Read below two materials.

300-500 words reflection on these readings. 

Consider how Stephen Jay Gould suggests that life doesn’t really have a clear progressive plan (in the first essay) but then also argues for ethical and environmental progress (in the second essay). In particular, how does Gould reconcile these very different themes (absence of clear evolutionary progress versus the need for ethical/environmental progress)? Relate to own viewpoints and values. Does any of this help you contextualize your educational, moral, and social pursuits? 

Gould,Stephen Jay, Natural History, 00280712, Sep90, Vol. 99, Issue 9

The Golden Rule – A Proper Scale for Our Environmental Crisis

One among millions of species, we have a parochial, but legitimate, interest in our own

survival. It takes a particular kind of genius or deep understanding to transcend this most

pervasive of all conceptual biases and to capture a phenomenon by grasping a proper

scale beyond the measuring rods of our own world. Phenomena unfold on their own

appropriate scales of space and time and may be invisible in our myopic world of

dimensions assessed by comparison with human height and times metered by human life

spans. So much of accumulating importance at earthly scales—the results of geological

erosion, evolutionary changes in lineages—is invisible by the measuring rod of a human

life. So much that matters to particles in the microscopic world of molecules—like the

history of a dust grain subject to Brownian motion—either averages out to stability at our

scale or simply stands below our limits of perception.

The case at hand is a classic representative of a genre (environmentalists versus

developers) made familiar in recent struggles to save endangered populations—the snail

darter of a few years back, or the northern spotted owl versus timber interests (decided,

properly in my view, for the birds).

The University of Arizona, with the backing of an international consortium of

astronomers, wishes to build a complex of telescopes atop Mount Graham in southeastern

Arizona. But the old-growth spruce-fir habitat on the mountaintop forms the heart of the

range for Tamiasciurus hudsonicus grahamensis, the Mount Graham red squirrel—a

distinct subspecies that lives nowhere else and that forms the southernmost population of

the entire species. The population has already been reduced to some 100 survivors, and

destruction of several acres of spruce-fir growth (to build the telescopes) within the 700

or so remaining acres of best habitat might well administer a coup de grace to this fragile

population.

I cannot state an expert opinion on details of this controversy. Many questions

need to be answered. Is the squirrel population already too small to survive in any case?

If not, could the population, with proper management, coexist with the telescopes in the

remaining habitat? (Environmentalists fear change of microclimate as much or more than

loss of acreage. Reduction of forest canopy will increase wind and sun, producing a drop

in humidity. The squirrels survive winter by storing unopened cones in food caches

beside trees. If humidity falls, cones may dry out and open, causing loss of seeds and

destruction of food.)

I do not think that, practically or morally, we can defend a policy of saving every

distinct local population of organisms. I can cite a good rationale for the preservation of

species—for each species is a unique and separate natural object that, once lost, can never

be reconstituted. But subspecies are distinct local populations of species with broader

geographical ranges. Subspecies are dynamic, interbreedable, and constantly changing;

what then are we saving by declaring them all inviolate? Thus, I confess that I do not

agree with all arguments advanced by defenders of the Mount Graham red squirrel. One

leaflet, for example, argues: “The population has been recently shown to have a fixed,

homozygous allele which is unique in Western North America.” Sorry folks. I will stoutly

defend species, but we cannot ask for the preservation of every distinctive gene, unless

we find a way to abolish death itself (for many organisms carry unique mutations).

No, I think that for local populations of species with broader ranges, the brief for

preservation must be made on a case-by-case basis, not a general principle of

preservation (lest the environmental movement ultimately lose popular support for trying

to freeze a dynamic evolutionary world in status quo). On this proper basis of individual

merit, I am entirely persuaded that the Mount Graham red squirrel should be protected

and the astronomical observatory built elsewhere—and for two reasons. First, the squirrel

itself: the Mount Graham red is an unusually interesting local population within an

important species. It is isolated from all other populations and forms the southernmost

extreme of the species’ range. Such peripheral populations, living in marginal habitats,

are of special interest to students of evolution.

Second, the habitat: environmentalists continually face the political reality that

support and funding can be won for soft, cuddly, and “attractive” animals, but not for

slimy, grubby, and ugly creatures (of potentially greater evolutionary interest and

practical significance) or for habitats. This situation has led to the practical concept of

“umbrella” or “indicator” species—surrogates for a larger ecological entity worthy of

preservation. Thus, the giant panda (really quite a boring and ornery creature despite its

good looks) raises money to save the remaining bamboo forests of China (and a plethora

of other endangered creatures with no political clout); the northern spotted owl has just

rescued some magnificent stands of old-growth giant cedars, Douglas fir, and redwoods;

and the Mount Graham red squirrel may save a rare and precious habitat of extraordinary

evolutionary interest.

The Pinaleno Mountains, reaching 10,720 feet at Mount Graham, are an isolated

fault-block range separated from others by alluvial and desert valleys that dip to less than

3,000 feet in elevation. The high peaks of the Pinalenos contain an important and unusual

fauna for two reasons. First, they harbor a junction of two biogeographic provinces: the

Nearctic, or northern, by way of the Colorado Plateau, and the Neotropical, or southern,

via the Mexican Plateau. The Mount Graham red squirrel (a northern species) can live

this far south because high elevations reproduce the climate and habitat found near sea

level in the more congenial north. Second, and more important to evolutionists, the old-

growth spruce-fir habitats on the high peaks of the Pinalenos are isolated “sky islands”—

10,000-year-old remnants of a habitat more widely spread over the region during the

height of the last Ice Age. In evolutionary terms, these isolated pieces of habitat are true

islands—patches of more northern microclimate surrounded by southern desert. They are

functionally equivalent to bits of land in the ocean. Consider the role that islands (like the

Galapagos) have played both in developing the concepts of evolutionary theory and in

acting as cradles of origin (through isolation) or vestiges of preservation for biological

novelties.

Thus, whether or not the telescopes will drive the Mount Graham red squirrel to

extinction (an unsettled question well outside my area of expertise), the sky islands of the

Pinalenos are precious habitats that should not be compromised. Let the Mount Graham

red squirrel, so worthy of preservation in its own right, also serve as an indicator species

for the unique and fragile habitat that it occupies.

But why should I, a confirmed eastern urbanite, choose to involve myself in the

case of the Mount Graham red squirrel? The answer, unsurprisingly, is that I have been

enlisted—involuntarily, unawares, and on the wrong side to boot. I am simply fighting

mad, and fighting back.

The June 7, 1990, Wall Street Journal ran a pro-development, anti-squirrel

opinion piece by Michael D. Copeland (identified as “executive director of the Political

Economy Research Center in Bozeman, Montana”) under the patently absurd title: “No

Red Squirrels? Mother Nature May Be Better Off.” (I can at least grasp, while still

rejecting, the claim that nature would be no worse off if the squirrels died, but I am

utterly befuddled at how anyone could argue that the squirrels inflict a positive harm

upon the mother of us all!) In any case, Copeland misunderstood my writings in

formulating a supposedly scientific argument for his position.

Now, scarcely a day goes by when I do not read a misrepresentation of my views

(usually by creationists, racists, or football fans, in order of frequency). My response to

nearly all misquotation is the effective retort of preference: utter silence. (Honorable

intellectual disagreement should always be addressed; misquotation should be ignored,

when possible and politically practical.) I make an exception in this case because

Copeland cited me in the service of a classic false argument—indeed, the standard,

almost canonical misuse of my profession of paleontology in debates about extinction.

Paleontologists have been enlisted again and again, in opposition to our actual opinions

and in support of attitudes that most of us regard as anathema, to uphold arguments by

developers about the irrelevance (or even, in this case, the benevolence) of modern

anthropogenic extinction. This standard error is a classic example of failure to understand

the importance of scale—thus I return to the premise and structure of my introductory

paragraphs.

Paleontologists do discuss the inevitability of extinction for all species—in the

long run and on the broad scale of geological time. We are fond of saying that 99 percent

or more of all species that ever lived are now extinct. (My colleague Dave Raup often

opens talks on extinction with a zinging one-liner: “To a first approximation, all species

are extinct.”) We do therefore identify extinction as the normal fate of species. We also

talk a lot—more of late since new data have made the field so exciting—about the mass

extinctions that punctuate the history of life from time to time. We do discuss the issue of

eventual “recovery” from these extinctions, in the sense that life does rebuild or surpass

its former diversity after several million years. Finally, we do allow that mass extinctions

break up stable faunas and, in this sense, permit or even foster evolutionary innovations

well down the road (including the dominance of mammals and the eventual origin of

humans, following the death of dinosaurs).

From this set of statements about extinction in the fullness of geological time (on

scales of millions of years), some apologists for development have argued that extinction

at any scale (even of local populations within years or decades) poses no biological worry

but, on the contrary, must be viewed as a comfortable part of an inevitable natural order.

Or so Copeland states:

Suppose we lost a species. How devastating would that be? “Mass extinctions

have been recorded since the dawn of paleontology,” writes Harvard paleontologist

Stephen Gould … the most severe of these occurred approximately 250 million years ago

… with an estimated 96 percent extinction of species, says Mr. Gould. … There is general

agreement among scientists that today’s species represent a small proportion of all those

that have ever existed—probably less than 1 percent. This means that more than 99

percent of all species ever living have become extinct.

From these facts, largely irrelevant to red squirrels on Mount Graham, Copeland

makes inferences about the benevolence of extinction in general (although the argument

applies only to geological scales):

Yet, in spite of these extinctions, both Mr. Gould and University of Chicago

paleontologist Jack Sepkoski say that the actual number of living species has probably

increased over time. [True, but not as a result of mass extinctions, despite Copeland’s

next sentence.] The “niches” created by extinctions provide an opportunity for a vigorous

development of new species…. Thus, evolutionary history appears to have been

characterized by millions of species extinctions and subsequent increases in species

numbers. Indeed, by attempting to preserve species living on the brink of extinction, we

may be wasting time, effort and money on animals that will disappear over time,

regardless of our efforts.

But all will “disappear over time, regardless of our efforts”—millions of years

from now for most species if we don’t interfere. The mean life span of marine

invertebrate species lies between 5 and 10 million years; terrestrial vertebrate species turn

over more rapidly, but still average in the millions. By contrast, Homo sapiens may be

only 250,000 years old or so and may enjoy a considerable future if we don’t self-

destruct. Similarly, recovery from mass extinction takes its natural measure in millions of

years—as much as 10 million or more for fully rekindled diversity after major

catastrophic events.

These are the natural time scales of evolution and geology on our planet. But what

can such vastness possibly mean for our legitimately parochial interest in ourselves, our

ethnic groups, our nations, our cultural traditions, our bloodlines? Of what conceivable

significance to us is the prospect of recovery from mass extinction 10 million years down

the road if our entire species, not to mention our personal family lineage, has so little

prospect of surviving that long?

Capacity for recovery at geological scales has no bearing whatever upon the

meaning of extinction today. We are not protecting Mount Graham red squirrels because

we fear for global stability in a distant future not likely to include us. We are trying to

preserve populations and environments because the comfort and decency of our present

lives, and those of fellow species that share our planet, depend upon such stability. Mass

extinctions may not threaten distant futures, but they are decidedly unpleasant for species

in the throes of their power (particularly if triggered by such truly catastrophic events as

extraterrestrial impact). At the appropriate scale of our lives, we are just a species in the

midst of such a moment. And to say that we should let the squirrels go (at our immediate

scale) because all species eventually die (at geological scales) makes about as much sense

as arguing that we shouldn’t treat an easily curable childhood infection because all

humans are ultimately and inevitably mortal. I love geological time—a wondrous and

expansive notion that sets the foundation of my chosen profession, but such immensity is

not the proper scale of my personal life.

The same issue of scale underlies the main contributions that my profession of

paleontology might make to our larger search for an environmental ethic. This decade, a

prelude to the millennium, is widely and correctly viewed as a turning point that will lead

either to environmental perdition or stabilization. We have fouled local nests before and

driven regional faunas to extinction, but we have never been able to unleash planetary

effects before our current concern with ozone holes and putative global warming. In this

context, we are searching for proper themes and language to express our environmental

worries.

I don’t know that paleontology has a great deal to offer, but I would advance one

geological insight to combat a well-meaning, but seriously flawed (and all too common),

position and to focus attention on the right issue at the proper scale. Two linked

arguments are often promoted as a basis for an environmental ethic:

1. That we live on a fragile planet now subject to permanent derailment and
disruption by human intervention;

2. That humans must learn to act as stewards for this threatened world.

Such views, however well intentioned, are rooted in the old sin of pride and

exaggerated self-importance. We are one among millions of species, stewards of nothing.

By what argument could we, arising just a geological microsecond ago, become

responsible for the affairs of a world 4.5 billion years old, teeming with life that has been

evolving and diversifying for at least three-quarters of that immense span? Nature does

not exist for us, had no idea we were coming, and doesn’t give a damn about us.

This assertion of ultimate impotence could be countered if we, despite our late

arrival, now held power over the planet’s future (argument number one above). But we

don’t, despite popular misperception of our might. We are virtually powerless over the

earth at our planet’s own geological time scale. All the megatonnage in our nuclear

arsenals yield but one ten-thousandth the power of the asteroid that might have triggered

the Cretaceous mass extinction. Yet the earth survived that larger shock and, in wiping

out dinosaurs, paved the road for the evolution of large mammals, including humans. We

fear global warming, yet even the most radical model yields an earth far cooler than many

happy and prosperous times of a prehuman past. We can surely destroy ourselves, and

take many other species with us, but we can barely dent bacterial diversity and will surely

not remove many million species of insects and mites. On geological scales, our planet

will take good care of itself and let time clear the impact of any human malfeasance.

People who do not appreciate the fundamental principle of appropriate scales

often misread such an argument as a claim that we may therefore cease to worry about

environmental deterioration—just as Copeland argued falsely that we need not fret about

extinction. But I raise the same counterargument. We cannot threaten at geological

scales, but such vastness is entirely inappropriate. We have a legitimately parochial

interest in our own lives, the happiness and prosperity of our children, the suffering of

our fellows. The planet will recover from nuclear holocaust, but we will be killed and

maimed by the billions, and our cultures will perish. The earth will prosper if polar

icecaps melt under a global greenhouse, but most of our major cities, built at sea level as

ports and harbors, will founder, and changing agricultural patterns will uproot our

populations.

We must squarely face an unpleasant historical fact. The conservation movement

was born, in large part, as an elitist attempt by wealthy social leaders to preserve

wilderness as a domain for patrician leisure and contemplation (against the image, so to

speak, of poor immigrants traipsing in hordes through the woods with their Sunday picnic

baskets). We have never entirely shaken this legacy of environmentalism as something

opposed to immediate human needs, particularly of the impoverished and unfortunate.

But the Third World expands and contains most of the pristine habitat that we yearn to

preserve. Environmental movements cannot prevail until they convince people that clean

air and water, solar power, recycling, and reforestation are best solutions (as they are) for

human needs at human scales—and not for impossibly distant planetary futures.

I have a decidedly unradical suggestion to make about an appropriate

environmental ethic—one rooted, with this entire essay, in the issue of appropriate human

scale versus the majesty, but irrelevance, of geological time. I have never been much

attracted to the Kantian categorical imperative in searching for an ethic—to moral laws

that are absolute and unconditional and do not involve any ulterior motive or end. The

world is too complex and sloppy for such uncompromising attitudes (and God help us if

we embrace the wrong principle, and then fight wars, kill, and maim in our absolute

certainty). I prefer the messier “hypothetical imperatives” that invoke desire, negotiation,

and reciprocity. Of these “lesser,” but altogether wiser and deeper, principles, one has

stood out for its independent derivation, with different words but to the same effect, in

culture after culture. I imagine that our various societies grope toward this principle

because structural stability, and basic decency necessary for any tolerable life, demand

such a maxim. Christians call this principle the “golden rule”; Plato, Hillel, and

Confucius knew the same maxim by other names. I cannot think of a better principle

based on enlightened self-interest. If we all treated others as we wish to be treated

ourselves, then decency and stability would have to prevail.

I suggest that we execute such a pact with our planet. She holds all the cards and

has immense power over us—so such a compact, which we desperately need but she does

not at her own time scale, would be a blessing for us, and an indulgence for her. We had

better sign the papers while she is still willing to make a deal. If we treat her nicely, she

will keep us going for a while. If we scratch her, she will bleed, kick us out, bandage up,

and go about her business at her planetary scale. Poor Richard told us that “necessity

never made a good bargain,” but the earth is kinder than human agents in the “art of the

deal.” She will uphold her end; we must now go and do likewise.

92 S C I E N T I F I C A M E R I C A N R e p r i n t e d f r o m t h e O c t o b e r 1 9 9 4 i s s u

e

ome creators announce their inventions with grand
éclat. God proclaimed, “Fiat lux,” and then flooded
his new universe with brightness. Others bring forth
great discoveries in a modest guise, as did Charles
Darwin in defining his new mechanism of evolu-

tionary causality in 1859: “I have called this principle, by which
each slight variation, if useful, is preserved, by the term Natur-
al Selection.”

Natural selection is an immensely powerful yet beautifully
simple theory that has held up remarkably well, under intense
and unrelenting scrutiny and testing, for 135 years. In essence,
natural selection locates the mechanism of evolutionary change
in a “struggle” among organisms for reproductive success, lead-
ing to improved fit of populations to changing environments.
(Struggle is often a metaphorical description and need not be
viewed as overt combat, guns blazing. Tactics for reproductive
success include a variety of nonmartial activities such as earlier
and more frequent mating or better cooperation with partners
in raising offspring.) Natural selection is therefore a principle of
local adaptation, not of general advance or progress.

Yet powerful though the principle may be, natural selection
is not the only cause of evolutionary change (and may, in many
cases, be overshadowed by other forces). This point needs em-
phasis because the standard misapplication of evolutionary the-
ory assumes that biological explanation may be equated with
devising accounts, often speculative and conjectural in practice,
about the adaptive value of any given feature in its original en-
vironment (human aggression as good for hunting, music and
religion as good for tribal cohesion, for example). Darwin him-
self strongly emphasized the multifactorial nature of evolu-
tionary change and warned against too exclusive a reliance on
natural selection, by placing the following statement in a max-

imally conspicuous place at the very end of his introduction: “I
am convinced that Natural Selection has been the most impor-
tant, but not the exclusive, means of modification.”

Reality versus Conceit
N A T U R A L S E L E C T I O N is not fully sufficient to explain evo-
lutionary change for two major reasons. First, many other caus-
es are powerful, particularly at levels of biological organization
both above and below the traditional Darwinian focus on or-
ganisms and their struggles for reproductive success. At the low-
est level of substitution in individual base pairs of DNA, change
is often effectively neutral and therefore random. At higher lev-
els, involving entire species or faunas, punctuated equilibrium
can produce evolutionary trends by selection of species based
on their rates of origin and extirpation, whereas mass extinc-
tions wipe out substantial parts of biotas for reasons unrelat-
ed to adaptive struggles of constituent species in “normal”
times between such events.

Second, and the focus of this article, no matter how ade-
quate our general theory of evolutionary change, we also yearn
to document and understand the actual pathway of life’s his-
tory. Theory, of course, is relevant to explaining the pathway
(nothing about the pathway can be inconsistent with good the-
ory, and theory can predict certain general aspects of life’s geo-
logic pattern). But the actual pathway is strongly underdeter-
mined by our general theory of life’s evolution. This point needs
some belaboring as a central yet widely misunderstood aspect
of the world’s complexity. Webs and chains of historical events
are so intricate, so imbued with random and chaotic elements,
so unrepeatable in encompassing such a multitude of unique
(and uniquely interacting) objects, that standard models of sim-
ple prediction and replication do not apply.

The history of life is not necessarily progressive; it is certainly

not predictable. The earth’s creatures have evolved through

a series of contingent and fortuitous events

evolution of
life on earth

By Stephen Jay Gould

S

the

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

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3

History can be explained, with satisfying rigor if evidence be
adequate, after a sequence of events unfolds, but it cannot be pre-
dicted with any precision beforehand. Pierre-Simon Laplace,
echoing the growing and confident determinism of the late 18th
century, once said that he could specify all future states if he could
know the position and motion of all particles in the cosmos at
any moment, but the nature of universal complexity shatters this
chimerical dream. History includes too much chaos, or extremely
sensitive dependence on minute and unmeasurable differences in
initial conditions, leading to massively divergent outcomes based
on tiny and unknowable disparities in starting points. And his-
tory includes too much contingency, or shaping of present results
by long chains of unpredictable antecedent states, rather than im-
mediate determination by timeless laws of nature.

Homo sapiens did not appear on the earth, just a geologic
second ago, because evolutionary theory predicts such an out-
come based on themes of progress and increasing neural com-
plexity. Humans arose, rather, as a fortuitous and contingent
outcome of thousands of linked events, any one of which could
have occurred differently and sent history on an alternative
pathway that would not have led to consciousness. To cite just
four among a multitude: (1) If our inconspicuous and fragile lin-
eage had not been among the few survivors of the initial radia-
tion of multicellular animal life in the Cambrian explosion 5

30

million years ago, then no vertebrates would have inhabited the
earth at all. (Only one member of our chordate phylum, the
genus Pikaia, has been found among these earliest fossils. This
small and simple swimming creature, showing its allegiance to
us by possessing a notochord, or dorsal stiffening rod, is among

the rarest fossils of the Burgess Shale, our best preserved Cam-
brian fauna.) (2) If a small and unpromising group of lobe-
finned fishes had not evolved fin bones with a strong central axis
capable of bearing weight on land, then vertebrates might nev-
er have become terrestrial. (3) If a large extraterrestrial body
had not struck the earth 65 million years ago, then dinosaurs
would still be dominant and mammals insignificant (the situa-
tion that had prevailed for 100 million years previously). (4) If
a small lineage of primates had not evolved upright posture on
the drying African savannas just two to four million years ago,
then our ancestry might have ended in a line of apes that, like
the chimpanzee and gorilla today, would have become ecolog-
ically marginal and probably doomed to extinction despite their
remarkable behavioral complexity.

Therefore, to understand the events and generalities of life’s
pathway, we must go beyond principles of evolutionary theory
to a paleontological examination of the contingent pattern of
life’s history on our planet—the single actualized version among
millions of plausible alternatives that happened not to occur.
Such a view of life’s history is highly contrary both to conven-
tional deterministic models of Western science and to the deep-
est social traditions and psychological hopes of Western culture
for a history culminating in humans as life’s highest expression
and intended planetary steward.

Science can, and does, strive to grasp nature’s factuality, but
all science is socially embedded, and all scientists record pre-
vailing “certainties,” however hard they may be aiming for pure
objectivity. Darwin himself, in the closing lines of On the Ori-
gin of Species, expressed Victorian social preference more than

SLAB CONTAINING SPECIMENS of Pteridinium from Namibia shows a
prominent organism from the earth’s first multicellular fauna, called
Ediacaran, which appeared some 600 million years ago. The Ediacaran
animals died out before the Cambrian explosion of modern life. These thin,

quilted, sheetlike organisms may be ancestral to some modern forms but
may also represent a separate and ultimately failed experiment in
multicellular life. The history of life tends to move in quick and quirky
episodes, rather than by gradual improvement.

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

nature’s record in writing: “As natural
selection works solely by and for the
good of each being, all corporeal and
mental endowments will tend to progress
towards perfection.”

Life’s pathway certainly includes
many features predictable from laws of
nature, but these aspects are too broad
and general to provide the “rightness”
that we seek for validating evolution’s
particular results—roses, mushrooms,
people and so forth. Organisms adapt to,
and are constrained by, physical princi-
ples. It is, for example, scarcely surpris-
ing, given laws of gravity, that the largest
vertebrates in the sea (whales) exceed the
heaviest animals on land (elephants to-
day, dinosaurs in the past), which, in
turn, are far bulkier than the largest ver-
tebrate that ever flew (extinct pterosaurs
of the Mesozoic era).

Predictable ecological rules govern the
structuring of communities by principles
of energy flow and thermodynamics
(more biomass in prey than in predators,
for example). Evolutionary trends, once
started, may have local predictability
(“arms races,” in which both predators
and prey hone their defenses and weap-
ons, for example—a pattern that Geerat
J. Vermeij of the University of California
at Davis has called “escalation” and doc-
umented in increasing strength of both
crab claws and shells of their gastropod

prey through time). But laws of nature do
not tell us why we have crabs and snails
at all, why insects rule the multicellular
world and why vertebrates rather than
persistent algal mats exist as the most
complex forms of life on the earth.

Relative to the conventional view of
life’s history as an at least broadly pre-
dictable process of gradually advancing
complexity through time, three features
of the paleontological record stand out in
opposition and shall therefore serve as
organizing themes for the rest of this ar-
ticle: the constancy of modal complexity
throughout life’s history; the concentra-
tion of major events in short bursts inter-
spersed with long periods of relative sta-
bility; and the role of external impositions,
primarily mass extinctions, in disrupting
patterns of “normal” times. These three
features, combined with more general
themes of chaos and contingency, require
a new framework for conceptualizing
and drawing life’s history, and this article
therefore closes with suggestions for a dif-
ferent iconography of evolution.

The Lie of “Progress”
T H E P R I M A R Y paleontological fact
about life’s beginnings points to pre-
dictability for the onset and very little for
the particular pathways thereafter. The
earth is 4.6 billion years old, but the old-
est rocks date to about 3.9 billion years

because the earth’s surface became molten
early in its history, a result of bombard-
ment by large amounts of cosmic debris
during the solar system’s coalescence and
of heat generated by radioactive decay of
short-lived isotopes. These oldest rocks
are too metamorphosed by subsequent
heat and pressure to preserve fossils (al-
though some scientists interpret the pro-
portions of carbon isotopes in these
rocks as signs of organic production).
The oldest rocks sufficiently unaltered to
retain cellular fossils—African and Aus-
tralian sediments dated to 3.5 billion
years old—do preserve prokaryotic cells
(bacteria and cyanophytes) and stroma-
tolites (mats of sediment trapped and
bound by these cells in shallow marine
waters). Thus, life on the earth evolved
quickly and is as old as it could be. This
fact alone seems to indicate an inevit-
ability, or at least a predictability, for
life’s origin from the original chemical
constituents of atmosphere and ocean.

No one can doubt that more com-
plex creatures arose sequentially after
this prokaryotic beginning—first eu-
karyotic cells, perhaps about two billion
years ago, then multicellular animals
about 600 million years ago, with a relay
of highest complexity among animals
passing from invertebrates, to marine
vertebrates and, finally (if we wish, albeit
parochially, to honor neural architecture
as a primary criterion), to reptiles, mam-
mals and humans. This is the conven-
tional sequence represented in the old
charts and texts as an “age of inverte-
brates,” followed by an “age of fishes,”
“age of reptiles,” “age of mammals,”
and “age of man” (to add the old gender
bias to all the other prejudices implied by
this sequence).

I do not deny the facts of the preced-
ing paragraph but wish to argue that our
conventional desire to view history as
progressive, and to see humans as pre-
dictably dominant, has grossly distorted
our interpretation of life’s pathway by
falsely placing in the center of things a
relatively minor phenomenon that arises
only as a side consequence of a physical-
ly constrained starting point. The most
salient feature of life has been the stabil-
ity of its bacterial mode from the begin-

94 S C I E N T I F I C A M E R I C A N D I N O S A U R S A N D O T H E R M O N S T E R S

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PROGRESS DOES NOT RULE (and is not even a primary thrust of) the evolutionary process. For reasons
of chemistry and physics, life arises next to the “left wall” of its simplest conceivable and preservable
complexity. This style of life (bacterial) has remained most common and most successful. A few
creatures occasionally move to the right, thus extending the right tail in the distribution of
complexity. Many always move to the left, but they are absorbed within space already occupied.
Note that the bacterial mode has never changed in position, but just grown higher.

Left wall of minimal complexity

Bacteria

Complexity

Present

Precambrian

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ning of the fossil record until today and,
with little doubt, into all future time so
long as the earth endures. This is truly the
“age of bacteria”—as it was in the be-
ginning, is now and ever shall be.

For reasons related to the chemistry
of life’s origin and the physics of self-
organization, the first living things arose
at the lower limit of life’s conceivable,
preservable complexity. Call this lower
limit the “left wall” for an architecture of
complexity. Because so little space exists
between the left wall and life’s initial bac-
terial mode in the fossil record, only one
direction for future increment exists—to-
ward greater complexity at the right.
Thus, every once in a while, a more com-
plex creature evolves and extends the
range of life’s diversity in the only avail-
able direction. In technical terms, the dis-
tribution of complexity becomes more
strongly right skewed through these oc-
casional additions.

But the additions are rare and epi-
sodic. They do not even constitute an evo-
lutionary series but form a motley se-
quence of distantly related taxa, usually
depicted as eukaryotic cell, jellyfish, trilo-
bite, nautiloid, eurypterid (a large relative
of horseshoe crabs), fish, an amphibian
such as Eryops, a dinosaur, a mammal
and a human being. This sequence can-
not be construed as the major thrust or
trend of life’s history. Think rather of an
occasional creature tumbling into the
empty right region of complexity’s space.
Throughout this entire time, the bacteri-
al mode has grown in height and re-
mained constant in position. Bacteria rep-
resent the great success story of life’s path-
way. They occupy a wider domain of
environments and span a broader range
of biochemistries than any other group.
They are adaptable, indestructible and
astoundingly diverse. We cannot even
imagine how anthropogenic intervention
might threaten their extinction, although
we worry about our impact on nearly
every other form of life. The number of
Escherichia coli cells in the gut of each hu-
man being exceeds the number of hu-
mans that has ever lived on this planet.

One might grant that complexifica-
tion for life as a whole represents a
pseudotrend based on constraint at the

left wall but still hold that evolution with-
in particular groups differentially favors
complexity when the founding lineage
begins far enough from the left wall to
permit movement in both directions. Em-
pirical tests of this interesting hypothesis
are just beginning (as concern for the sub-
ject mounts among paleontologists), and
we do not yet have enough cases to ad-
vance a generality. But the first two stud-
ies—by Daniel W. McShea of the Uni-
versity of Michigan on mammalian ver-
tebrae and by George F. Boyajian of the
University of Pennsylvania on ammonite
suture lines—show no evolutionary ten-
dencies to favor increased complexity.

Moreover, when we consider that for
each mode of life involving greater com-
plexity, there probably exists an equally
advantageous style based on greater sim-
plicity of form (as often found in para-
sites, for example), then preferential evo-
lution toward complexity seems unlikely
a priori. Our impression that life evolves
toward greater complexity is probably
only a bias inspired by parochial focus on
ourselves, and consequent overattention
to complexifying creatures, while we ig-

nore just as many lineages adapting
equally well by becoming simpler in
form. The morphologically degenerate
parasite, safe within its host, has just as
much prospect for evolutionary success
as its gorgeously elaborate relative cop-
ing with the slings and arrows of outra-
geous fortune in a tough external world.

Steps, Not Inclines
E V E N I F C O M P L E X I T Y is only a drift
away from a constraining left wall, we
might view trends in this direction as
more predictable and characteristic of
life’s pathway as a whole if increments of
complexity accrued in a persistent and
gradually accumulating manner through
time. But nothing about life’s history is
more peculiar with respect to this com-
mon (and false) expectation than the ac-
tual pattern of extended stability and
rapid episodic movement, as revealed by
the fossil record.

Life remained almost exclusively uni-
cellular for the first five sixths of its his-
tory—from the first recorded fossils at
3.5 billion years to the first well-doc-
umented multicellular animals less than

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NEW ICONOGRAPHY OF LIFE’S TREE shows that maximal diversity in anatomical forms (not in number
of species) is reached very early in life’s multicellular history. Later times feature extinction of most
of these initial experiments and enormous success within surviving lines. This success is measured
in the proliferation of species but not in the development of new anatomies. Today we have more
species than ever before, although they are restricted to fewer basic anatomies.

Anatomical Diversity
Ti

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e

STEPHEN JAY GOULD taught biology, geology and the history of science at Harvard Uni-
versity from 1967 until his death in 2002 at age 60. The influential and provocative evo-
lutionary biologist had a Ph.D. in paleontology from Columbia University. Well known for
his popular writings, in particular a monthly column in Natural History magazine, he was
the author of more than a dozen books, including Full House: The Spread of Excellence from
Plato to Darwin and The Mismeasure of Man. T

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COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

600 million years ago. (Some simple
multicellular algae evolved more than a
billion years ago, but these organisms be-
long to the plant kingdom and have no
genealogical connection with animals.)
This long period of unicellular life does
include, to be sure, the vitally important
transition from simple prokaryotic cells
without organelles to eukaryotic cells
with nuclei, mitochondria and other com-
plexities of intracellular architecture—
but no recorded attainment of multicel-
lular animal organization for a full three
billion years. If complexity is such a good
thing, and multicellularity represents its
initial phase in our usual view, then life
certainly took its time in making this cru-
cial step. Such delays speak strongly
against general progress as the major
theme of life’s history, even if they can be
plausibly explained by lack of sufficient
atmospheric oxygen for most of Precam-

brian time or by failure of unicellular life
to achieve some structural threshold act-
ing as a prerequisite to multicellularity.

More curiously, all major stages in or-
ganizing animal life’s multicellular archi-
tecture then occurred in a short period be-
ginning less than 600 million years ago
and ending by about 530 million years
ago—and the steps within this sequence
are also discontinuous and episodic, not
gradually accumulative. The first fauna,
called Ediacaran to honor the Australian
locality of its initial discovery but now
known from rocks on all continents, con-
sists of highly flattened fronds, sheets and
circlets composed of numerous slender
segments quilted together. The nature of
the Ediacaran fauna is now a subject of
intense discussion. These creatures do not
seem to be simple precursors of later
forms. They may constitute a separate
and failed experiment in animal life, or

34. Sidneyia
35. Odaraia
36. Eiffelia
37. Mackenzia
38. Odontogriphus
39. Hallucigenia
40. Elrathia
41. Anomalocaris
42. Lingulella
43. Scenella
44. Canadaspis
45. Marrella
46. Olenoides

22. Emeraldella
23. Burgessia
24. Leanchoilia
25. Sanctacaris
26. Ottoia
27. Louisella
28. Actaeus
29. Yohoia
30. Peronochaeta
31. Selkirkia
32. Ancalagon
33. Burgessochaeta

11. Micromitra
12. Echmatocrinus
13. Chancelloria
14. Pirania
15. Choia
16. Leptomitus
17. Dinomischus
18. Wiwaxia
19. Naraoia
20. Hyolithes
21. Habelia

1. Vauxia (gracile)
2. Branchiocaris
3. Opabinia
4. Amiskwia
5. Vauxia (robust)
6. Molaria
7. Aysheaia
8. Sarotrocercus
9. Nectocaris

10. Pikaia

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they may represent a full range of di-
ploblastic (two-layered) organization, of
which the modern phylum Cnidaria
(corals, jellyfishes and their allies) remains
as a small and much altered remnant.

In any case, they apparently died out
well before the Cambrian biota evolved.
The Cambrian then began with an as-
semblage of bits and pieces, frustrating-
ly difficult to interpret, called the “small
shelly fauna.” The subsequent main
pulse, starting about 530 million years
ago, constitutes the famous Cambrian ex-
plosion, during which all but one modern
phylum of animal life made a first ap-
pearance in the fossil record. (Geologists
had previously allowed up to 40 million
years for this event, but an elegant study,
published in 1993, clearly restricts this pe-
riod of phyletic flowering to a mere five
million years.) The Bryozoa, a group of
sessile and colonial marine organisms, do
not arise until the beginning of the sub-
sequent, Ordovician period, but this ap-
parent delay may be an artifact of failure
to discover Cambrian representatives.

Although interesting and portentous
events have occurred since, from the flow-
ering of dinosaurs to the origin of human
consciousness, we do not exaggerate
greatly in stating that the subsequent his-
tory of animal life amounts to little more

than variations on anatomical themes es-
tablished during the Cambrian explosion
within five million years. Three billion
years of unicellularity, followed by five
million years of intense creativity and then
capped by more than 500 million years
of variation on set anatomical themes
can scarcely be read as a predictable, in-
exorable or continuous trend toward
progress or increasing complexity.

We do not know why the Cambrian
explosion could establish all major ana-
tomical designs so quickly. An “external”
explanation based on ecology seems at-
tractive: the Cambrian explosion repre-
sents an initial filling of the “ecological
barrel” of niches for multicellular organ-
isms, and any experiment found a space.
The barrel has never emptied since; even
the great mass extinctions left a few spe-
cies in each principal role, and their oc-
cupation of ecological space forecloses
opportunity for fundamental novelties.
But an “internal” explanation based on
genetics and development also seems nec-
essary as a complement: the earliest mul-
ticellular animals may have maintained a
flexibility for genetic change and embry-
ological transformation that became
greatly reduced as organisms “locked in”
to a set of stable and successful designs.

Either way, this initial period of both
internal and external flexibility yielded a
range of invertebrate anatomies that may
have exceeded (in just a few million years
of production) the full scope of animal
form in all the earth’s environments to-
day (after more than 500 million years of
additional time for further expansion).
Scientists are divided on this question.
Some claim that the anatomical range of
this initial explosion exceeded that of
modern life, as many early experiments
died out and no new phyla have ever
arisen. But scientists most strongly op-

posed to this view allow that Cambrian
diversity at least equaled the modern
range—so even the most cautious opin-
ion holds that 500 million subsequent
years of opportunity have not expanded
the Cambrian range, achieved in just five
million years. The Cambrian explosion
was the most remarkable and puzzling
event in the history of life.

Dumb Luck
M O R E O V E R , W E D O N O T know why
most of the early experiments died, while
a few survived to become our modern
phyla. It is tempting to say that the vic-
tors won by virtue of greater anatomical
complexity, better ecological fit or some
other predictable feature of convention-
al Darwinian struggle. But no recognized
traits unite the victors, and the radical al-
ternative must be entertained that each
early experiment received little more
than the equivalent of a ticket in the
largest lottery ever played out on our
planet—and that each surviving lineage,
including our own phylum of verte-
brates, inhabits the earth today more by
the luck of the draw than by any pre-
dictable struggle for existence. The his-
tory of multicellular animal life may be
more a story of great reduction in initial
possibilities, with stabilization of lucky
survivors, than a conventional tale of
steady ecological expansion and mor-
phological progress in complexity.

Finally, this pattern of long stasis,
with change concentrated in rapid epi-
sodes that establish new equilibria, may
be quite general at several scales of time
and magnitude, forming a kind of fractal
pattern in self-similarity. According to
the punctuated equilibrium model of spe-
ciation, trends within lineages occur by
accumulated episodes of geologically in-
stantaneous speciation, rather than by
gradual change within continuous pop-
ulations (like climbing a staircase rather
than rolling a ball up an inclined plane).

Even if evolutionary theory implied a
potential internal direction for life’s path-
way (although previous facts and argu-

GREAT DIVERSITY quickly evolved at the dawn of
multicellular animal life during the Cambrian
period (530 million years ago). The creatures
shown here are all found in the Middle Cambrian
Burgess Shale fauna of Canada. They include
some familiar forms (sponges, brachiopods)
that have survived. But many creatures (such
as the giant Anomalocaris, at the lower right,
largest of all the Cambrian animals) did not live
for long and were so anatomically peculiar
(relative to survivors) that we cannot classify
them among known phyla.

41

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ments in this article cast doubt on such
a claim), the occasional imposition of a
rapid and substantial, perhaps even tru-
ly catastrophic, change in environment
would have intervened to stymie the pat-
tern. These environmental changes trigger
mass extinction of a high percentage of
the earth’s species and may so derail any
internal direction and so reset the path-
way that the net pattern of life’s history
looks more capricious and concentrated
in episodes than steady and directional.

Mass extinctions have been recog-
nized since the dawn of paleontology; the
major divisions of the geologic time scale
were established at boundaries marked
by such events. But until the revival of in-
terest that began in the late 1970s, most
paleontologists treated mass extinctions
only as intensifications of ordinary
events, leading (at most) to a speeding up
of tendencies that pervaded normal
times. In this gradualistic theory of mass
extinction, these events really took a few
million years to unfold (with the appear-
ance of suddenness interpreted as an ar-
tifact of an imperfect fossil record), and
they only made the ordinary occur faster
(more intense Darwinian competition in
tough times, for example, leading to even
more efficient replacement of less adapt-
ed by superior forms).

The reinterpretation of mass extinc-
tions as central to life’s pathway and

radically different in effect began with
the presentation of data by Luis and
Walter Alvarez in 1979, indicating that
the impact of a large extraterrestrial ob-
ject (they suggested an asteroid seven to
10 kilometers in diameter) set off the last
great extinction at the Cretaceous-Ter-
tiary boundary 65 million years ago. Al-
though the Alvarez hypothesis initially
received very skeptical treatment from
scientists (a proper approach to highly
unconventional explanations), the case
now seems virtually proved by discovery
of the “smoking gun,” a crater of appro-
priate size and age located off the Yu-
catán peninsula in Mexico.

This reawakening of interest also in-
spired paleontologists to tabulate the
data of mass extinction more rigorously.
Work by David M. Raup, J. J. Sepkoski,
Jr., and David Jablonski of the Universi-
ty of Chicago has established that multi-
cellular animal life experienced five ma-
jor (end of Ordovician, late Devonian,
end of Permian, end of Triassic and end
of Cretaceous) and many minor mass ex-
tinctions during its 530-million-year his-
tory. We have no clear evidence that any
but the last of these events was triggered
by catastrophic impact, but such careful
study leads to the general conclusion that
mass extinctions were more frequent,
more rapid, more extensive in magnitude
and more different in effect than paleon-

tologists had previously realized. These
four properties encompass the radical
implications of mass extinction for un-
derstanding life’s pathway as more con-
tingent and chancy than predictable and
directional.

Mass extinctions are not random in
their impact on life. Some lineages suc-
cumb and others survive as sensible out-
comes based on presence or absence of
evolved features. But especially if the trig-
gering cause of extinction be sudden and
catastrophic, the reasons for life or death
may be random with respect to the orig-
inal value of key features when first
evolved in Darwinian struggles of nor-
mal times. This “different rules” model
of mass extinction imparts a quirky and
unpredictable character to life’s pathway
based on the evident claim that lineages
cannot anticipate future contingencies of
such magnitude and different operation.

To cite two examples from the im-
pact-triggered Cretaceous-Tertiary ex-
tinction 65 million years ago: First, an
important study published in 1986 not-
ed that diatoms survived the extinction
far better than other single-celled plank-
ton (primarily coccoliths and radiolaria).
This study found that many diatoms had
evolved a strategy of dormancy by en-
cystment, perhaps to survive through
seasonal periods of unfavorable condi-
tions (months of darkness in polar spe-

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CLASSICAL REPRESENTATIONS OF LIFE’S HISTORY reveal the severe biases of
viewing evolution as embodying a central principle of progress and
complexification. In these paintings by Charles R. Knight from a 1942 issue
of National Geographic, the first panel shows invertebrates of the Burgess
Shale. But as soon as fishes evolve, no subsequent scene ever shows

another invertebrate, although they did not go away or stop evolving.
When land vertebrates arise ( panel 2), we never see another fish, even
though return of land vertebrate lineages to the sea may be depicted
( panel 3). The sequence always ends with mammals—even though fishes,
invertebrates and reptiles are still thriving—and, of course, humans.

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.

cies as otherwise fatal to these photosyn-
thesizing cells; sporadic availability of sil-
ica needed to construct their skeletons).
Other planktonic cells had not evolved
any mechanisms for dormancy. If the ter-
minal Cretaceous impact produced a
dust cloud that blocked light for several
months or longer (one popular idea for a
“killing scenario” in the extinction), then
diatoms may have survived as a fortu-
itous result of dormancy mechanisms
evolved for the entirely different function
of weathering seasonal droughts in ordi-
nary times. Diatoms are not superior to
radiolaria or other plankton that suc-
cumbed in far greater numbers; they
were simply fortunate to possess a fa-
vorable feature, evolved for other rea-
sons, that fostered passage through the
impact and its sequelae.

Second, we all know that dinosaurs
perished in the end Cretaceous event and
that mammals therefore rule the verte-
brate world today. Most people assume
that mammals prevailed in these tough
times for some reason of general superi-
ority over dinosaurs. But such a conclu-
sion seems most unlikely. Mammals and
dinosaurs had coexisted for 100 million
years, and mammals had remained rat-
sized or smaller, making no evolutionary
“move” to oust dinosaurs. No good ar-
gument for mammalian prevalence by
general superiority has ever been ad-
vanced, and fortuity seems far more like-
ly. As one plausible argument, mammals
may have survived partly as a result of
their small size (with much larger, and
therefore extinction-resistant, popula-
tions as a consequence, and less ecologi-
cal specialization with more places to hide,
so to speak). Small size may not have been
a positive mammalian adaptation at all,
but more a sign of inability ever to pene-
trate the dominant domain of dinosaurs.
Yet this “negative” feature of normal
times may be the key reason for mamma-
lian survival and a prerequisite to my writ-
ing and your reading this article today.

Sigmund Freud often remarked that
great revolutions in the history of science
have but one common, and ironic, fea-
ture: they knock human arrogance off
one pedestal after another of our previous
conviction about our own self-impor-

tance. In Freud’s three examples, Coper-
nicus moved our home from center to pe-
riphery; Darwin then relegated us to “de-
scent from an animal world”; and, final-
ly (in one of the least modest statements
of intellectual history), Freud himself dis-
covered the unconscious and exploded
the myth of a fully rational mind.

In this wise and crucial sense, the Dar-
winian revolution remains woefully in-
complete because, even though thinking
humanity accepts the fact of evolution,
most of us are still unwilling to abandon
the comforting view that evolution means
(or at least embodies a central principle
of) progress defined to render the ap-
pearance of something like human con-
sciousness either virtually inevitable or at
least predictable. The pedestal is not
smashed until we abandon progress or
complexification as a central principle
and come to entertain the strong possi-
bility that H. sapiens is but a tiny, late-
arising twig on life’s enormously ar-
borescent bush—a small bud that would
almost surely not appear a second time if
we could replant the bush from seed and
let it grow again.

Parochial Evolution
P R I M A T E S A R E V I S U A L A N I M A L S ,
and the pictures we draw betray our
deepest convictions and display our cur-
rent conceptual limitations. Artists have
always painted the history of fossil life
as a sequence from invertebrates, to fish-
es, to early terrestrial amphibians and
reptiles, to dinosaurs, to mammals and,
finally, to humans. There are no excep-
tions; all sequences painted since the in-
ception of this genre in the 1850s follow
the convention.

Yet we never stop to recognize the al-
most absurd biases coded into this uni-
versal mode. No scene ever shows an-
other invertebrate after fishes evolved,
but invertebrates did not go away or stop
evolving! After terrestrial reptiles emerge,
no subsequent scene ever shows a fish

(later oceanic tableaux depict only such
returning reptiles as ichthyosaurs and
plesiosaurs). But fishes did not stop
evolving after one small lineage managed
to invade the land. In fact, the major
event in the evolution of fishes, the origin
and rise to dominance of the teleosts, or
modern bony fishes, occurred during the
time of the dinosaurs and is therefore
never shown at all in any of these se-
quences—even though teleosts include
more than half of all species of verte-
brates. Why should humans appear at
the end of all sequences? Our order of
primates is ancient among mammals,
and many other successful lineages arose
later than we did.

We will not smash Freud’s pedestal
and complete Darwin’s revolution until
we find, grasp and accept another way of
drawing life’s history. J.B.S. Haldane
proclaimed nature “queerer than we can
suppose,” but these limits may only be
socially imposed conceptual locks rather
then inherent restrictions of our neurol-
ogy. New icons might break the locks.
Trees—or rather copiously and luxuri-
antly branching bushes—rather than lad-
ders and sequences hold the key to this
conceptual transition.

We must learn to depict the full range
of variation, not just our parochial per-
ception of the tiny right tail of most com-
plex creatures. We must recognize that
this tree may have contained a maximal
number of branches near the beginning
of multicellular life and that subsequent
history is for the most part a process of
elimination and lucky survivorship of a
few, rather than continuous flowering,
progress and expansion of a growing
multitude. We must understand that lit-
tle twigs are contingent nubbins, not pre-
dictable goals of the massive bush be-
neath. We must remember the greatest of
all biblical statements about wisdom:
“She is a tree of life to them that lay hold
upon her; and happy is every one that re-
taineth her.”

100 S C I E N T I F I C A M E R I C A N D I N O S A U R S A N D O T H E R M O N S T E R S

Extinction: A Scientific American Book. Steven M. Stanley. W. H. Freeman and Company, 1987.

Wonderful Life: The Burgess Shale and the Nature of History. S. J. Gould. W. W. Norton, 1989.

The Book of Life. Edited by Stephen Jay Gould. W. W. Norton, 1993.

The Structure of Evolutionary Theory. Stephen Jay Gould. Harvard University Press, 2002.

M O R E T O E X P L O R E

COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.