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

layers of rock, in the order, from youngest to oldest, of ABCDE. This principle of superposition was first devised in the seventeenth century by the Danish polymath Nicolaus Steno, who later became an archbishop and was canonized by Pope Pius XI in 1988—surely the only case of a saint making an important scientific contribution. Using Steno’s principle, the geological record was painstakingly ordered in the eighteenth and nineteenth centuries: all the way from the very old Cambrian to the Recent. So far, so good. But this tells you only the relative ages of rocks, not their actual ages.
    Since about 1945 we have been able to measure the actual ages of some rocks—using radioactivity. Certain radioactive elements (“radioisotopes”) are incorporated into igneous rocks when they crystallize out of molten rock from beneath the earth’s surface. Radioisotopes gradually decay into other elements at a constant rate, usually expressed as the “half-life”—the time required for half of the isotope to disappear. If we know the half-life, how much of the radioisotope was there when the rock formed (something that geologists can accurately determine), and how much remains now, it’s relatively simple to estimate the age of the rock. Different isotopes decay at different rates. Old rocks are often dated using uranium-238 (U-238), found in the common mineral zircon. U-238 has a half-life of around 700 million years. Carbon-14, with a half-life of 5,730 years, is used for much younger rocks, or even human artifacts such as the Dead Sea Scrolls. Several radioisotopes usually occur together, so the dates can be cross-checked, and the ages invariably agree. The rocks that bear fossils, however, are not igneous but sedimentary, and can’t be dated directly. But we can obtain the ages of fossils by bracketing the sedimentary layers with the dates of adjacent igneous layers that contain radioisotopes.
    Opponents of evolution often attack the reliability of these dates by saying that rates of radioactive decay might have changed over time or with the physical stresses experienced by rocks. This objection is often raised by “young-earth” creationists, who hold the earth to be six to ten thousand years old. But it is specious. Since the different radioisotopes in a rock decay in different ways, they wouldn’t give consistent dates if decay rates changed. Moreover, the half-lives of isotopes don’t change when scientists subject them to extreme temperatures and pressures in the laboratory. And when radiometric dates can be checked against dates from the historical record, as with the carbon-14 method, they invariably agree. It is radiometric dating of meteorites that tells us that the earth and solar system are 4.6 billion years old. (The oldest earth rocks are a bit younger—4.3 billion years in samples from northern Canada—because older rocks have been destroyed by movements of the earth’s crust.)
    There are yet other ways to check the accuracy of radiometric dating. One of them uses biology, and involved an ingenious study of fossil corals by John Wells of Cornell University. Radioisotope dating showed that these corals lived during the Devonian period, about 380 million years ago. But Wells could also find out when these corals lived simply by looking closely at them. He made use of the fact that the friction produced by tides gradually slows the earth’s rotation over time. Each day—one revolution of the earth-is a tiny bit longer than the last one. Not that you would notice: to be precise, the length of a day increases by about two seconds every 100,000 years. Since the duration of a year—the time it takes the earth to circle the sun—doesn’t change over time, this means that the number of days per year must be decreasing over time. From the known rate of slowing, Wells calculated that when his corals were alive—380 million years ago if the radiometric dating was correct—each year would have contained about