PALEONTOLOGY AND EVOLUTION
2008; Oxford University Press; Volume: 62; Issue: 6 Linguagem: Inglês
10.1111/j.1558-5646.2008.00382.x
ISSN1558-5646
Autores Tópico(s)Genomics and Phylogenetic Studies
ResumoReview of Evolution: What the Fossils Say and Why It Matters , by Prothero, Donald R. . 2007 . Columbia University Press , New York . 408 pp . Cloth, $29.50. ISBN 978-0-231-13962-5 . Paleontology has long occupied a somewhat marginal place in the evolutionary biological sciences—a situation (almost nobody will dispute) arising from its treatment in our Founding Document: Darwin's (1859)On the Origin of Species by Means of Natural Selection. Darwin essentially invented taphonomy (the study of all the things that can prevent a once-living organism from ending up on a paleontologist's laboratory table) to explain why the fossil record supported his theory only in the broadest terms. Yet, ironically, what is often overlooked is that Darwin got his first taste of what was soon to become his theory when the Beagle stopped at St. Jago in the Cape Verde Islands in mid-January 1832, only 2-1/2 weeks after leaving England. There he collected the shells of marine invertebrates along the strand, with a nearby limestone outcrop providing fossils of some of the very same species. The fossil specimens and their modern equivalents appeared to him to be identical. And after an interlude of captivation by the Brazilian Atlantic rainforest (arguably the birth of “biodiversity”) Darwin was truly hooked when, in the Fall of 1832, he saw that endemic taxa (e.g., edentate mammals and South American rodents) showed both replacement patterns in time (the extinct glyptodonts and giant ground sloths succumbing to extinction, but still represented by modern species of armadillos and tree sloths) whereas his fossil cavy from Bahia Blanca seemed to sail right through, persisting into the modern fauna. Then, receiving Lyell's just recently published anti-Lamarckian/anti-transmutational diatribe (Volume 2 of Lyell'sPrinciples of Geology) a few weeks later, Darwin was off and running: he had his own original data to compare with the wisdom of his elders. It is worthwhile to recall that Darwin had heard of transmutation from both his grandfather Erasmus' Zoonomia, and in person from the young invertebrate zoologist Robert Grant—a fan of both the elder Darwin and of Lamarck. Hard on the heels of his paleontological experiences at Bahia Blanca, Darwin quickly learned his Lamarck from Lyell, who was judicious in his summary of Lamarck's views even as he was scathingly opposed: Lyell was a barrister after all! From then on, Darwin was fast on his way to becoming more convinced of the likely truth of transmutation rather than creationism long before he got to the Galapagos in the late summer of 1835. Thus paleontology, at least as much as anything else, is what got Darwin there in the first place. And yet his Origin effectively gave permission to his successors to ignore paleontology in favor of the near-exclusive contemplation of variation, selection and reproductive processes in general to understand the very nature of what soon came to be called the “evolutionary” process. As George Gaylord Simpson put it in the Introduction to his brilliant 1944 Tempo and Mode in Evolution, in the minds of his “neontological” contemporaries (he had geneticists specifically in his sights), paleontology had little more to offer than the “completed demonstration of the truth of evolution.” In one of the great wry comments ever written by any evolutionary biologist, Simpson completed the thought by saying that geneticists believe that a paleontologist is “like a man who undertakes to study the principles of the internal combustion engine by standing on a street corner and watching the motor cars whizz by.” Simpson begged to differ: His entire point in the ensuing chapters was to demonstrate that patterns in evolutionary history characteristically repeat themselves regardless of position in time, place, or clade. The pattern he focused on was the rapid-seeming appearance of higher taxa with their defining adaptations/synapomorphies already well in place in the earliest known fossils—implying to Simpson a very rapid evolutionary origin, often leaving no trace of intermediates. His two-part punch line was that what he called the “determinants of evolution”—the factors that account for small-scale, microevolutionary change—must perforce be assumed to be the same determinants that underlie large-scale, “macroevolutionary” (or “megaevolutionary”) change. His list of determinants included such factors as population size, nature, and rate of mutation, etc.; curiously, although selection played a big role in his thinking, it was not itself listed as a “determinant.” But (Part 2 of Simpson's punch line), the larger-scale patterns in history suggest alternative theoretical postulates on how such determinants combine to work in what Simpson saw to be the special circumstances underlying the origins of higher taxa—such as horses, bats and whales. This was the sort of pattern Simpson's “Quantum Evolution”—the centerpiece of Tempo and Mode in Evolution—was devised to explain. It was law-like in its insistence that patterns in evolutionary history repeat themselves (again, without regard to spatiotemporal location or cladal identity) and, as well, an attempt to demonstrate that a paleontologist can make fundamental, even elemental, contributions to evolutionary process theory. The core postulate of Simpson's approach is that large-scale events in evolutionary history are not exactly like the patterns produced in selection experiments under laboratory conditions—although of course the two are related. Simpson was in effect holding evolutionary theory responsible to actual patterns seen in the fossil record. Think of mass extinctions—and the smaller scale “turnovers” of regional species-level extinctions followed by evolutionary reproliferations as ecosystems are rebuilt—that absolutely dominate the entire history of Phanerozoic life. “Star-burst” phylogenies based on extant taxa can be expected to reflect monophyletic subcomponents of such turnovers-cum-rapid evolutionary diversification. The persistent lack of resolution of the relationships of the three major clades of coelomate animals, for example, arguably reflects rapid diversification of the “Cambrian Explosion.” But although such patterns literally leap out of the fossil record, and have been known since the days of Cuvier in the early 19th century, phylogenetic systematists still routinely dismiss “unresolved” polychotomies simply as products of faulty data or analysis. Don Prothero's Evolution. What the Fossils Say and Why It Matters is written to make clear just what the “completed demonstration of the truth of evolution” looks like now—two-thirds of a century after Simpson's Tempo and Mode appeared. It's subtitle stands in firm opposition to Duane Gish's (1978) title Evolution. The Fossils Say No!—although Gish's pithier subtitle, and use of an exclamation point, are hallmarks of the more ferocious passion that underlies the rhetoric of the Religious Right in general, and the creationists/intelligent design advocates in particular. As a showcase of what the fossil record now has to tell us about closely connected morphological transformation series lying in good, expected stratigraphic order, this book is a huge success. I wish it were on line—so that high-school teachers the world over could Google it, download sections (especially the rich and convincing illustrations) and front-load this ammunition into their daily classroom presentations on evolution. Prothero ably reviews issues in scientific methodology and evolutionary biology in the first section of his book. Most of his prose seems judicious—although I did feel he is trying to have it both ways as he cites the cladist canon that ancestors pose intractable problems in their recognition as scientifically testable hypotheses. Yet, given that they did exist (as species, I would say), Prothero tells the reader that he will go ahead and use the term “ancestor” anyway, albeit loosely—even to the point (as it turns out) of occasionally acknowledging supraspecific taxa as probable ancestors. The uninitiated, shall we say, are bound to be a bit confused by this borderline doublespeak. The book picks up both speed and passion as the really good parts hove into view. The origin of life and the Cambrian explosion—as well as marine invertebrate evolution (where Prothero has had some hands-on experience) are handled well enough. But it is when we get to vertebrates that Prothero (himself a paleomammalogist) truly shines. This is the real meat of the book, as Prothero makes clear that the “demonstrated truth of evolution” is in reality no such thing as far as half of the population of the United States is concerned. Paleontology still needs to do its job—and Prothero delivers as he summarizes all the really marvelous finds in the past few decades. In the same way that visitors to the American Museum's Darwin exhibition (now traveling the world in several iterations) have tended to resonate deeply with the comparative osteology of vertebrates (“I GET it!,” more than one visitor has announced when confronted with a series of vertebrate arm skeletons), Prothero's clear, compelling and well-illustrated presentation of The Fish With Legs (the Upper Devonian Tiktaalik) comes replete with a reconstruction and a cladogram showing where it fits in phylogenetically. Other highlights include his discussion of that still-and-increasingly-marvelous succession of “mammal-like” reptiles from the Permo-Triassic (“Karroo”) of South Africa; early whales, not only with legs, but also with tell-tale artiodactyl “double-pulley” astragali pointing to their terrestrial origins; and insectivorous bats without echolocation—a discovery so new that that it did not even make it into Prothero's text. And of course, Prothero goes to town with that bane of every creationist's existence (because, of course, all they really care about is where we humans came from)—the spectacularly rich, dense, and diverse stratigraphic record of hominids over the past 4 million years. The hominid fossil record is a tremendous boon in the unending battle against the anti-evolution forces. Simpson would, I think, have been gratified: his “quantum evolution” explained why it would be unlikely to find such clear-cut intermediates (living in transitional environments for short times in highly localized places)—and not that such intermediates never existed. Finding them is the icing on the cake—not a refutation of Quantum Evolution. So Simpson, I think, would be pleased. But he would still wonder—as I do—if that is all there is to the story. Prothero does not address the repeated-pattern phenomena that arrested Simpson and those, like myself, who have gotten the message that here lies the path where paleontology can make a real difference in understanding how evolution has produced the history of life that it has. (Or even, as Steve Gould might have said had he read the preceding sentence, alternative histories that might have been!). Beyond a discussion of punctuated equilibria (a passage in part geared to refuting creationist claims that punk eek is merely a storyline made up to hide the absence of transitional, “missing-link” fossils), there is nothing of Simpson's second theme connecting paleontology with evolutionary thought in a far more dynamic way. This of course is not Prothero's purpose—which is to get in every creationist's face with the message that there are plenty of “missing links” in the fossil record—and we are finding more and more of them all the time. But still the plaintive lines from the Threepenny Opera (Lotte Lenya singing “Is that all there is, my friend…#?”) echo in my ears as I type these words. Van Dam et al. (2006) recently published an analysis of rodent evolutionary history in the fossil record of Spain between 24.5 and 2.5 million years ago. Most of the evolution is tied up in turnovers—where extinction takes out most of the existing fauna, and evolution thereupon kicks in and produces new species in cross-genealogical spasms of replacement that Darwin actually knew about (i.e., as a general pattern) but found threatening to his theory. It is not: except in the sense that it implies (as does the basic postulate of punctuated equilibria) that most morphological change in evolutionary history (and remember that morphological change is the Ur-probleme of evolution) occurs in conjunction with speciation events. If that is so, that is way not expected from conventional evolutionary theory. Then there was the work by Mark Pagel et al. (2006) published a month or so earlier than Van Dam et al.—claiming that substitutional changes in a number of molecular databases seem to have occurred at nodes some 22% of the time, lending (from their perspective) large support to “punctuational” change in evolution. To me the percentage seems low—simply because the fossils seem to be saying that most morphological change occurs in conjunction with speciation. And I was perplexed that, in spite of this molecular support for punctuational change, Pagel et al. reported no evidence of stasis. But then I read that their analysis made no distinction between coding and noncoding substitutions. As a paleontologist, I agreed with Pagel et al. that, were that distinction to be made, there would be a higher correlation of substitutions at coding loci with nodes (speciation)—and the noncoding stuff would be left to drift in molecular clock (or nearly so) fashion, blurring any signs of stasis. Or so it seems. Until the work is done, who knows if it is “true?” But it is a sign that Simpson was right: a paleontological perspective can shed some real light—or at least raise some real possibilities and some testable questions—on potential interest to all biologists interested in how the evolutionary process actually works. Patterns in the fossil record can and should be taken seriously—and examined with the entire wide gamut of tools available in all disciplines of modern evolutionary biology. Book Review Editor: John Thompson
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