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In the deep history of our planet, there have been at least five short intervals in which the majority of living species suddenly went extinct. Biologists are used to thinking about how environmental pressures slowly select the organisms most fit for survival through natural selection, shaping life on Earth like an artist sculpting clay. However, mass extinctions are drastic examples of natural selection at its most ruthless, killing off vast numbers of species at one time in a way that is hardly typical of evolution.
In the 1980s, Nobel Prize-winning physicist Luis Alvarez and his son first hypothesized that the impact of comets or asteroids caused the mass extinctions of the past. Most scientists slowly came to accept this theory of extinction, and since then a great scar in the Earth--an impact crater--has been discovered off the coast of Mexico that dates to around the time the dinosaurs went extinct. An asteroid probably did kill off the dinosaurs, but the causes of the other four mass extinctions are still obscured beneath the accumulated weight of hundreds of millions of years, and no one has found any other credible evidence of impact craters.
But now, together with Mark Roth of the Fred Hutchinson Cancer Research Center in Seattle, I believe we have found a possible biochemical scar, present within living animals, that links Earth's greatest mass extinction to a single substance: hydrogen sulfide (H2S). Hydrogen sulfide is a relatively simple molecule that gives rotten eggs their distinctive foul odor and is quite toxic--in high concentrations a single breath can kill. And it looks like that is what happened: Hundreds of millions of years ago, hydrogen sulfide probably saturated our oceans and atmosphere, poisoning nearly every creature on Earth.
Yet some creatures, like our very distant ancestors, must have somehow survived this toxic environment. What Roth has discovered is that H2S, incredibly, also has the ability to preserve and save lives. In small doses the chemical puts many animals into a state of "suspended animation," a useful adaptation that would have allowed creatures to, in essence, hibernate through the catastrophe of mass extinction. If this idea is correct, our understanding of the deep past could lead to a dramatic medical revolution very soon.
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