Life on a Young Planet by Andrew H. Knoll Read Free Book Online Page B

archaeal and bacterial limbs. This suggests that modern organisms are descended from ancestors that lived in hot environments. Thus, when we encounter microbial communities in the colorful hot springs of Yellowstone Park or in the midocean ridges that traverse the deep seafloor, we are glimpsing some of our earliest ancestors (color plate 1 ).
    (Recently, geneticists have added an intriguing twist to this story. The amino acid sequences in proteins from living microorganisms can be used to reconstruct ancient proteins likely to have been present in the last common ancestor. Surprisingly, the putatively ancestral proteins synthesized from these reconstructions are not stable at high temperatures. If true, the last common ancestor of bacteria and archaeans could not have been hyperthermophilic at all. How to reconcile this finding with the Tree of Life remains a subject of debate. One possibility is that the earliest organisms evolved at moderate temperatures, but gave rise to (at least) two groups of descendants that colonized energy-rich hot springs. Now all we need is an exterminating angel to wipe out all life save the handful of lineages sheltered in hydrothermal foxholes. Giant meteor impacts would do nicely, and crater histories of the Moon and Mars indicate that early in its history (before 3.9 billion years ago), the inner solar system was pummeled again and again by gigantic meteors. Earth could not have escaped this drubbing. Indeed, Norman Sleep of Stanford University long ago proposed that life’s only refuge on the primitive Earth would have been hydrothermal vents in the deep seafloor. Thus, the deepest branches of the tree may tell us of both evolution and extinction when life was young.)
    As implied earlier, the Tree of Life provides a road map for history of life, its branching order reflecting the successive radiations of biological diversity. The tree suggests that early ecosystems were centered on hydrothermal vent and spring systems, with the later appearance of photosynthesis enabling life to spread across the planet. Large, complex organismslike plants and animals are evolutionary latecomers, confined to distal twigs on a eukaryotic branch formed mainly by microscopic organisms.
    There is another way to interpret the tree. Because organisms in general, and microorganisms in particular, are commonly tied to specific habitats, the tree can be read as an environmental history of the Earth. For example, most early branching organisms do not use oxygen in metabolism, and many are killed by exposure to O 2 at even part-per-million levels. Organisms that can thrive when oxygen is present in moderate amounts branch later, and only at the tips of the tree do we find organisms like ourselves that require oxygen in high concentrations.
    The Tree of Life, thus, makes predictions about Earth history that can be tested against the geological record. The first essential point of the tree is that the organisms and environments of our common experience are relatively recent features; the deep history of life is microbial. The other main point is that life has not evolved on a static planetary surface. Rather, life and environments have evolved together throughout our planet’s history, inexorably linked by the biogeochemical cycles in which both participate.
    Armed with predictions from comparative biology, we can turn our attention back to the Cambrian Explosion, captured so vividly in the cliffs along the Kotuikan River. The Tree of Life supports Charles Darwin’s intuition that the Cambrian radiation of animals must have been preceded by a long antecedent history of life. Paleontologists wishing to reconstruct this history must focus on rocks deposited before the Cambrian period—on Pre cambrian rocks that document Earth’s early planetary development. We also need to replace zoological search images by pictures drawn from microbiology. But bacteria, archaeans, and simple eukaryotic microorganisms are tiny and