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Feb, 2000What does the dreaded "E" word mean, anyway.
Author/s: Stephen Jay Gould
A reverie for the opening of the new Hayden Planetarium.
Evolution posed no terrors in the liberal constituency of New York City when I studied biology at Jamaica High School in 1956. But our textbooks didn't utter the word either--a legacy of the statutes that had brought William Jennings Bryan and Clarence Darrow to legal blows at Tennessee's trial of John Scopes in 1925. The subject remained doubly hidden within my textbook--covered only in chapter 63 (of 66) and described in euphemism as "the hypothesis of racial development."
The antievolution laws of the Scopes era, passed during the early 1920s in several southern and border states, remained on the books until 1968, when the Supreme Court declared them unconstitutional. The laws were never strictly enforced, but their existence cast a pall over American education, as textbook publishers capitulated to produce "least common denominator" versions acceptable in all states--so schoolkids in New York got short shrift because the statutes of some distant states had labeled evolution dangerous and unteachable.
Ironically, at the very end of this millennium (I am writing this essay in late November 1999), demotions, warnings, and anathemas have again come into vogue in several regions of our nation. The Kansas school board has reduced evolution, the central and unifying concept of the life sciences, to an optional subject within the state's biology curriculum--an educational ruling akin to stating that English will still be taught but that grammar may henceforth be regarded as a peripheral frill, permitted but not mandated as a classroom subject. Two states now require that warning labels be pasted (literally) into all biology textbooks, alerting students that they might wish to consider alternatives to evolution (although no other well-documented scientific concept evokes similar caution). Finally, at least two states have retained all their Darwinian material in official pamphlets and curricula but have replaced the dreaded "e" word with a circumlocution, thus reviving the old strategy of my high school text.
As our fight for good (and politically untrammeled) public education in science must include our forceful defense of a key word--for inquisitors have always understood that an idea can be extinguished most effectively by suppressing all memory of a defining word or an inspirational person--we might consider an interesting historical irony that, properly elucidated, might even aid us in our battle. We must not compromise our showcasing of the "e" word, for we give up the game before we start if we grant our opponents control over basic terms. But we should also note that Darwin himself never used the word "evolution" in his epochal book of 1859. In Origin of Species, he calls this fundamental biological process "descent with modification." Darwin, needless to say, did not shun "evolution" from motives of fear, conciliation, or political savvy but rather for an opposite and principled reason that can help us appreciate the depth of the intellectual revolution that he inspired and some of the reasons (understandable if indefensible) for the persistent public unease.
Pre-Darwinian terminology for evolution--a widely discussed, if unorthodox, view of life in early nineteenth-century biology--generally used such names as transformation, transmutation, or the development hypothesis. In choosing a label for his own, very different account of genealogical change, Darwin would never have considered "evolution" as a descriptor, because that vernacular English word implied a set of consequences contrary to the most distinctive features of his proposed revolutionary mechanism of change.
"Evolution," from the Latin evolvere, literally means "an unrolling"--and clearly implies an unfolding in time of a predictable or prepackaged sequence in an inherently progressive, or at least directional, manner (the "fiddlehead" of a fern unrolls and expands to bring forth the adult plant--a true evolution of preformed parts). The Oxford English Dictionary traces the word "evolution" to seventeenth-century English poetry. Here the word's key meaning--the sequential exposure of prepackaged potential--inspired the first recorded usages in our language. For example, Henry More (1614-87), the British philosopher responsible for several of the seventeenth-century citations in the OED entry, stated in 1664,"I have not yet evolved all the intangling superstitions that may be wrapt up."
The few pre-Darwinian English citations of genealogical change as "evolution" all employ the word as a synonym for predictable progress. For example, in describing Lamarck's theory for British readers (in the second volume of his Principles of Geology, 1832), Charles Lyell generally uses the neutral term "transmutation"--except in one passage, where he wishes to highlight a claim for progress: "The testacea of the ocean existed first, until some of them by gradual evolution were improved into those inhabiting the land."
Although the word "evolution" does not appear in the first edition of Origin of Species, Darwin does use the verbal form "evolved" clearly in the vernacular sense and in an especially crucial spot: the very last word of the book! Most students have failed to appreciate the incisive and intended "gotcha" of these closing lines, which have generally been read as a poetic reverie, a harmless linguistic flourish essentially devoid of content, however rich in imagery. In fact, the canny Darwin used this maximally effective location to make a telling point about the absolute glory and comparative importance of natural history as a calling.
We usually regard planetary physics as the paragon of rigorous science, while dismissing natural history as a lightweight exercise in dull, descriptive cataloging that any person with sufficient patience might accomplish. But Darwin, in his closing passage, identified the primary phenomenon of planetary physics as a dull and simple cycling to nowhere, in sharp contrast with life's history, depicted as a dynamic and upwardly growing tree. The Earth revolves in uninteresting sameness, but life evolves by unfolding its potential for ever expanding diversity along admittedly unpredictable, but wonderfully various, branchings:
Whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
But Darwin could not have described the process regulated by his mechanism of natural selection as "evolution" in the vernacular meaning then conveyed by the word. For the mechanism of natural selection yields only increasing adaptation to changing local environments, not predictable progress in the usual sense of cosmic or general betterment expressed as growing complexity, augmented mentality, or whatever. In Darwin's causal world, an anatomically degenerate parasite, reduced to a formless clump of feeding and reproductive cells within the body of a host, may be just as well adapted to its surroundings, and just as well endowed with prospects for evolutionary persistence, as is the most intricate creature, exquisitely adapted in all parts to a complex and dangerous external environment. Moreover, since natural selection can adapt organisms only to local circumstances, and since local circumstances change in an effectively random manner through geological time, the pathways of adaptive evolution cannot be predicted.
Thus, on these two fundamental grounds--lack of inherent directionality and lack of predictability--the process regulated by natural selection could scarcely have suggested, to Darwin, the label "evolution," an ordinary English word for sequences of predictable and directional unfolding. We must then, and obviously, ask how "evolution" achieved its coup in becoming the name for Darwin's process--a takeover so complete that the word has now almost (but not quite, as we shall soon see) lost its original English meaning of "unfolding" and has transmuted (or should we say "evolved"?) into an effective synonym for biological change through time.
This interesting shift, despite Darwin's own reticence, occurred primarily because a great majority of his contemporaries, while granting the overwhelming evidence for evolution's factuality, could not accept Darwin's radical views about the causes and patterns of biological change. Most important, they could not bear to surrender the comforting and traditional view that human consciousness must represent a predictable (if not a divinely intended) summit of biological existence. If scientific discoveries enjoined an evolutionary reading of human superiority, then one must bow to the evidence. But Darwin's contemporaries (and many people today as well) would not surrender their traditional view of human domination, and therefore could conceptualize genealogical transmutation only as a process defined by predictable progress toward a human acme--in short, as a process well described by the term "evolution" in its vernacular meaning of "unfolding an inherent potential."
Herbert Spencer's progressivist view of natural change probably exerted the greatest influence in establishing "evolution" as the general name for Darwin's process, for Spencer held a dominating status as Victorian pundit and grand panjandrum of nearly everything conceptual. In any case, Darwin had too many other fish to fry and didn't choose to fight a battle about words rather than things. He felt confident that his views would eventually prevail, even over the contrary etymology of a word imposed upon his process by popular will. (He knew, after all, that meanings of words can transmute within new climates of immediate utility, just as species transform under new local environments of life and ecology!) Darwin never used the "e" word extensively in his writings, but he did capitulate to a developing consensus by referring to his process as evolution for the first time in Descent of Man, published in 1871. (Still, Darwin never used the word "evolution" in the title of any book--and he chose, in his book on human history, to emphasize the genealogical "descent" of our species, not our "ascent" to higher levels of consciousness.)
When I was a young boy, growing up on the streets of New York City, the American Museum of Natural History became my second home and inspiration. I loved two exhibits most of all--the Tyrannosaurus skeleton on the fourth floor and the star show at the adjacent Hayden Planetarium. I juggled these two passions for many years and eventually became a paleontologist; Carl Sagan, my near-contemporary from the neighboring neverland of Brooklyn (I grew up in Queens) weighed the same two interests in the same building but opted for astronomy as a calling. (I have always suspected a basic biological determinism behind our opposite choices. Carl was tall and looked up toward the heavens; I am shorter than average and tend to look down at the ground.)
My essays may be known for their tactic of selecting odd little tidbits as illustrations of general themes. But why, to mark the reopening of the Hayden Planetarium, would I highlight such a quirky and apparently irrelevant subject as the odyssey of the term "evolution" in scientific, and primarily biological, use---thus seeming, once again, to reject the cosmos in favor of the dinosaurs? Method does inhere in my apparent madness (whether or not I succeed in conveying this reasoning to my readers). I am writing about the term "evolution" in the domain I know in order to explicate its strikingly different meaning in the profession that I put aside but still love avocationally. A discussion of the contrasts between biological evolution and cosmological evolution might offer some utility as a commentary about alternative worldviews and as a reminder that many supposed debates in science arise from confusion engendered by differing uses of words and not from deep conceptual muddles about the nature of things.
Interdisciplinary unification represents a grand and worthy goal of intellectual life, but greater understanding can often be won by principled separation and mutual respect, based on clear definitions and distinctions among truly disparate processes, rather than by false unions forged with superficial similarities and papered over by a common terminology In our understandable desire to unify the sciences of temporal change, we have too often followed the Procrustean strategy of enforcing a common set of causes and explanations upon the history or a species and the life or a star--partly, at least, for the very bad reason that both professions use the term "evolution" to denote change through time. In this case, the fundamental differences trump the superficial similarities-and true unity will be achieved only when we acknowledge the disparate substrates that, taken together, probe the range of possibilities for theories of historical order.
The Darwinian principle of natural selection yields temporal change--evolution in the biological definition--by the twofold process of producing copious and undirected variation within a population and then passing along only a biased (selected) portion of this variation to the next generation. In this manner, the variation within a population at any moment can be converted into differences in mean values (average size, average braininess) among successive populations through time. For this fundamental reason, we call such theories of change variational as opposed to the more conventional, and more direct, models of transformational change imposed by natural laws that mandate a particular trajectory based on inherent (and therefore predictable) properties of substances and environments. (A ball rolling down an inclined plane does not reach the bottom because selection has favored the differential propagation of moving versus stable elements of its totality but because gravity dictates this result when round balls roll down smooth planes.)
To illustrate the peculiar properties of variational theories like Darwin's in an obviously caricatured, but not inaccurate, description: Suppose that a population of elephants inhabits Siberia during a warm interval before the advance of an ice sheet. The elephants vary, at random and in all directions, in their amount of body hair. As the ice advances and local conditions become colder, elephants with more hair will tend to cope better, by the sheer good fortune of their superior adaptation to changing climates--and they will leave more surviving offspring on average. (This differential reproductive success must be conceived as broadly statistical and not guaranteed in every case: in any generation, the hairiest elephant of all may fall into a crevasse and die.) Because offspring inherit their parents' degree of hairiness, the next generation will contain a higher proportion of more densely clad elephants (who will continue to be favored by natural selection as the climate becomes still colder). This process of increasing average hairiness may continue for many generations, leading to the evolution of woolly mammoths.
This little fable can help us understand how peculiar and how contrary to all traditions of Western thought and explanation the Darwinian theory of evolution, and variational theories of historical change in general, must sound to the common ear. All the odd and fascinating properties of Darwinian evolution--the sensible and explainable but quite unpredictable nature of the outcome (dependent upon complex and contingent changes in local environments), the nonprogressive character of the alteration (adaptive only to these unpredictable local circumstances and not inevitably building a "better" elephant in any cosmic or general sense)--flow from the variational basis of natural selection.
Transformational theories work in a much simpler and more direct manner. If I want to go from A to B, I will have so much less conceptual (and actual) trouble if I can postulate a mechanism that will just push me there directly than if I must rely upon the selection of "a few good men" from a random cloud of variation about point A, then constitute a new generation around an average point one step closer to B, then generate a new cloud of random variation about this new point, then select "a few good men" once again from this new array--and then repeat this process over and over until I finally reach B.
When one adds the oddity of variational theories in general to our strong cultural and psychological resistance against their application to our own evolutionary origin (as an unpredictable and not necessary progressive little twig on life's luxuriant tree), then we can better understand why Darwin's revolution surpassed all other scientific discoveries in reformatory power and why so many people still fail to understand, and may even actively resist, its truly liberating content. (I must leave the issue of liberation for another time, but once we recognize that the specification of morals and the search for a meaning to our lives cannot be accomplished by scientific study in any case, then Darwin's variational mechanism will no longer seem threatening and may even become liberating in teaching us to look within ourselves for answers to these questions and to abandon a chimerical search for the purpose of our lives, and for the source of our ethical values, in the external workings of nature.)
These difficulties in grasping Darwin's great insight became exacerbated when our Victorian forebears made their unfortunate choice of a defining word--"evolution"--with its vernacular meaning of "directed unfolding." We would not face this additional problem today if "evolution" had undergone a complete transformation to become a strict and exclusive definition of biological change--with earlier and etymologically more appropriate usages then abandoned and forgotten. But important words rarely undergo such a clean switch of meaning, and "evolution" still maintains its original definition of "predictable unfolding" in several nonbiological disciplines-including astronomy.
When astronomers talk about the evolution of a star, they clearly do not have a variational theory like Darwin's in mind. Stars do not change through time because mama and papa stars generate broods of varying daughter stars, followed by the differential survival of daughters best adapted to their particular region of the cosmos. Rather, theories of stellar "evolution" could not be more relentlessly transformational in positing a definite and predictable sequence of changes unfolding as simple consequences of physical laws. (No biological process operates in exactly the same manner, but the life cycle of an organism certainly works better than the evolution of a species as a source of analogy.)
Ironically, astronomy undeniably trumps biology in faithfulness to the etymology and the vernacular definition of "evolution"--even though the term now holds far wider currency under the radically altered definition of the biological sciences. In fact, astronomers have been so true to the original definition that they confine "evolution" to historical sequences of predictable unfolding and resolutely shun the word when describing cosmic changes exhibiting the key features of biological evolution--unpredictability and lack of inherent directionality.
As an illustration of this astronomical usage, consider the most standard and conventional of all sources--the Encyclopaedia Britannica article "Stars and Star Clusters" (15th edition, 1990 printing). The section entitled "Star Formation and Evolution" begins by analogizing stellar "evolution" to a preprogrammed life cycle, with the degree of evolution defined as the position along the predictable trajectory:
Throughout the Milky Way Galaxy ... astronomers haw' discovered stars that are well evolved or even approaching extinction, or both, as well as occasional stars that must be very young or still in the process of formation. Evolutionary effects on these stars are not negligible.
The fully predictable and linear sequence of stages in a stellar lifetime (evolution, to astronomers) records the consequences of a defining physical process in the construction and history of stars: the conversion of mass to energy by nuclear reactions deep within stars, leading to the transformation of hydrogen into helium.
The spread of luminosities and colors of stars within the main sequence can be understood as a consequence of evolution.... As the stars evolve, they adjust to the increase in the helium-to-hydrogen ratio in their cores.... When the core fuel is exhausted, the internal structure of the star changes rapidly; it quickly leaves the main sequence and moves towards the region of giants and supergiants.
The same basic sequence unfolds through stellar lives, but the rate of change (evolution, to astronomers) varies as a predictable consequence of differences in mass:
Like the rate of formation of a star, the subsequent rate of evolution on the main sequence is proportional to the mass of the star; the greater the mass, the more rapid the evolution.
More complex factors may determine variation in some stages of the life cycle, but the basic directionality (evolution, to astronomers) does not alter, and predictability from natural law remains precise and complete:
The great spread in luminosities and colors of giant, supergiant, and subgiant stars is also understood to result from evolutionary events. When a star leaves the main sequence, its future evolution is precisely determined by its mass, rate of rotation (or angular momentum), chemical composition, and whether or not it is a member of a close binary system.
In the most revealing verbal clue of all, the discourse of this particular scientific culture seems to shun the word "evolution" when historical sequences become too meandering, too nondirectional, or too complex to explain as simple consequences of controlling laws--even though the end result may be markedly different from the beginning state, thus illustrating significant change through time. For example, the same Britannica article on stellar evolution notes that one can often reach conclusions about the origin of a star or a planet from the relative abundance of chemical elements in its present composition.
Earth, however, has become so modified during its geological history that we cannot use this inferential method to reconstruct the initial state of our own planet. Because the current configuration of Earth's surface developed through complex contingencies and could not have been predicted from simple laws, this style of change apparently does not rank as evolution--but only, in astronomical parlance, as being "affected":
The relative abundances of the chemical elements provide significant clues regarding their origin. The Earth's crust has been affected severely by erosion, fractionation, and other geologic events, so that its present varied composition offers few clues as to its early stages.
I don't mention these differences to lament, to complain, or to criticize astronomers in any way. After all, their use of "evolution" remains more faithful to etymology and the original English definition, whereas our Darwinian reconstruction has virtually reversed the original meaning. In this case, since neither side will or should give up its understanding of "evolution" (astronomers because they have retained an original and etymologically correct meaning, and evolutionists because their redefinition expresses the very heart of their central and revolutionary concept of life's history), our best solution lies simply in exposing the legitimate differences and explaining the good reasons behind the disparity in usage.
In this way, at least, we may avoid confusion and also the special frustration generated when prolonged wrangles arise from misunderstandings of words rather than from genuine disputes about things and causes in nature. We evolutionary biologists must remain especially sensitive to this issue, because we still face considerable opposition, based on conventional hopes and fears, to our insistence that life evolves in unpredictable directions, with no inherent goal. Since astronomical evolution upholds both contrary positions--predictability and directionality--evolutionary biologists need to emphasize their own distinctive meaning, especially since the general public feels much more comfortable with the astronomical sense and will therefore impose this more congenial definition upon the history of life if we do not clearly explain the logic, the evidence, and the sheer fascination of our challenging conclusion.
Two studies published within the past month led me to this topic, because each discovery confirms the biological, variational, and Darwinian "take" on evolution while also, and quite explicitly, refuting a previous, transformational interpretation--rooted in our culturally established prejudices for the more comforting, astronomical view--that had blocked our understanding and skewed our thoughts about an important episode in life's history:
1. Vertebrates "all the way down." In one of the most crucial and enigmatic episodes in the history of life--and a challenge to the older, more congenial idea that life has progressed in a basically stately, linear manner through the ages--nearly all animal phyla made their first appearance in the fossil record at essentially the same time, an interval of some 5 million years (about 525 million to 530 million years ago) called the Cambrian explosion. (Geological firecrackers have long fuses when measured by the inappropriate scale of human time.) Only one major phylum with prominent and fossilizable hard parts did not appear in this incident or during the Cambrian period at all--the Bryozoa, a group of colonial marine organisms unknown to most nonspecialists today (although still relatively common in shallow oceanic waters) but prominent in the early fossil record of animal life.
One other group, until last month, also had no record within the Cambrian explosion, although late Cambrian representatives (well after the explosion itself) have been known for some time. Whereas popular texts have virtually ignored the Bryozoa, the absence of this other group has been prominently showcased and proclaimed highly significant. No vertebrates had ever been recovered from deposits of the Cambrian explosion, although close relatives within our phylum (the Chordata), if not technically vertebrates, had been collected (the Chordata includes three major subgroups: the tunicates, Amphioxus and its relatives, and the vertebrates proper).
This absence of vertebrates from strata bearing nearly all other fossilizable animal phyla provided a strong ray of hope for people who wished to view our own group as "higher" or more evolved--a predictable direction. If evolution implies linear progression, then later is better--and uniquely later (or almost uniquely, given those pesky bryozoans) can only enhance the distinction. But the November 4, 1999, issue of Nature includes a persuasive article ("Lower Cambrian Vertebrates from South China," by D-G. Shu, H-L. Luo, S. Conway Morris, X-L. Zhang, S-X. Hu, L. Chen, J. Han, M. Zhu, Y. Li, and L-Z. Chen) reporting the discovery of two vertebrate genera within the Lower Cambrian Chengjiang formation of southern China, right within the temporal heart of the Cambrian explosion. (The Burgess Shale of western Canada, the celebrated site for most previous knowledge of early Cambrian animals, postdates the actual explosion by several million years. The recently discovered Chengjiang fauna, with equally exquisite preservation of soft anatomy, has been yielding comparable or even greater treasures for more than a decade. See "On Embryos and Ancestors," Natural History, July-August 1998.)
These two creatures--each only an inch or so in length and lacking both jaws and a backbone and in fact possessing no bony skeleton at all--might not strike a casual student as worthy of inclusion within our exalted lineage. But these features, however much they may command our present focus, arose later in the history of vertebrates and do not enter the central and inclusive taxonomic definition of our group. The vertebrate jaw, for example, evolved from hard parts that originally fortified the gill openings and then moved forward to surround the mouth. All early fishes--and two modern survivors of this initial radiation, the lampreys and the hagfishes--lacked jaws.
The two Chengjiang genera possess all the defining features of vertebrates: the stiff dorsal supporting rod, or notochord (subsequently lost in adults after the vertebral column evolved); the arrangement of flank musculature in a series of zigzag elements from front to back; the set of paired openings piercing the pharynx (operating primarily as respiratory gills in later fishes but used mostly for filter feeding in ancestral vertebrates). In fact, the best reconstruction of branching order on the vertebrate tree places the origin of these two new genera after the inferred ancestors of modern hagfishes but before the presumed forebears of lampreys. If this inference holds, then vertebrates already existed in substantial diversity within the Cambrian explosion. In any case, we now have two distinct and concrete examples of vertebrates "all the way down"--that is, in the very same strata that include the first known fossils of nearly all phyla of modern multicellular animals. We vertebrates do not stand higher and later than our invertebrate cousins, for all "advanced" animal phyla made their first appearance in the fossil record at essentially the same time. The vaunted complexity of vertebrates did not require a special delay to accommodate a slow series of progress-did not require a special delay to accommodate a slow series of progressive steps, predictable from the general principles of evolution.
2. An ultimate parasite, or "how are the mighty fallen." The phyla of complex multicellular animals enjoy a collective designation as Metazoa (literally, "higher animals"). Mobile, single-celled creatures bear the name Protozoa ("first animals"--actually a misnomer, since many of these creatures, in terms of genealogical branching, rank as close to multicellular plants and fungi as to multicellular animals). In a verbal in-between stand the Mesozoa ("middle animals"). Many taxonomic and evolutionary schemes for the organization of life rank the Mesozoa by the literal implication of their name--that is, as a persistently primitive group, intermediate between the single-celled and the multicellular animals and illustrating a necessary transitional step in a progressivist reading of life's history.
But the Mesozoa have always been viewed as enigmatic, primarily because they live as parasites within truly multicellular animals, and parasites often adapt to their protected surroundings by evolving an extremely simplified anatomy, sometimes little more than a glob of absorptive and reproductive tissue cocooned within the body of a host. Thus, the extreme simplicity of parasitic anatomy could represent the evolutionary degeneration of a complex, free-living ancestor rather than the maintenance of a primitive state.
The major group of mesozoans, the Dicyemida, live as microscopic parasites in the renal organs of squid and octopuses. Their adult anatomy could hardly be simpler: a single axial cell (which generates the reproductive cells) in the center, enveloped by a single layer of ciliated outer cells (some ten to forty in number) arranged in a spiral around the axial cell, except at the front end, where two the tissues of the host.
The zoological status of the dicyemids has always been controversial. Some scientists, including Libbie H. Hyman, who wrote the definitive, multivolume text on invertebrate anatomy for her generation, regarded their simplicity as primitive and their evolutionary status as intermediate in the rising complexity of evolution. As she noted in 1940, "Their characters are in the main primitive and not the result of parasitic degeneration." But even those researchers who viewed the dicyemids as parasitic descendants of more complex free-living ancestors never dared to derive these ultimately simple multicellular creatures from a very complex metazoan. For example, Horace W. Stunkard, the leading student of dicyemids in the generation of my teachers, thought that these mesozoans had descended from the simplest of all Metazoa above the grade of sponges and corals--the platyhel-minth flatworms.
Unfortunately, the anatomy of dicyemids has become so regressed and specialized that no evidence remains to link them firmly with other animal groups, so the controversy of persistently primitive versus degeneratively parasitic could never be settled until now. But newer methods of gene sequencing can solve this dilemma, because even though visible anatomy may fade or transform into something unrecognizable, evolution can hardly erase all traces of complex gene sequences. If genes known only from advanced Metazoa--and known to operate only in the context of organs and functions unique to Metazoa--also exist in dicyemids, then these creatures are probably degenerated metazoans. But if, after extensive search, no sign of distinctive metazoan genomes can be detected in dicyemids, then the Mesozoa may well be intermediate between single and multicelled life after all.
In the October 21, 1999, issue of Nature, M. Kobayashi, H. Furuya, and P. W. H. Holland present an elegant solution to this old problem ("Dicyemids Are Higher Animals"). These researchers located a Hox gene--a member of a distinctive subset known only from metazoans and operating in the differentiation of body structures along the antero-posterior (front to back) axis--in Dicyema orientale. These particular Hox genes occur only in triploblastic, or "higher," metazoans with body cavities and three cell layers, and not in any of the groups (such as the Porifera, or sponges, and the Cnidaria, or corals and their relatives) traditionally placed "below" triploblasts. Thus, the dicyemids are descended from "higher," triploblastic animals and have become maximally simplified in anatomy by adaptation to their parasitic lifestyle. They do not represent primitive vestiges of an early stage in the linear progress of life.
In short, if the traditionally "highest" of all triploblasts--the vertebrate line, including our exalted selves--appears in the fossil record at the same time as all other triploblastic phyla in the Cambrian explosion, and if the most anatomically simplified of all parasites can evolve (as an adaptation to local ecology) from a free-living lineage within the "higher," triploblastic phyla, then the biological, variational, and Darwinian meaning of "evolution" as unpredictable and nondirectional gains powerful support from two cases that, in a former and now disproven interpretation, once bolstered an opposite set of transformational prejudices.
As a final thought to contrast the predictable unfolding of stellar evolution with the contingent nondirectionality of biological evolution, I should note that Darwin's closing line about "this planet ... cycling on according to the fixed law of gravity;' while adequate for now, cannot hold for all time. Stellar evolution will, one day, enjoin a predictable end, at least to life on Earth. Quoting one more time from Britannica:
The Sun is destined to perish as a white dwarf. But before that happens, it will evolve into a red giant, engulfing Mercury and Venus in the process. At the same time, it will blow away the earth's atmosphere and boil its oceans, making the planet uninhabitable.
The same predictability also allows us to specify the timing of this catastrophe--about 5 billion years from now! A tolerably distant future, to be sure, but consider the issue another way, in comparison with the very different style of change known as biological evolution. Earth originated about 4.6 billion years ago. Thus, half of our planet's potential history unfolded before contingent biological evolution produced even a single species with consciousness sufficient to muse over such matters. Moreover, this single lineage arose within a marginal group of mammals--the primates, which include about 200 of the 4,000 or so mammalian species. By contrast, the world holds at least half a million species of beetles. If a meandering process consumed half of all available time to build such an adaptation even once, then mentality at a human level certainly doesn't seem to rank among the "sure bets," or even the mild probabilities, of history.
We must therefore contrast the good fortune of our own evolution with the inexorable evolution of our nurturing Sun toward a spectacular climax that might make our further evolution impossible. True, the time may be too distant to inspire any practical concern, but we humans do like to muse and to wonder. The contingency of our evolution offers no guarantees against the certainties of the Sun's evolution. We shall probably be long gone by then, perhaps taking a good deal of life with us and perhaps leaving those previously indestructible bacteria as the highest mute witnesses to a stellar expansion that will finally unleash a unicellular Armageddon. Or perhaps we, or our successors, will have colonized the universe by then and will shed only a brief tear for the destruction of a little cosmic exhibit entitled "the museum of our geographic origins." Somehow I prefer the excitement of wondering and cogitation--not to mention the power inherent in acting upon things that can be changed--to the certainty of distant dissolution.
Stephen Jay Gould teaches biology, geology, and the history of science at Harvard University. He is also Frederick P. Rose Honorary Curator in Invertebrates at the American Museum of Natural History.
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