Why Evolution Is True by Jerry A. Coyne Read Free Book Online Page A

requires a very specific set of circumstances. First, the remains of an animal or plant must find their way into water, sink to the bottom, and get quickly covered by sediment so that they don’t decay or get scattered by scavengers. Only rarely do dead plants and land-dwelling creatures find themselves on the bottom of a lake or ocean. This is why most of the fossils we have are of marine organisms, which live on or in the ocean floor, or naturally sink to the floor when they die.
    Once buried safely in the sediments, the hard parts of fossils become infiltrated or replaced by dissolved minerals. What remains is a cast of a living creature that becomes compressed into rock by the pressure of sediments piling up on top. Because soft parts of plants and animals aren’t easily fossilized, this immediately creates a severe bias in what we can know about ancient species. Bones and teeth are abundant, as are shells and the hard outer skeletons of insects and crustaceans. But worms, jellyfish, bacteria, and fragile creatures like birds are much rarer, as are all terrestrial species compared to aquatic ones. Over the first 80 percent of the history of life, all species were soft-bodied, so we have only a foggy window into the earliest and most interesting developments in evolution, and none at all into the origin of life.
    Once a fossil is formed, it has to survive the endless shifting, folding, heating, and crushing of the earth’s crust, processes that completely obliterate most fossils. Then it must be discovered. Buried deeply beneath the earth’s surface, most are inaccessible to us. Only when the sediments are raised and exposed by the erosion of wind or rain can they be attacked with the paleontologist’s hammer. And there is only a short window of time before these semiexposed fossils are themselves effaced by wind, water, and weather.
    Taking into account all of these requirements, it’s clear that the fossil record must be incomplete. How incomplete? The total number of species that ever lived on earth has been estimated to range between 17 million (probably a drastic underestimate given that at least 10 million species are alive today) and 4 billion. Since we have discovered around 250,000 different fossil species, we can estimate that we have fossil evidence of only 0.1 percent to 1 percent of all species—hardly a good sample of the history of life! Many amazing creatures must have existed that are forever lost to us. Nevertheless, we have enough fossils to give us a good idea of how evolution proceeded, and to discern how major groups split off from one another.
    Ironically, the fossil record was originally put in order not by evolutionists but by geologists who were also creationists, and who accepted the account of life given in the book of Genesis. These early geologists simply ordered the different layers of rocks that they found (often from canal excavations that accompanied the industrialization of England) using principles based on common sense. Because fossils occur in sedimentary rocks that begin as silt in oceans, rivers, or lakes (or more rarely as sand dunes or glacial deposits), the deeper layers, or “strata,” must have been laid down before the shallower ones. Younger rocks lie atop older ones. But not all layers are laid down at any one place—sometimes there’s no water to form sediments.
    To establish a complete ordering of rock layers, then, you must cross-correlate the strata from different localities around the world. If a layer of the same type of rock, containing the same type of fossils, appears in two different places, it’s reasonable to assume that the layer is of the same age in both places. So, for example, if you find four layers of rock in one location (let’s label them, from shallowest to deepest, as ABDE), and then you find just two of those same layers in another place, interspersed with yet another layer—BCD—you can infer that this record includes at least five