Tee Origin Of Species



by charles darwin, mjl, rt:j,u3w op the aaril, oeomxjjcal, likxybak, inc., societiks;

m'thrth • jovrnai. op jiesrarcht» dumbo ii, fi, rk.aomt'8 votaor ROtrST» TIIK ffomo.1


Title page of On the Origin of Species by Charles Darwin, 1859. (Library of Congress)

of close relatives with their remoter kin, and so forth, until we find features in common that link up absolutely all kinds of organisms on earth.

And that, in fact, is what we observe. Another way to look at it (as Darwin did in his only diagram in his book) is to see that as the tree of life grows up and branches out, new features acquired in evolutionary history are shared only with subsequent descendants— and not with other organisms belonging to groups that had branched off earlier. Indeed, a century before Darwin wrote, the great Swedish naturalist Karl von Linné (Linnaeus) showed that life is organized in a hierarchical fashion; organisms seem to fall into natural groups based on their appearance, and these natural groups seem to cluster together into more inclusive natural groups, and so on. Linnaeus—along with nearly everyone else in the eighteenth century—thought that a supernatural Creator (God) had fashioned life this way. Darwin was able to suggest a natural explanation for these "nested sets" of organisms that we see in the biological realm—and to suggest that these sets of similar organisms must be there if the fundamental idea of evolution is true.

It is the experience of more than 150 years of research in systematic biology ("systemat-ics") and paleontology that life is indeed organized into progressively nested sets of species. Although these repeated observations do not prove the truth of evolution—absolute proof of anything in science is in principle impossible—failure to falsify the idea means that evolution is as well corroborated a notion as any other in science.

Here is an example of how life is nested into increasingly larger groups, eventually including absolutely all of life. You can start with any species you want: a dog species, a whale, a species of grass, a fungal species, anything. Let's start with our own species, Homo sapiens. We look very much like extinct fossil species that go back some 4 to 5 million years, looking more like the younger fossil species, and sharing fewer features the farther back you look in the fossil record. Confining our gaze, however, to living species, we find that we share more than 98 percent of our genes with chimpanzees; we share other similarities with the tail-less great apes. Great apes and humans (collectively, the hominids, Family Hominidae) share still more resemblances with monkeys—and all together, the hominids, monkeys, lemurs, and a few other groups belong in the Order Primates. Primates, though, are linked with rodents, whales, bats, carnivores, and others into the Class Mammalia, all of whose members have hair and mammary glands. Mammals, though, share with fishes, amphibians, reptiles, and birds a basic body plan that includes a vertebral column—forming the Vertebrata. Vertebrates are animals. Animals share with plants, fungi, and certain kinds of micro-organisms a basic structure to their cells, in which DNA and RNA (the "nucleic acids" of heredity) are isolated from the rest of the cell in a distinct nucleus set off by a double-layered wall. But DNA is found in most bacteria, while the similar molecule RNA is found in absolutely all of life. So there you have it—a nested set of groupings of organisms that link human beings up with all other forms of life on earth. Had we started with any other species, we would have found the same pattern of progressive inclusion based on shared possession of similar features—shared features that arose in the course of evolution.

The other great prediction that comes from the basic idea that all species are descended from a single common ancestor is that the fossil record of the history of life ought to show a progression from simple to more complex organisms. Assuming that life arose by natural chemical processes more than 3.5 billion years ago, we would expect the earliest life forms to have simple DNA or RNA strands covered by a simple protein coating. (Whether DNA and RNA, which carry the instructions for protein assembly, arose first, or rather proteins [whose component amino acids can be formed and strung together into chains in simple laboratory experiments] came first, is still a matter of debate among scientists who study the origin of life.) The simplest forms of life we know are bacteria, assumed to be much more complex than the earliest life forms. But

Skeletons of five primates (Bettmann/Corbis)

among the organisms that we know living around us today, we would predict that the simplest of them, the bacteria, would be the first to show up in the fossil record. We would not predict that, say, elephants would have come first, and fungi and microbes later!

This second prediction is also abundantly confirmed by our observations of the fossil record. The oldest fossils are some 3.5 billion years old—and are indeed bacteria. Bacteria were the sole forms of life for at least a billion years. The next big step in life's history was the appearance of more complex (eukaryotic) cells, the kind with a distinct nucleus. And, as we would predict, the first evidence of such cells (more than 2 billion years ago) shows them to belong to relatively simple, single-celled micro-organisms, not the more complex, multicellular forms of life we know as animals, fungi, and plants.

Although the famous Cambrian explosion showed a very rapid proliferation of forms of animal life over a 10-million-year period beginning some 540 million years ago, evidence of even earlier fossils shows that the more simple kinds of animals—such as the group to which corals and jellyfish belong—did indeed come before the more complex, advanced animals such as arthropods (which include crustaceans, insects, and the extinct trilobites). If we consider our own group, the vertebrates, we encounter, as we would predict, various forms of fishes first in the fossil record, followed by amphibians, whose reproduction is still tied to the water. The appearance of the "amniote egg," with membranes that retain water and nutrients for the developing embryo, came later. The oldest, most primitive amniotes were scaly reptiles. Birds arose later from a specific group of reptiles (dinosaurs, apparently), while mammals arose from a separate lineage of reptiles, as is beautifully shown by a sequence of fossils in South Africa that are more than 200 million years old. Primates arose later, and advanced primates— such as our own lineage—arose only about 4 to 5 million years ago.

Thus the second prediction of evolution is also well corroborated. The basic idea of evolution, that all life has descended from a single common ancestor in the remote geological past, is thus abundantly confirmed. Evolution is as well corroborated as any other scientific theory—such as plate tectonics or special relativity. It is as well confirmed as the notion that the earth is round, spins on its axis, and revolves around the sun. For that reason, most biologists are content to call evolution a "fact"—for that is what facts really are, well-confirmed ideas about the nature of things.

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