A short history of nearly everything - Bill Bryson [172]
In between the big kill-offs, there have also been many smaller, less well-known extinction episodes—the Hemphillian, Frasnian, Famennian, Rancholabrean, and a dozen or so others—which were not so devastating to total species numbers, but often critically hit certain populations. Grazing animals, including horses, were nearly wiped out in the Hemphillian event about five million years ago. Horses declined to a single species, which appears so sporadically in the fossil record as to suggest that for a time it teetered on the brink of oblivion. Imagine a human history without horses, without grazing animals.
In nearly every case, for both big extinctions and more modest ones, we have bewilderingly little idea of what the cause was. Even after stripping out the more crackpot notions there are still more theories for what caused the extinction events than there have been events. At least two dozen potential culprits have been identified as causes or prime contributors: global warming, global cooling, changing sea levels, oxygen depletion of the seas (a condition known as anoxia), epidemics, giant leaks of methane gas from the seafloor, meteor and comet impacts, runaway hurricanes of a type known as hypercanes, huge volcanic upwellings, catastrophic solar flares.
This last is a particularly intriguing possibility. Nobody knows how big solar flares can get because we have only been watching them since the beginning of the space age, but the Sun is a mighty engine and its storms are commensurately enormous. A typical solar flare—something we wouldn't even notice on Earth—will release the energy equivalent of a billion hydrogen bombs and fling into space a hundred billion tons or so of murderous high-energy particles. The magnetosphere and atmosphere between them normally swat these back into space or steer them safely toward the poles (where they produce the Earth's comely auroras), but it is thought that an unusually big blast, say a hundred times the typical flare, could overwhelm our ethereal defenses. The light show would be a glorious one, but it would almost certainly kill a very high proportion of all that basked in its glow. Moreover, and rather chillingly, according to Bruce Tsurutani of the NASA Jet Propulsion Laboratory, “it would leave no trace in history.”
What all this leaves us with, as one researcher has put it, is “tons of conjecture and very little evidence.” Cooling seems to be associated with at least three of the big extinction events—the Ordovician, Devonian, and Permian—but beyond that little is agreed, including whether a particular episode happened swiftly or slowly. Scientists can't agree, for instance, whether the late Devonian extinction—the event that was followed by vertebrates moving onto the land—happened over millions of years or thousands of years or in one lively day.
One of the reasons it is so hard to produce convincing explanations for extinctions is that it is so very hard to exterminate life on a grand scale. As we have seen from the Manson impact, you can receive a ferocious blow and still stage a full, if presumably somewhat wobbly, recovery. So why, out of all the thousands of impacts Earth has endured, was the KT event so singularly devastating? Well, first it was positively enormous. It struck with the force of 100 million megatons. Such an outburst is not easily imagined, but as James Lawrence Powell has pointed out, if you exploded one Hiroshima-sized bomb for every person alive on earth today you would still be about a billion bombs short of the size of the KT impact. But even that alone may not have been enough to wipe out 70 percent of Earth's life, dinosaurs included.
The KT meteor had the additional advantage—advantage if you are a mammal, that is—that it landed in a shallow sea just ten meters deep, probably at just the right angle, at a time when oxygen levels were 10 percent higher than at present and so the world was more combustible. Above all the floor of the sea where it landed was made of rock rich in sulfur. The