Читать книгу The Tangled Tree: A Radical New History of Life - David Quammen, David Quammen - Страница 25

Chaos” was the name of the group into which Linnaeus, the great systematizer, in the 1774 edition of his Systema Naturae, had lumped Leeuwenhoek’s bacteria and other little creatures. That was a durable judgment. Even well into the twentieth century, decades after Ferdinand Cohn, experts were still arguing about whether bacterial taxonomy was a meaningful enterprise or hopelessly chaotic.

Beginning in 1923, the standard source for identifying bacteria was a thick compendium, Bergey’s Manual of Determinative Bacteriology, edited by the bacteriologist David Hendricks Bergey. But as microbiology progressed, it became clear that the Bergey’s system was vague, inconsistent, and, on some fundamentals, inaccurate. It didn’t offer a tree of bacterial life. It was only a glorified field guide. Still, other researchers who critiqued Bergey’s Manual, and then tried to improve on it, found the critiquing much easier than the improving. The task of bacterial classification was just so difficult. There was almost no fossil record of bacterial ancestors. There weren’t enough differences of external shape and internal anatomy, even as seen through powerful microscopes, to support fine distinctions. Physiological characters could also be misleading, if they reflected parallel adaptations rather than shared ancestry. What did that leave for a classifier to use? (Hint: Carl Woese would offer an answer, but not until 1977.) This conundrum came to a head in 1962, when two of the world’s leading microbiologists, C. B. van Niel and Roger Stanier, essentially threw up their hands in despair.

Van Niel was a Dutchman, educated in Delft, who in 1928 decamped to California, where he taught at a marine biological station that was part of Stanford University. His particular interests were bacterial physiology and taxonomy. Roger Stanier was a younger Canadian who became van Niel’s student, then his special protégé, then his collaborator. In 1941, when Stanier was still just twenty-five years old, he and van Niel coauthored an influential paper on bacterial classification.

That paper stood as definitive for a generation—until both authors renounced it. Stanier himself later admitted some embarrassment about it, all the more so because he had arm-twisted van Niel to sign on as coauthor—student and teacher together, although the work was mainly Stanier’s. What the paper contained, besides a pointed critique of Bergey’s Manual, was a shiny new proposal for classifying bacteria—not just a checklist or a field guide but a “natural” system reflecting their evolutionary relationships. That system divided the familiar bacteria into four major groups (as Ferdinand Cohn had done) and placed them in a kingdom of simple creatures along with just one other group: the blue-green algae.

Algae? Yes, the blue-green algae, as they were then called, had long been an ambiguous group, because they seemed to straddle the line between bacteria and plants. (This was partly what allowed Cohn to believe that all bacteria were plants—the blurry lines around blue-green algae.) Algae was a catchall term for a loose assemblage of creatures that photosynthesize, including these tiny blue-green creatures, but that didn’t mean all algae shared a single common ancestor. Did they? Stanier and van Niel said no. By their new definition of things, blue-green algae were more similar to bacteria than to other algae, and these two groups should be lumped together in a kingdom of their own, apart from everything else. Eventually they labeled such cells procaryotic—meaning “before kernel,” as I’ve mentioned—and set them in contrast to eucaryotic cells, comprising all else. (Their spellings were later corrected, from more accurate transliteration of the Greek roots, to prokaryotic and eukaryotic.) The kernel in question was a cell nucleus. Just as a bacterium doesn’t have one, neither do the creatures that were then known as blue-green algae (and are now classified as cyanobacteria). Advances in microscopy since the end of World War II, including electron microscopy, had given microbiologists a better view of those distinctions and others, making possible a fresh analysis of what a bacterium is—and what it isn’t. Stanier and van Niel offered that fresh analysis along with the prokaryote category in a new paper, published in 1962, titled “The Concept of a Bacterium.” By their lights, the “abiding intellectual scandal of bacteriology” was that no such concept had ever been clearly delineated. What was a bacterium? Um, hard to say.

They tried to correct that by placing bacteria and blue-green algae together as prokaryotes, and setting them in contrast to the alternative category, eukaryote, which encompasses all other forms of cellular life. The chief distinguishing features of a prokaryote, according to Stanier and van Niel, were: (1) no cell nucleus, (2) cell division by simple fission, rather than the elaborate process of chromosome pairing known as mitosis, and (3) a cell wall strengthened by a certain sort of latticework molecule with a fancy name, peptidoglycan. I know, it looks like the moniker of a flying reptile from the Jurassic. Forget about it for now, and when peptidoglycan comes back as an important clue toward understanding the deepest structure of the tree of life, and the twig on the branch on the limb from which we humans have sprouted, I’ll remind you.

The dichotomy between prokaryotes and eukaryotes, creatures without cell nuclei and those with, relatively simple beings and relatively complex, became a fundamental organizing principle of biology. Stanier and his two coauthors of a textbook would later say that it “probably represents the greatest single evolutionary discontinuity to be found in the present-day living world.” It was also a salubrious reminder to humans of our inescapable linkage to other creatures, including some very humble ones. We are, at the most basic level of classification, eukaryotes. So are amoebae. So are yeasts. So are jellyfish, sea cucumbers, the little parasites that cause malaria, and rhododendrons. To an average person, the gap between an amoeba and a bacterium may seem narrow (partly because most of us have never, or at least not since high school biology, looked through a microscope at either), but the prokaryote-eukaryote distinction reveals it as oceanic. You could think of the living world—and, beginning from Stanier and van Niel’s 1962 paper, biologists did think of the living world—as divided into proks and euks.

Besides putting that idea into play, “The Concept of a Bacterium” is notable for having signaled surrender, by Stanier and van Niel, in the battle of bacterial taxonomy. About this they were candid, confessional, and brusque. Ever since Leeuwenhoek, microbiologists had been seeking the best way to classify bacteria. Ever since Darwin, they had been arguing about how one bacterium was related to another. Enough was enough. “Any good biologist finds it intellectually distressing to devote his life to the study of a group that cannot be readily and satisfactorily defined.” C. B. van Niel himself had devoted forty years. He and Stanier now alluded to the “elaborate taxonomic proposal” they had published back in 1941, “which neither of us cares any longer to defend.” Never mind that. They admitted having “become sceptical about the value” of any such formal systems, or the effort spent to develop them, although they still affirmed the importance of figuring out just what the devil bacteria are.

This skepticism, this taxonomist’s despair, had been wiggling up inside van Niel for a long time. Two decades earlier, even as he was signing onto that first elaborate proposal, he had confessed his gloom to Stanier in a letter: “Many, many years ago I often went around with a sense of futility of all our (my) efforts. It made me sick to go around in the laboratory (this was in Delft) and talk and think about names and relations of microorganisms.” Was any of it real? Was there any value to putting bacteria into labeled boxes? “During those periods I would go home after a day at the lab, and wish that I might be employed somewhere as a high-school teacher.” Not that he would enjoy such teaching, he realized, but at least “it would give me some assurances that what I was doing was considered worth-while.” Nowadays we might see that as a signal of bipolar disorder, but it’s just as likely that van Niel simply viewed bacterial taxonomy with great clarity.

Under their revised spellings, prokaryote and eukaryote, those two became enshrined for a generation as the most fundamental categories of life. Eukaryotes had cell nuclei. Prokaryotes did not. That dichotomy seemed to represent, as Stanier and his coauthors had written, the greatest single evolutionary divide in the living world. There were two basic kinds of creature, the proks and the euks, and there was nothing between.

What makes this worth knowing is that Carl Woese proved it wrong.