actors in different garb.
Burnet began his inquiry to determine where the waters of Noah’s flood came from. He was convinced that the modern oceans could not drown the earth’s mountains. “I can as soon believe,” wrote a contemporary, “that a man could be drowned in his own spittle as that the world should be deluged by the water in it.” Burnet rejected the idea that Noah’s flood might have been a merely local event, falsely extended by witnesses who could not have traveled widely—for that would contravene the authority of sacred scripture. But he rejected even more strongly the notion that God had simply created the extra water as a miracle—for that would dispute the rational world of science. He was led, instead, to the following account of earth history.
From the chaos of the primeval void, our earth precipitated as a perfectly ordered sphere. Its materials sorted themselves according to their densities. Heavy rocks and metals formed a spherical core at the center with a liquid layer above and a sphere of volatiles above the liquid. The volatile layer consisted mostly of air, but it also included terrestrial particles. These precipitated in time to form a perfectly smooth, featureless earth atop the liquid layer.
In this smooth Earth were the first scenes of the world, and the first generation of Mankind; it had the Beauty of Youth and blooming Nature, fresh and fruitful, and not a Wrinkle, Scar or Fracture in all its body; no Rocks nor Mountains, no hollow Caves, nor gaping Channels, but even and uniform all over.
But the earth’s own evolution required the destruction of this earthly paradise, and it happened naturally just when disobedient mankind required punishment. Rainfall was light, and the earth began to dry up and crack. The sun’s heat vaporized some of the water below the surface. It rose through the cracks, clouds formed, and the rains began. But even forty days and nights could not supply enough water, and more had to rise from the abyss. The falling rain sealed the cracks, forming a pressure cooker without a relief valve as the vaporizing water below pushed upward. The pressure built, and the surface finally burst, causing floods, tidal waves, and the rupture and displacement of the earth’s original surface to form mountains and ocean basins. So violent were these disruptions that the earth was wrenched to its current axial tilt (vide Velikovsky—essay 19). The waters finally retreated to the abyssal caverns, leaving “a gigantic and hideous ruin … a broke and confused heap of bodies.” Man, alas, had been made for Eden, and the patriarchal life-span of circa nine-hundred years declined more than tenfold.
And so, according to Reverend Thomas, we inhabitants of a “dirty little planet” await its transformation as promised by scripture and reasoned from planetary physics. The earth’s volcanoes will erupt all at once, and the universal conflagration will begin. Protestant Britain, with its reserves of coal (then largely unmined) will burn with a fury, but the fire will surely start in Rome, the papist home of antichrist. The charred particles will precipitate slowly back to earth, forming once again a perfect sphere without relief. And so the one-thousand-year reign of Christ will commence. At its end, the giants Gog and Magog will appear, forcing a new battle between good and evil. The saints will ascend to the bosom of Abraham, and the earth, having run its course, will become a star.