Life on a Young Planet Read Online Free PDF

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The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species.… As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.
    We can explain the similarities between humans and chimps by descent from a common ancestor that possessed the various features the two groups share. Their differences have arisen since they diverged. This makes the paleontological prediction that the oldest fossils of humanlike primates should resemble the last common ancestor of chimps and humans more closely than modern people do; the features that make us distinctly human should appear only in younger fossils of our lineage. The fossil record of human ancestry is notoriously sketchy, but skeletal remains unearthed in Africa and Asia confirm this prediction. (Note that there is no expectation that successively older members of our lineage should close in on chimp morphology. Humans didn’t descend from chimpanzees; humans and chimps both diverged from a common ancestor that was neither Homo nor Pan .)
    Not all shared features are equally helpful in determining “propinquity of descent” (another delightful Darwinism). For example, birds, bats, and the extinct pterosaurs all sport wings, but their wings have different skeletal structures, and many other features show that these airborne animals are not closely related. Wings evolved independently in each group as an adaptation for flight; in the parlance of systematic biology, these features are convergent . Only features that are shared because of common ancestry ( homologies , in evolution-speak) can be used to assess evolutionary relationships. In practice, we don’t always know whether similar features are convergent or homologous and so rely on sophisticated computer algorithms to sort out large sets of comparative biological data.
    It is relatively easy to see how morphological characteristics might be used to articulate a hypothesis of evolutionary relatedness, or phylogeny , for all primates, all mammals, or even all vertebrate animals. We can also grant that an expert, at least, could do the same for mollusks or arthropods. But how can we place mollusks, arthropods, and vertebrates within a greater evolutionary tree of all animals? And, much harder, how can we reconstruct the whole of Darwin’s great Tree of Life, a phylogeny that encompasses all living things?
    Wandering through an alpine forest or snorkeling above a coral reef, we observe an ecology shaped by plants (or seaweeds) and animals, with large vertebrates at the top of the food chain and other creatures below. Ecosystems also contain many organisms that we can’t see, but concern for their contributions is generally fleeting—surely bacteria and other microorganisms, tiny and simple, eke out their living in a world of our making?
    As large animals, we can be forgiven for holding a worldview that celebrates ourselves, but, in truth, this outlook is dead wrong. We have evolved to fit into a bacterial world, and not the reverse. Why this should be is, in part, a question of history, but it is also an issue of diversity and ecosystem function. Animals may be evolution’s icing, but bacteria are the cake.
    Plants, animals, fungi, algae, and protozoa are eukaryotic organisms, genealogically linked by a pattern of cell organization in which genetic material occurs within a membrane-bounded structure called the nucleus. Bacteria and other prokaryotes are different—their cells lack nuclei. In terms of