Herbivores Carnivores
Early Tertiary (archaic) 0.18 0.44
Early Tertiary (advanced) 0.38 0.61
Middle to late Tertiary 0.63 0.76
Modern 0.95 1.10
Both herbivores and carnivores displayed continual increase in brain size during their evolution, but at each stage, the carnivores were always ahead. Animals that make a living by catching rapidly moving prey seem to need bigger brains than plant eaters. And, as the brains of herbivores grew larger (presumably under intense selective pressure imposed by their carnivorous predators), the carnivores also evolved bigger brains to maintain the differential.
South America provides a natural experiment to test this claim. Until the Isthmus of Panama rose just a couple of million years ago, South America was an isolated island continent. Advanced carnivores never reached this island, and predatory roles were filled by marsupial carnivores with low encephalization quotients. Here, the herbivores display no increase in brain size through time. Their average encephalization quotient remained below 0.5 throughout the Tertiary; moreover, these native herbivores were quickly eliminated when advanced carnivores crossed the isthmus from North America. Again, brain size is a functional adaptation to modes of life, not a quantity with an inherent tendency to increase. When we document an increase, we can relate it to specific requirements of ecological roles. Thus, we should not be surprised that “primitive” sharks have such large brains; they are, after all, the top carnivores of the sea, and brain size reflects mode of life, not time of evolutionary origin. Likewise, carnivorous dinosaurs like Allosaurus and Tyrannosaurus had larger brains than herbivores like Brontosaurus.
But what about our preoccupation with ourselves: does anything about the history of vertebrates indicate why one peculiar species should be so brainy? Here’s a closing item for thought. The most ancient brain cast of a primate belongs to a 55-million-year-old creature named Tetonius homunculus. Jerison has calculated its encephalization quotient as 0.68. This is, to be sure, only two-thirds the size of an average living mammal of the same body weight, but it is by far the largest brain of its time (making the usual correction for body weight); in fact, it is more than three times as large as an average mammal of its period. Primates have been ahead right from the start; our large brain is only an exaggeration of a pattern set at the beginning of the age of mammals. But why did such a large brain evolve in a group of small, primitive, tree-dwelling mammals, more similar to rats and shrews than to mammals conventionally judged as more advanced? And with this provocative query, I end, for we simply do not know the answer to one of the most important questions we can ask.
24 | Planetary Sizes and Surfaces
CHARLES LYELL expressed in no uncertain terms the guiding concept of his geologic revolution. In 1829, he wrote to his colleague and scientific opponent Roderick Murchison:
My work … will endeavor to establish the principle of reasoning in the science … that no causes whatever have from the earliest time to which we can look back, to the present, ever acted, but those now acting; and that they never acted with different degrees of energy from that which they now exert.
The doctrine of slow, stately, essentially uniform rates of change had a profound influence on nineteenth-century thought. Darwin adopted it thirty years later, and paleontologists ever since have been searching for cases of slow and steady evolution in the fossil record. But where did Lyell’s preference for gradual change originate?
All cosmic generalizations have complex roots. In part, Lyell merely “discovered” his own political prejudices in nature—if the earth proclaims that change must proceed slowly and gradually, encumbered by the weight of events long past, then liberals might take comfort in a world increasingly threatened by social unrest. Nature, however, is not merely an empty stage upon which scientists display their prior preferences; nature also speaks back. Many of the forces that shape the surface of our planet do act slowly and continuously. Lyell could measure the accumulation of silt in river bottoms and the gradual erosion of hillslopes. Lyell’s gradualism, while far too extreme in his formulation, does express a large part of the earth’s history.
Our planet’s gradual processes arise from the action of what my colleagues Frank Press and Raymond Siever call the external and internal heat engines of the earth. Our sun powers the external engine, but its influence depends upon the earth’s atmosphere. Press and Siever write:
Solar energy drives the atmosphere in a complex pattern of winds to give us our climates and weather, and it drives the ocean’s circulation in a pattern that is coupled to the atmosphere. The water and gases of the oceans and atmosphere chemically react with the solid surface and physically transport material from one place to another.
Most of these processes work gradually, in a classic Lyellian manner; their large results are an accumulation of minute changes. Running water wears the land away; dunes march over deserts; waves destroy the coastline in some places, while currents transport sand to extend it elsewhere.
Heat derived from radioactive decay powers the internal engine. Some of its results—earthquakes and volcanic eruptions, for example—strike us as sudden and catastrophic, but the basic process, discovered only a decade ago, must be a source of joy for Lyell’s shade. Internal heat puts the earth’s surface in motion, driving the continents apart at minute rates of centimeters per year. This gradual motion, extended over 200 million years, has separated the single land of Pangaea into our present, widely dispersed continents.
Yet our earth is decidedly atypical among the other inner planets of our solar system: Mercury, Mars, and our own moon. (I exclude Venus because we know almost nothing about its surface; only one Russian probe has successfully penetrated its dense atmosphere to send back but two ambiguous photos. I also exclude Jupiter and the large planets beyond. They are so much larger and less dense than the inner planets that they belong to a very different class of cosmic bodies.) No geologist, no matter how strong his prior preferences, could have preached a doctrine of uniformity on the surface of any inner planet except the earth.
Craters made by meteoritic bombardment dominate the surfaces of Mars, Mercury, and our moon. Indeed, the surface of Mercury is little more than a field of tightly packed and superimposed craters. The moon’s surface is divided into two major areas: densely cratered highlands and the more sparsely cratered maria (“seas” of basaltic lava). Lyellian gradualism, so applicable to our earth, cannot possibly describe the history of our planetary neighbors.
Consider, for example, our moon’s history, as inferred from data collected during the Apollo missions and summarized by Columbia University geologist W. Ian Ridley: The moon’s crust rigidified more than 4 billion years ago. By 3.9 billion years ago, the greatest period of meteoritic bombardment had ended, the mare basins had been excavated, and the major craters formed. Between 3.1 and 3.8 billion years ago, radioactively generated heat produced the basaltic lava that filled the mare basins. Then the generation of new heat failed to match its loss at the lunar surface and the crust rigidified again; by 3.1 billion years ago, the crust became too thick to permit the ascent of any more basalt, and activity at the lunar surface essentially ended. Since then, nothing much has happened beyond the very occasional impact of a large meteorite and the constant influx of very small ones.
We view the moon today much as it looked 3 billion years ago. It has no atmosphere to erode and recycle the material of its surface, and it cannot generate the internal heat to churn up and change its visage. The moon is not dead, but it is certainly quiescent. The concentration of moonquakes at 800–1,000 km below the surface suggests a rigid crust of this thickness, compared with 70 km or so for the earth’s lithosphere. A partially molten zone may exist below the lunar crust, but it is too far down to influence the surface. The moon’s surface is ancient, and its record tells the story of its catastrophes—massive meteorites and upwelling lava. Its early history was marked by violent change; its last 3 billion years by
very little indeed.
Why is the earth so different from its neighbors in recording a history marked in large part by cumulative gradual processes, rather than ancient catastrophes? Readers might be temped to think that the answer lies in some complicated difference of composition. But all the inner planets are basically similar, so far as we can tell, in density and mineralogical content. I wish to argue that the difference arises from a disarmingly simple fact—size itself, and nothing else: the earth is a good deal larger than its neighborts.
Galileo first discussed the cardinal importance of size in determining the form and operation of all physical objects (see essays 21 and 22). As a basic fact of geometry, large bodies are not subject to the same balance of forces as small objects of the same shape (all planets are, necessarily, roughly spherical). Consider the ratio of surface to volume in two spheres of different radii. Surface is measured by a constant times the radius squared; volume by a different constant times the radius cubed. Hence, volumes increase faster than surfaces as objects of the same shape become larger.
I maintain that Lyell’s insight is a contingent result of the earth’s relatively low surface/volume ratio, not a general characteristic of all change, as he would have argued. We begin by assuming that the earth’s early history did not differ much from that of its neighbors. At one time, our planet must have been scarred by abundant craters. But they were effaced billions of years ago, destroyed by the earth’s two heat machines: churned up by the internal machine (uplifted in mountains, covered by lava, or buried in the depths of the earth by subduction at the descending borders of lithospheric plates) or quickly obliterated in atmospheric or fluvial erosion by the external machine.
These two heat machines operate only because the earth is large enough to possess a relatively small surface and large gravitational field. Mercury and the moon have neither atmosphere nor an active surface. The external machine requires an atmosphere for its work. Newton’s equation relates the force of gravity directly to the mass of two bodies and inversely to the square of the distance separating them. To calculate the gravitational force holding a molecule of water vapor on the earth and moon, we need only consider the mass of the planet (since the mass of the molecule is constant) and the distance from the planet’s surface to its center. As a planet gets larger, its mass increases as the cube of its radius, while the squared distance from surface to center is simply the radius squared. Hence, as a planet gets larger, its gravitational pull on an atmospheric particle increases as r3/r2 (where r is the planet’s radius). On the moon and Mercury, this force is too small to hold an atmosphere; even the heaviest particles do not abide long. The earth’s gravity is sufficiently strong to hold a large, permanent atmosphere, to act as a medium for its external heat machine.
Internal heat is generated radioactively over the volume of a planet. It is radiated out into space at a planet’s surface. Small planets, with their high ratio of surface to volume, quickly lose their heat and solidify their outer layers to relatively great depths. Larger planets retain their heat and the mobility of their surfaces.
The ideal test for this hypothesis would be a planet of intermediate size, for we predict that such a body would display a mix of early catastrophes and gradual processes. Mars, obligingly, is just the right size, nicely intermediate between the earth and our moon or Mercury. About half the Martian surface is cratered; the rest reflects the activity of rather limited internal and external heat machines. Martian gravity is weak compared to that of the earth, but it is strong enough to hold a slight atmosphere (about 200 times thinner than ours). High winds course over the Martian surface and dune fields have been observed. The evidence for fluvial erosion is even more impressive, if somewhat mysterious, given the paucity of water vapor in the Martian atmosphere. (The mystery has been much alleviated by the discovery that Mars’s polar caps are predominantly frozen water, not carbon dioxide, as previously conjectured. It also seems likely that a considerable amount of water lies frozen as permafrost in the Martian soil. Carl Sagan has shown me photos of relatively small craters with lobate extensions in all directions. It is hard to interpret these features as anything but liquefied mud, flowing away from the crater following localized melting of permafrost upon impact. They cannot be made of lava because the meteorites that formed the craters were too small to generate enough heat on impact to melt rock.)
Evidence for internal heat is also abundant (and rather spectacular), while some recent speculation plausibly links it with the processes that move the earth’s plates. Mars has a volcanic province with giant mountains surpassing anything on earth. Olympus Mons has a base 500 km wide, a height of 8 km and a crater 70 km in diameter. The nearby Vallis Marineris dwarfs any canyon on earth: it is 120 km wide, 6 km deep and more than 5,000 km long.
Now, the speculation: Many geologists believe that the earth’s plates are moved by plumes of heat and molten material rising from deep within the earth (perhaps even at the core-mantle boundary, 3,200 km below the surface). These plumes emerge at the surface at relatively fixed “hot spots,” and the earth’s plates ride over the plumes. The Hawaiian Islands, for example, are an essentially linear chain increasing in age toward the northwest. If the Pacific plate is slowly moving over a fixed plume, then the Hawaiian Islands might have formed one by one.
Mars, at its intermediate size, should be more dynamic than the moon, less so than the earth. The moon’s crust is too thick to move at all; internal heat does not reach the surface. The earth’s crust is thin enough to break into plates and move continuously. Suppose that the crust of Mars is thin enough to allow heat to rise, but too thick to break up and move extensively. Suppose also that plumes exist both on the earth and Mars. Giant Olympus Mons may represent the locus of a plume, rising under a crust that cannot move—Olympus Mons, if you will, may be like all the Hawaiis, piled one atop the other. The Vallis Marineris may represent an unsuccessful “try” at plate tectonics—the crust fractured, but could not move.
Science, at its best, is unifying. It strikes my intellectual fancy to learn that the principle regulating a fly on my ceiling also determines the uniqueness of our earth among the inner planets (flies, as small animals, have a high ratio of surface to volume; gravitational forces, acting upon volume, are not strong enough to overcome the strength of surface adhesion holding a fly’s foot to the ceiling). Pascal once remarked, in planetary metaphor, that knowledge is like a sphere in space; the more we learn—that is, the larger the sphere—the greater our contact with the unknown (the planet’s surface). True enough—but remember the principle of surfaces and volumes! The larger the sphere, the greater the ratio of known (volume) to unknown (surface). May absolutely increased ignorance continue to flourish with relatively increased knowledge.
7 | Science in Society—A Historical View
25 | On Heroes and Fools in Science
AS A ROMANTIC teen-ager, I believed that my future life as a scientist would be justified if I could discover a single new fact and add a brick to the bright temple of human knowledge. The conviction was noble enough; the metaphor was simply silly. Yet that metaphor still governs the attitude of many scientists toward their subject.
In the conventional model of scientific “progress,” we begin in superstitious ignorance and move toward final truth by the successive accumulation of facts. In this smug perspective, the history of science contains little more than anecdotal interest—for it can only chronicle past errors and credit the bricklayers for discerning glimpses of final truth. It is as transparent as an old-fashioned melodrama: truth (as we perceive it today) is the only arbiter and the world of past scientists is divided into good guys who were right and bad guys who were wrong.
Historians of science have utterly discredited this model during the past decade. Science is not a heartless pursuit of objective information. It is a creative human activity, its geniuses acting more as artists than as information processors. Changes in theory are not simply the derivative results of new d
iscoveries but the work of creative imagination influenced by contemporary social and political forces. We should not judge the past through anachronistic spectacles of our own convictions—designating as heroes the scientists whom we judge to be right by criteria that had nothing to do with their own concerns. We are simply foolish if we call Anaximander (sixth century B.C.) an evolutionist because, in advocating a primary role for water among the four elements, he held that life first inhabited the sea; yet most textbooks so credit him.
(Joseph Scrofani. Reproduced with permission, from Natural History Magazine, August-September 1974. © The American Museum of Natural History, 1974)
In this essay, I will take the most notorious of textbook baddies and try to display their theory as both reasonable in its time and enlightening in our own. Our villains are the eighteenth century “preformationists,” adherents to an outmoded embryology. According to the textbooks, preformationists believed that a perfect miniature homunculus inhabited the human egg (or sperm), and that embryological development involved nothing more than its increase in size. The absurdity of this claim, the texts continue, is enhanced by its necessary corollary of emboîtement or encasement—for if Eve’s ovum contained a homunculus, then the ovum of that homunculus contained a tinier homunculus, and so on into the inconceivable—a fully formed human smaller than an electron. The preformationists must have been blind, antiempirical dogmatists supporting an a priori doctrine of immutability against clear evidence of the senses—for one only has to open a chicken’s egg in order to watch an embryo develop from simplicity to complexity. Indeed, their leading spokesman, Charles Bonnet, had proclaimed that “preformationism is the greatest triumph of reason over the senses.” The heroes of our textbooks, on the other hand, were the “epigeneticists”; they spent their time looking at eggs rather than inventing fantasies. They proved by observation that the complexity of adult form developed gradually in the embryo. By the mid-nineteenth century, they had triumphed. One more victory for unsullied observation over prejudice and dogma.
Ever Since Darwin: Reflections in Natural History Page 17