Surviving a Perfect Storm
THE GREAT EXTINCTIONS: What Causes Them and How They Shape Life. Norman MacLeod. Firefly Books, 2013. $29.95.
In Charles Darwin’s day, scientists did not yet know about giant asteroid impacts, flood basalt volcanic eruptions, and most of the other violent extremes of Earth’s geological history. Consequently, Darwin didn’t consider that extinction could happen on a mass scale, or that it played a major role in evolution. Indeed, Victorian-era scientists were reluctant to accept any kind of “catastrophe theory,” in part because it smacked of biblical fundamentalism. The irony of that resistance is not lost on Norman MacLeod, paleontology curator at London’s Natural History Museum.
In his new book, The Great Extinctions, MacLeod highlights research showing that extinction rates over millions of years have remained relatively constant, which seems counter to the dynamics of natural selection. After all, if a species is evolving to become better suited to its environment, shouldn’t it also get better at avoiding extinction? As MacLeod points out, random catastrophic events could account for the seeming paradox: Survival under such extreme circumstances is largely a matter of chance. And since Darwin’s time, it has become clear that catastrophic events have occurred repeatedly over the history of life on Earth.
MacLeod examines what are called the “Big Five” mass extinction events seen in the fossil record, examining the role that unpredictable catastrophe and chance played in the outcomes. He also discusses “the sixth extinction,” a hypothetical future event that could drastically reduce the biodiversity of life on Earth.
In the Big Five extinctions, MacLeod shows that the estimated percentage of species lost ranges from 71 to 81 percent at the end of the Cretaceous period to 93 to 97 percent at the end of the Permian—an astonishing level of loss. However, the data also show that an estimated 95 percent of all extinctions occurred outside of these intervals, from noncatastrophic causes. So are the Big Five really more consequential? One distinction: Mass extinctions affect marine and land organisms equally, indicating a sweeping global change. The great extinction periods also saw habitat destruction that affected not only “ecological consumers” but also primary producers: species that most others depend on, in some form, for their own survival.
MacLeod admits that paleontologists are working with incomplete information. The fossil record is woefully incomplete, he laments. The proportion of currently living species is less than 1 percent of all species that have ever lived during Earth’s history, suggesting a total number in the hundreds of millions, but, based on existing knowledge of animal body types that fossilize well and their habitats, scientists estimate that recovery of only about 12 million species’ fossils is possible. Of these, perhaps only 500,000 species have thus far been found and described.
Despite science’s limited view of the past, MacLeod argues convincingly against the idea that mass extinctions typically arose from a single cause, such as a meteor strike. More compelling evidence suggests that multiple factors working in concert, often over the course of a million or more years (a brief moment in geologic time), underlie major species die-offs. Although at least one major asteroid impact coincided with an extinction event (the one at the end of the Cretaceous, which wiped out all nonavian dinosaurs), MacLeod makes a strong argument that a combination of factors that destroy habitat—such as volcanic activity, sea-level change, ocean oxygen levels, and global temperature change—is more likely at the root of mass extinctions. In each case, he says, a “perfect storm” of environmental contingencies appears to have come together as the result of simple coincidence. That would explain why most of the other known impacts are not clearly associated with any extinction event.
Looking to the future, MacLeod details how human activity could spark a “sixth extinction” that would extinguish half of Earth’s current species in the next 100 to 1,000 years—a very short timescale compared to the Big Five extinctions. A 50 percent loss may not equal what the Earth has withstood in the past, but that is no cause for comfort, MacLeod says. The modern human impact could still rival that of major past volcanic activity, and the consequences could include major habitat destruction and disruption of the food supply. Elsewhere in the book, MacLeod takes the geologist’s long view. In discussing the devastating Permian extinction, he writes, “It is well to recall life evolved originally from simple bacterial beginnings… Even if the evolutionary clock had been reset back to that point…life would, in all likelihood, simply have picked itself up, dusted itself off and resumed evolving.” Then he adds, “There is little evidence life itself was ever in any danger of going extinct.” So even if we humans wipe ourselves out at some point, we can rest assured that life on the planet will very likely continue humming along without us.
Fenella Saunders is managing editor of American Scientist, where she covers work in the physical sciences, among other subjects. She received her M.A. in psychology and animal behavior from Hunter College of the City University of New York.