Life on a Young Planet Read Online Free PDF Page A

early life, we must first learn about Bacteria and Archaea, the tiny architects of terrestrial ecosystems.
    M OST OF US learn about Richard III through Shakespeare’s eponymous drama, but as history, this account is suspect—after all, Shakespeare’s patrons won the War of the Roses. Biased, selective, incomplete, and even incomprehensible documents are the daily bread of historians. Despite the shortcomings of individual accounts, however, scholars can arrive at a balanced understanding of the past by sifting through a number of different records for points of agreement and complementary perspectives.
    The study of biological history works much the same way. The fossiliferous cliffs along the Kotuikan River served to introduce one great library of Earth’s evolutionary past—the geological record. Sedimentary rocks preserve a remarkable record of life and environments through time, but as we’ve already observed, this accounting is episodic, not continuous. It is also highly selective, brightly illuminating some groups of organisms while leaving others in darkness. For example, we know a great deal about the paleontology of horses, but little about the earthworms beneath their feet.
    Fortunately, we can consult a second library—the biological diversity that surrounds us today. Comparative biology offers rich resources forevolutionary analysis, providing genealogy to complement paleontology’s record of time, and physiology to match geology’s chronicle of environmental change. The great cell biologist Christian de Duve has gone so far as to suggest that the genes of living organisms contain a full accounting of evolutionary history. If so, however, it is—like Shakespeare’s histories—limited to an account of life’s winners. Only paleontology can tell us about trilobites, dinosaurs, and other biological wonders that no longer grace the Earth. If we wish to understand life’s history, then, we must weave together insights drawn from geology and comparative biology, using living organisms to reanimate fossils and fossils to learn how the diversity of our own moment came to be.
    Despite an almost bewildering diversity of form and function, all cells share a common core of molecular features, including ATP (life’s principal energy currency), DNA, RNA, a common (with a few minor exceptions) genetic code, molecular machinery for transcribing genetic information from DNA into RNA, and more machinery to translate RNA messages into proteins that provide structure and regulate cell function. The reciprocal observation is equally striking. In spite of their fundamental unity of molecular structure, organisms display extraordinary variation in size, shape, physiology, and behavior. Life’s unity and diversity are both remarkable in their own ways; together they comprise the two great themes of comparative biology.
    Even a casual observer will notice the pattern of nested similarity displayed by Earth’s biological diversity. Humans and chimpanzees are clearly distinct, but they share many features of anatomy and physiology, resembling each other far more than either does, say, a horse. Humans, chimps, and horses, in turn, share features such as hair, lungs, and limbs that separate them from catfish. Yet, all animals with bony skeletons share a basic pattern of anatomical organization that unites them as a group and differentiates them from other sets of species built on different design principles—insects, for example, or spiders.
    The nested similarity of species was well known to early naturalists. Linnaeus codified it in the 1730s, proposing a hierarchical system of taxonomic classification that is still in use today. It was Charles Darwin, however, who explicitly recognized the genealogical nature of this pattern. Biological differences have arisen through time, he wrote, becauseof “descent, with modification,” that is, by evolutionary divergence from common ancestors under the influence of natural