Read an Excerpt
Time Frames
The Evolution of Punctuated Equilibria
By Niles Eldredge PRINCETON UNIVERSITY PRESS
Copyright © 1985 Niles Eldredge
All rights reserved.
ISBN: 978-0-691-02435-6
CHAPTER 1
EVOLUTION NOWADAYS
UTMOST CLARITY
The California coastline is volatile. The huge Pacific chunk of crustal plate surfaces as a thin rind of the North American continent, a sliver of rock running from Mexico up through L.A. and San Francisco before slipping once again below the waves. In the last 120 million years this outer rim of California coast has moved north perhaps as much as 350 miles — a rate of .2 inches per year. But that is only the average rate: what really happens is far more sporadic. Tension constantly mounts as the Pacific plate tries to slip north past the rest of the continent. Periodically the tension is released, and the sharp snaps of major earthquakes provide most of the movement along the famous San Andreas fault system. The net rate may be slow, but the movement itself comes in small steps that are often sudden and violent.
The rocks along that outer Californian rim are a mixed lot. Some are hundreds of millions of years old, accumulations of muds and sands that piled up when the crust was still submerged. There are balloon-shaped intrusions of granite — molten, igneous magma injected into the surrounding sediments. The magma has long since cooled to form massive topographic lumps, such as Bodega Head, 60 miles north of San Francisco. And there are younger sediments, some marine and others the accumulations of lakes and rivers, that have filled the basins and inlets which have appeared off and on as the earthquakes kept shifting the land around. Some of these younger sediments are jammed with fossils.
Not all the movement along the San Andreas is lateral: the rocks are sometimes shoved up or sunk down. The younger silts and sands have hardly had time to become firmly cemented into hard rock, and California is as notorious for its landslides as it is for its earthquakes. As a terrain, it is young and unstable — a difficult circumstance for a homeowner but a delight for the fossil collector.
The little town of Capitola, 10 miles south of Santa Cruz on Monterey Bay, has all the earmarks of a typical west-coast village. South of the beach, boardwalk and surfers, cliffs rapidly climb to an imposing height. Houses are precariously perched above (and even more dangerously tucked in below) the cliff wall and the beach is littered with huge chunks that keep spalling off the cliff face. Here is a vibrant, fresh exposure of the Purisima Formation — a thick sequence of sedimentary rock that surfaces periodically up and down the central California coast. Named for another town not too far from Capitola (geologic names are usually based on geographic features near a typical occurrence), the Purisima at Capitola is a light gray-green deposit of sands and silts that positively teems with fossils. Most of the beds are so soft that clamshells can easily be removed with a pocketknife.
"Purisima" is a truly apt name for these rocks, for they reveal the very essence of life's fossil record with utter clarity and simplicity. The fossils of the Purisima are mostly clams and snails: periwinkles, slipper shells, razor clams and so forth. A pale pearly pink, the shells have mostly lost the outermost, darkly colored layer living mollusks usually sport. They are a bit more delicate than the shells of recently dead organisms that litter strand lines along modern beaches. So a little care is in order in extracting a specimen from the friable, silty matrix. But otherwise the fossils are manifestly what they seem to be: simply the buried shells of clams and snails, plus crab carapaces, sand dollar skeletons, and the bones of whales, seals and porpoises. These animals — plus sea anemones, annelid worms and other creatures without hard skeletons to lend themselves to preservation — lived and died in some former coastal Californian sea. Occasionally their shells were buried in the bottom muds after death, in some cases trapped in rapidly formed deposits as the waves were agitated by a passing storm. Others simply accumulated quietly on the sea bottom, and were gently buried as the silts and muds built up.
It was something of a triumph for the human intellect to see fossils for what they are. It seems obvious to us today that an oyster jutting from a cliff, or a horn sticking out of a mountainside, must be the preserved remains of some creature long dead. It was by no means so obvious to our remote forebears — nor is it intuitively obvious, probably, to a majority of the 4 billions of us on earth today. Herodotus, the Greek historian and traveler, thought that the presence of shells far inland in the hills encircling the Mediterranean implied the former presence of a much more extensive seaway. And Leonardo, with utterly characteristic verve and brilliance, realized that the fossil shark teeth he had picked up along his daily peregrinations must be simply what they seem: the teeth of sharks that had inhabited a now-vanished sea. But the received truth of the Middle Ages regarded fossils as either the work of the devil or the petrified remains of thunderbolts.
The Purisima Formation actually poses the problem in reverse. The shells are so fresh, so familiar, that it is not entirely obvious that they are not the shells of living creatures. Except, of course, that you stand with your back to Monterey Bay delicately prising specimen after specimen from a layer covered with 50 additional feet of sediment. Even more dramatic, perhaps, are the rocks and fossils of roughly the same age on the east coast. The western shore of Chesapeake Bay, along Maryland's Calvert Cliffs, produces a beautiful array of fossil shells — shells that periodically weather out, hit the beach and mingle with the shells of the mollusks still living there today. Sometimes only the realization that a particular species of clam or snail no longer lives in the Chesapeake tells you that the shell lying on the beach must be a fossil.
It is naturally a matter of simple curiosity to ask a few questions of this Purisima Formation. How old is it, for one thing? I have been calling rocks of Purisima ilk "young" — and they are, by reasonable geological standards. The Purisima at Capitola is Pliocene in age — which makes these rocks and fossils somewhere around 3 million years old. Most of the fossils I know best, in contrast, are from the Paleozoic, a grand division of geologic time that began close to 600 million years ago and ran for nearly 350 million years. Most of my fossils — trilobites — are about 400 million years old. That's why the Purisima seems relatively young to me. But, on the other hand, even someone steeped in the earlier phases of the 600-million-year-long history of complex life must admit that 3 million years is a respectable length of time. Three million years, yet the shells remain fresh — and distinctly modern in appearance.
And so we ask another question: how many of the species we see in the Purisima — all the various crabs, mammals and mollusks — still live out there in Monterey Bay, or along the more southerly reaches of the California coast? The answer — simple as it is, and somehow as expectable as it may be, given the fresh look of the fossils — nonetheless is a bit shocking: perhaps as many as 90 percent. The vast majority of those 3-million-year-old species have modern names, names given to species of crabs, clams and snails of the modern California shallow-water marine invertebrate fauna. This is not to say that those Purisima species still considered alive today are identical in every feature to their fossil counterparts. But it does mean that the paleontologists who first found, studied and classified those Purisima fossils realized that the fossils were so similar to modern species that separate names for the fossils were simply not warranted. There just is no rationale, no purpose to be served in giving different names to such virtually identical creatures just because they are separated by 3 million years of time. Yet that is the natural propensity of paleontologists: collections of otherwise similar, if not completely identical, fossils tend to get different names for no reason other than their supposedly significant age differences. Evidently the similarities between Purisima fossils and living shells is so compelling that normal paleontological procedures just could not be followed.
And here is the profound significance of this modest exposure of Pliocene rocks and fossils at Capitola, California: the simple facts of the matter fly squarely in the face of the most deeply entrenched canons of the paleontological profession — for if evolution is anything, it is change through time. We have become convinced of the sheer inevitability of change given the mere passage of time. So deep are the roots of this belief that paleontologists have been searching with great frustration and largely in vain for "insensibly graded series" — perfectly, progressively intergrading series of fossils that exhibit gradual, directional change up through a sequence of layered rocks. "Insensibly graded series" is Darwin's phrase, but the expectation that change is inevitable given the mere passage of time goes back well before Darwin. Paleontologists have literally been caught between a rock and a hard place: Evolutionary theory, or so they have thought, predicted that most evolutionary change would be slow, steady, gradual and progressive. We would reasonably expect that, at least in exceptionally fossiliferous sequences, we should find sufficient examples to bear out this prediction. But paleontologists (and, as we shall see, even Darwin himself) realized that examples of gradual, progressive change are rare in the fossil record. Thus the conflict: the actual history of evolutionary events, though universally conceded to be but imperfectly recorded in the rocks, nonetheless has always seemed strongly at odds with the conventional predictions Darwin and nearly all later evolutionists (paleontologists and other biologists alike) maintained must be true.
Matching observations with predictions derived from abstract theory is, of course, the lifeblood, the central process of all science. Science is a system of ideas, a body of statements about the material universe. Only logical consistency and especially the evidence of our senses can serve as a check, a way of assessing the accuracy of our ideas about the world around us. It is a procedural norm of science — though, curiously enough, a dictum honored most often in the breach — that when one's thoughts about the way things are supposed to be conflict with what seems to be the actual truth of the matter, we reexamine those notions which led us to erroneous predictions. So we must ask why the conflict between expectations and simple phenomena such as the Purisima fossils has persisted so long. Part of the answer lies in the very problems Darwin had to face just to convince the rest of the world that life had in fact evolved.
DARWIN'S INHERITANCE
Darwin didn't invent the idea of evolution. Alfred Russel Wallace, who later independently came up with the idea of natural selection, apparently set out for the Amazon forests in the 1840s with both his companion Henry Walter Bates and a firm conviction that life had evolved. As Frederick J. Teggart pointed out in his Theory of History (1925), the very idea of evolution stems from the Cartesian vision of a universe in constant motion. According to Teggart (p. 133), it was an achievement of eighteenth-century science in general to see the modern world as the result of changes that took place in the past. Darwin inherited a legacy of general notions of historical change — and, as is well known, his own grandfather was among his direct forerunners in shaping a definitive picture of the evolution of life.
Teggart's analysis of evolutionary thought up to his day makes absorbing reading. As a historian, Teggart saw an analogy between the biological theory of evolution and similar notions in the social sciences. Diffidently disavowing any biological expertise, Teggart nonetheless delivered a stunning exegesis of biological theory — and a critique that would have been devastating had it ever been read by biologists. Charged with the task of developing a coherent social studies curriculum for secondary schools in the early decades of this century, Teggart saw this social science in disarray. There seemed to be no coherent theory that united views of social organization with notions of how social systems change through time. And in particular, Teggart saw the two dominant approaches to the study of historical change — the work of historians on the one hand and that of evolutionarily minded sociologists and anthropologists on the other — as both severely flawed. The problem, simply, was the failure to integrate the actual events of history with general theories of historical change.
Teggart saw the description of the events of history as the special province of historiographers, people actually engaged in writing what we conventionally call "history." Historiography done well makes compelling reading, but Teggart was concerned with the lack of what we might call "rigor" in the output of the typical historian. There was then (and still is only emerging now) little in the way of comparative history, in which patterns of change among two or more societies might be contrasted and their similarities and differences recorded with an eye to developing general theories of social change through time. Rather (or so it seemed to Teggart), the goal of history seemed far more the glorification of a nation's pride, and historiography too often read like literary criticism, with too much emphasis on attempting to divine the motives underlying the actions of singular historical individuals.
But "evolutionism" seemed hardly any better. Teggart saw the general idea of evolution as linked with the notion of progress, and indeed it has recently become all the rage to point to Darwin's near infatuation with the view that evolution typically represents progressive change (see, for example, Jeremy Rifkin's generally compelling argument along these lines in his Algeny, and a similar, if more restrained, discussion in Eldredge and Tattersall's The Myths of Human Evolution). But the connection between evolution and progress was made long before Darwin. For example, Teggart writes (p. 133):
The theory of "evolution" rests, in the first place, upon the assumption that "progressive change" is "natural" and to be taken for granted, and that the aim of this progressive movement is the attainment of perfection. In the judgment of Erasmus Darwin [Darwin's grandfather] (1794), "it would appear that all nature exists in a state of perpetual improvement by laws impressed on the atoms of matter by the great Cause of Causes; and that the world may still be in its infancy, and continue to improve forever and ever."
But as a further and perhaps more fundamental objection to eighteenth- and nineteenth-century notions of evolution, Teggart worried about biologists in effect taking time for granted. "The assumption [Teggart writes, p. 134] that change is invariably slow, gradual, and continuous entails the very important condition that we may neglect the element of time." He cites the great French biologist Jean-Baptiste de Lamarck precisely to this effect. In his 1802 Hydrogéologie, Lamarck wrote: "For nature, time is nothing. It is never a difficulty, she always has it at her disposal; and it is for her the means by which she has accomplished the greatest as well as the least of her results." But there is a danger here. The peril that arises when one ignores the actual events of history derives from this simple assumption that given enough time, all things may eventually happen:
When, however, it is assumed that Nature always has unlimited time at her disposal, and that change is invariably slow and gradual, the statement is equivalent to the assertion that, in the study of evolution, the possibility of "events" may be ruled out of consideration. The dictum that "Nature never makes leaps" thus comes to be accepted as assurance that there never have been "events" in the history of the forms of life [Teggart, 1925, p. 135].
And somehow, to Teggart (and to many of us), the events of history, whether biological or social, must surely have something to tell us about how that change actually occurs.
(Continues...)
Excerpted from Time Frames by Niles Eldredge. Copyright © 1985 Niles Eldredge. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
