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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