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Nature’s Clocks: How Scientists Measure the Age of Almost Everything
Doug Macdougall
University of California Press, June 2008
278 pages, $24.95
In 1650, the archbishop of Ireland, James Ussher, added the first new material to the Bible in about fifteen centuries. In part by working backwards from the “begat” lists in the Bible’s first few books (“And Peleg lived thirty years, and begat Re’u ... And Re’u lived two and thirty years, and begat Serug ...”), Ussher determined that the Earth had come into existence on exactly October 23, 4004 B.C. Standard editions of the Bible officially incorporated his dates as marginalia, to guide readers through historical accounts of Adam, Abraham, and the whole crew.
Of course, Ussher’s calculations have sort of fallen apart since then. Not long after 1650, a pious if idiosyncratic Christian named Isaac Newton proved that Earth was at least fifty thousand years old, based on its rate of cooling. A century after that, Scottish geologist James Hutton was confident enough in his knowledge of rock and soil formation to declare that the Earth had “no vestige of a beginning, and no prospect of an end.” During the Enlightenment, begats didn’t cut it.
Still, although it’s easy to mock Ussher’s “precision,” he used the best tools available to him. And most scientists after Ussher—whether using fossil records, rates of biological evolution, speed of salt accumulation in the ocean, or any other measure—also chronically underestimated the age of our planet by factors of one hundred or more. Humans just weren’t prepared to grasp such immense spans of time. Circumspect scientists like Hutton didn’t hazard guesses and confined themselves to tepid estimates of “relative age”—the true but unhelpful notion that what’s further down in the ground is older, and that’s all we can say.
But as Doug Macdougall recounts in Nature’s Clocks, one discovery changed everything. Radioactivity finally allowed geologists, physicists, and paleontologists to quantify how unfathomably old the Earth is. Scientists can root out traces of radioactive atoms in pretty much anything, and it’s easy to translate those traces into historical dates: “Whether radioactive atoms sit on the frigid surface of Mars, are scorched in the active lava of a volcano, or are subjected to the immense pressures of the Earth’s deep interior; they still decay [i.e., disintegrate] at the same rate,” Macdougall writes. “Radioactivity is like a clock that never needs adjusting—every second, minute, hour, ticked off precisely, forever.” Indeed, just after 1900, the heyday of Marie Curie and radioactivity studies, the best estimate for Earth’s age swelled from 20 million years to 1.5 billion, and the planet has only grown older since.
Probably the most incredible fact is that radioactive dating works equally well for spans of time that really aren’t congruent—as if you could measure your bedroom and the distance to the moon with the same tool. Anthropology wants to look back forty thousand years, biology 4 million, paleontology 40 million, and geology 4 billion. Yet by applying the same basic tests to different elements—carbon-14 for young samples; potassium and argon for mid-range samples; uranium and lead for old samples—scientists can date them with equal precision. Modern methods of dating were worked out at the University of Chicago in the 1950s, and since then radioactive dating has become almost as widespread in the earth sciences as mathematics, and equally effective.
Macdougall delves into the technical details of dating samples, but he balances that with good science history about the sleuths who developed these methods. Chemist Clair Patterson, who in 1953 was the first person to gauge the true age of Earth (4.55 billion years), grew so obsessed with the airborne industrial lead that contaminated his samples of meteors that he became an environmental crusader, and he’s largely responsible for the shudder we feel upon hearing “lead paint” or “leaded gasoline.”
But amid the stories, like a fish below the water’s surface, we see flashes of conflict. Geologist Hutton was ostracized as an atheist for his “vestige” verdict—for if the calculations of even a learned man like Ussher fell laughably short, that hardly supports a close reading of the Bible. (Other religions did better: According to Hinduism and its Brahma cycles of eternal creation and destruction, the Earth lasts about 4.32 billion years.) Debates over the Earth’s origins were once as hotly contested as evolution is today. And it becomes clear that time has been the key issue in many cultural conflicts—especially whether enough of it has elapsed to support materialistic processes that seem to erode belief in a creator.
Some scientists recognized the potential conflicts of dating samples early on, and ran. One member of the team that developed carbon dating—which works well for once-living objects less than fifty thousand years old—refused to handle religious objects. (Or at least Christian objects; he was fine with Egyptian relics.) But inevitably, as more people grew to trust radioactive dating, scholars started bringing in safely authentic objects like the Dead Sea Scrolls, to get a fix on their date. By the 1980s, scientists were salivating to get swatches of suspected fakes, like the Shroud of Turin, the reputed burial cloth of Jesus.
Curiously, though, when scientists proved the shroud was almost certainly a forgery—its linen was woven between 1260 and 1320—many Christians shrugged. There are still attempts, some rather ingenious, to explain away the carbon-dating results, and the debates are undeniably fascinating. But the Shroud’s losing its status as a venerable object mattered far less in 1989 than it would have one hundred years before. Ussher’s dates as well have long since disappeared from the Bible. In fact, Nature’s Clocks also provides a timeline for, albeit inadvertently, how quickly most people of faith stopped contesting the “atheistic” implications of geology and other natural sciences.
Some of those findings, at least. Others, like the fossil record and its implications for evolution, still receive scrutiny. Macdougall discusses fossils less than ancient rocks and tree rings, but he does raise interesting points. For one, microfossils—the trails of bacteria that ate their way into rocks—extend back 3.5 billion years. This suggests that the planet was either seeded with extraterrestrial life early on, or that life arose alarmingly quickly. What’s more, given the profusion of new fossils at certain dates (most notably the “Cambrian explosion”), evolution seems to act more quickly than many scientists assume. It may not be gradual at all!
On the other hand, Macdougall disappoints in the area of human evolution. A few nice tidbits do exist: Dating skulls pushed back the evolution of hominids by millions of years, and dating campsites and graves allowed archaeologists to trace how Homo sapiens and Neanderthals migrated out of Africa. But by focusing only on radioactivity (he’s a retired geologist), Macdougall all but ignores another fascinating natural clock: DNA.
Random mutations accumulate in the DNA of all living things at a consistent rate. (Most mutations are harmless.) By comparing two groups—and seeing how many of the billions of rungs on the double helix they still share—scientists can figure out how long ago they diverged on the tree of life. With this procedure, genetists have determined that the family trees of all humans start to overlap only about three thousand years ago, meaning we all share at least one remote ancestor from that time. What’s more, DNA evidence implies that human ancestors, after splitting off into their own branch for a million years, probably reverted back to breeding with chimpanzees. That’s because although most of our twenty-three chromosomes separated from apes over 6 million years, at least one, the X-chromosome, didn’t split until around 5 million years ago. There’s no other way that a younger chromosome would have been reintroduced.
Unfortunately, none of this finds its way into Macdougall’s book. The radioactive dating he describes is hardly dead. (See the “News” item on page 14 about using radiocarbon dating on human feces to determine when the first people arrived in North America.) Any breakthroughs now will depend on finding new samples, not refining its techniques. Radioactive dating has remapped our ideas of how humans fit into the universe, which is why an admirable history like Nature’s Clocks can be written. But to fill in any more details we’ll need different hourglasses, like the chronometers in our chromosomes.
Sam Kean is the associate editor of Science & Spirit.
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