WHAT THE MEDIA DON'T TELL YOU ABOUT EVOLUTION Review by Kevin Padian In Search of Deep Time: Beyond the Fossil Record to a New History of Life BY HENRY GEE Free Press, New York, 1999 ($26) This is a subversive book. Read it only if you want to know how scientists actually do their work, as opposed to the mythology of textbooks and documentaries. In it, you will discover how and why the beloved Linnean system of taxonomy--the one that gave us classes and orders and families, oh my!--is being replaced by a wholly evolutionary way of looking at nature. Be warned: you won't learn this on TV. But you will be able to understand what scientists are talking about when, for example, they claim that birds are dinosaurs. Henry Gee (pronounced like the letter) is an editor and senior writer at Nature, one of the two weekly journals that every scientist pores over faithfully. His training is as a paleontologist, but he got his education during one of those interesting times in the history of a discipline when a paradigm shift is occurring. As it happened, he got caught in the middle of it, so he writes with the viewpoint of someone who sees both sides. The discipline that shifted was not so much paleontology but systematics, or the reasoning behind classification. Classifications changed in all branches of biology, but the revolution began in vertebrate paleontology. Here's what happened. In the 1950s a German entomologist named Willi Hennig put forth a system of classifying organisms in which only evolutionary innovations could be used to reconstruct relationships. Hennig's reasoning, which was also Charles Darwin's, was that classification should be based strictly on ancestry, and because these innovations were the closest tracers of most recent common ancestry, they should be used to the exclusion of all other features. Before this time, from Linnaeus in the 1750s to the present, general similarities (not strictly innovative ones) were also used, because general similarities among organisms (as long as they connoted a common genetic basis) tended to hold groups together as ecological as well as evolutionary units. Thus, for example, although biologists had long realized that birds evolved from some kind of reptile, they did not subsume birds as a group within reptiles. Rather, because birds were so different from reptiles, the terms "bird" and "reptile" denoted two separate and equal classes of vertebrates. A more consistent approach, true to evolutionary history, is always to rank descendant groups as parts of their ancestral units (humans are unique, but they are also members of the larger ape, primate and mammal groups). The systematists can then avoid constructing taxonomic groups that have no natural counterpart (such as the conventional meaning of "reptiles" without birds). The classic example, recounted by Gee, is in classifying the salmon, the lungfish and the cow. Traditionally, the salmon and lungfish are grouped as fishes, and the cow is a mammal. But Hennig's system recognizes that the features we use to group the salmon and lungfish are only general fishlike things related to living in water that applied to the original vertebrates. So the salmon and lungfish are not related by any evolutionary innovations. Instead the lungfish and the cow share some heretofore unique features that the salmon lacks, such as the presence of nasal passages that open into the throat and the bony configuration of the limbs, so they are grouped together as choanates. To many, the latter arrangement seems pointless, but if the point of classification is to uncover the history of life and to group it accordingly, this arrangement succeeds better than traditional methods. A Radical Upbringing As a Ph.D. student in the mid-1980s, Gee found himself thrown in with the most radical group of Hennigians in paleontology: the fossil fish section of London's Natural History Museum. Cladistics, as Hennig's system came to be called (after the Greek word for "branch"), was mother's milk to him, and he tells many amusing tales of heresy and hogwash in the pubs, bars, correspondence columns and scientific conferences where cladistics was argued, championed and disparaged. Those were heady days indeed. But if this book simply recounted the paradigm shift in systematic philosophy, it would be of limited interest. Instead it describes the development of a whole new way of thinking about biology and particularly about what we know of the history of life, what we can and can't know and study scientifically, and how this knowledge affects even the way we narrate our stories about the evolution of life. In Search of Deep Time would be a good title for a book about the history of geochronology, which this is not. An earlier working title was Thirty Ghosts, an allusion to a quotation from Arthur C. Clarke and Stanley Kubrick's 2001: A Space Odyssey to the effect that behind every human being now living stand thirty ghosts. The number is of course much greater, but the point is that our lineal chain of ancestry is both unimaginably extensive and largely unknowable. Gee describes the difficulty of reconstructing the past using his experience searching for hominid fossils in Africa with Meave Leakey's crew. A bone you pick up might be a hominid and might persuasively be not far from the direct line to living humans. But you can never really know, because not enough information is preserved. Deep Time, with its attendant destruction of information from the geologic past, has wiped away direct evidence. We have to reconstruct evolutionary history, as we reconstruct human history, from the bits and pieces we have available to us. But there is more: we have to have a method in order to do testable science. Gee shows that many traditional explanations of major evolutionary transitions are not testable and therefore have no scientific content. For example, let's say that you don't agree with the overwhelming evidence that birds evolved from small carnivorous dinosaurs (see my article with Luis M. Chiappe in Scientific American, February 1998), because as far as you're concerned, flight had to evolve in the trees, and dinosaurs couldn't climb trees. This statement may be true or false, but it's not scientific, because you're making a statement about the process of evolution (how flight had to evolve) that you're not allowing to be tested by any contradictory patterns of evolution. Testing Evolutionary Scenarios In the case of how flight evolved, the patterns of evolution tell a different story, and here is where cladistics comes in. Every cladistic analysis of the relationships of birds to other animals, involving dozens of fossil animals and hundreds of characteristics, has placed birds squarely within small carnivorous dinosaurs. Gee's point is that maybe bird ancestors could climb trees and maybe they couldn't, but we'll never know for sure. The origin of birds is a question of pattern that can be tested by the distribution of innovative features that indicate closest evolutionary relationships; ideas about how flight must have evolved, he says, rely on faith in the particular workings of natural selection or other evolutionary processes. So which kind of knowledge is a more reliable guide to the ancestry of birds? Or, as Chico Marx once said in a movie in which he is discovered in a compromising position with another man's wife, "Who are you going to believe--me, or your own eyes?" Does this mean that we can't know about anything but genealogy in extinct organisms? Not so, I think, and here I would be a little less hard-nosed than Gee's orthodox cladists. A good phylogeny can test hypotheses about evolutionary processes and particular historical pathways. If phylogenies show that humans evolved from semiarboreal limb-swingers rather than from quadrupedal sprinters, that would help explain why we have hands rather than hooves and why we can pitch baseballs better than most cows can. Gee shows, however, that in explaining evolutionary history we have to be careful not to see our thirty ghosts as trying to become "like us." It is often assumed that the "small" brains of earlier hominids were inferior to ours, but these creatures saw their world in different terms than we do ours, and after all, their world was different. Nevertheless, their brains were clearly adequate to the task, not striving to become what textbook authors, apparently without irony, call "fully human." Divorcing evolutionary pattern from process in this way, we see that the origin of tetrapods and their emergence onto land, the origin of birds and the evolution of flight, and the origin of humans and the inception of speech are pairs of evolutionarily coupled but logically separate problems. If we assume that the second member of each pair was the reason for the first, we will never learn anything new about evolutionary history. This is an important book because it clearly explains the workings and applications of the most versatile new implement in the toolbox of evolutionary biology. Twenty years after the hegemony of cladistics was established, the public remains almost completely ignorant of Hennig's method and how it is applied to problems in the history of life. Gee explains it all congenially and clearly, with wit, originality and self-deprecating humor. It is like having an affable, bemused, literate and somewhat peripatetic cousin take you around his workplace; only in this case, the workplace is a zoo full of extinct animals, the keepers at first seem like weirdos with some kind of secret knowledge that they're trying to impart to you, and the signs on the cages make you blink and look twice at the animals. Oprah may not select this one for her book club, but I'll tell you what: if you've been reading newspaper and science magazine accounts of contentious issues in paleontology and evolutionary biology and wondering what's really behind so much of the debate, this is the book for you. -------------------------------------------------------------------------------- KEVIN PADIAN is professor of integrative biology and curator in the Museum of Paleontology at the University of California, Berkeley. He is the editor, with Philip J. Currie, of Encyclopedia of Dinosaurs (Academic Press, 1997). --------------------------------------------------------------------------------