The Wild File


Q) If humans evolved from apes, why do we still have apes?
Tom Adair, Minneapolis, Minnesota

UM, TO THROW THINGS at us when we visit the zoo? Seriously, though, "evolution produces a pattern like a tree with many branches," says Marian Dagosto, a biologist at Northwestern University, in Evanston, Illinois. In other words, she says, "The best way to view the evolutionary relationship between a chimpanzee and a human is not that humans evolved from chimps, but that humans and chimpanzees share a common ancestor." In the Miocene Epoch, which extended from about 23 million to five million years ago, there were untold numbers of ape species. Scientists believe that one of these unnamed species evolved into Homo sapiens about 300,000 years ago. The remaining apes either went extinct or evolved into one of the four species that exist today: the chimpanzee, gorilla, orangutan, and bonobo. "According to fossil records," adds Dagosto, "that Miocene ancestor probably looked more like a chimpanzee than a human." That explains why, in the family album, your great-great-grandmother might look a little hairy.

Q) I just finished reading John Steinbeck's The Grapes of Wrath and was wondering, are we in for another Dust Bowl anytime soon?
Dave Cox, Boulder, Colorado

AS ANY TRANSPLANTED Okie will tell you, one Dust Bowl is plenty for a lifetime. While experts warn us not to be surprised if we see another catastrophic drought like the one that scoured the Great Plains during the 1930s, they also say it's not likely we'll experience the sky-obscuring "black blizzards" that blotted out the sun for days at a time. "If you look at long-term records, decade-long droughts happen twice every 100 years," says Donald Wilhite, director of the National Drought Mitigation Center, in Lincoln, Nebraska. But unlike farmers of the thirties—who believed that tilling soil would release moisture that would then fall as rain—today's farmers attempt to reduce the exposure of valuable topsoil to the wind, a battle fought by agriculturists worldwide. So while we may not experience Dust Bowl Redux, much of the nation is already a few years into what could become a catastrophic drought. This dry spell, says Wilhite, could be especially damaging to the millions of people who have migrated to southwestern cities, where water supplies are dubious. Our advice: Forget about migrating to California, and stock up on Evian while you still can.

Q) How steep can a mountain get before it becomes unstable?
Robert Keil, Los Angeles, California

IF THE CONDITIONS are right, any slope can succumb to a landslide—even surfaces that are practically horizontal. The general rule of thumb is this: If the angle of an unconsolidated slope is greater than the angle of repose (defined as the maximum angle at which loose material—say, sand or rock debris—will remain in place without sliding), the slope is in danger of failing. Sand, for instance, has an angle of repose of roughly 34 degrees, so if a windstorm whips a dune into a 45-degree precipice, chances are it'll slide. The rules change, however, in the presence of one or more "landslide initiation processes," a blanket term for factors that act to decrease the static friction that holds loose material together. These include rapid snowmelt, water-level changes, volcanic eruptions, earthquakes, and excessive rainfall. Burned areas, which are stripped of vegetation and have chemically altered soil that's more vulnerable to water saturation, and hillsides with distinct layers—clay on top of volcanic soil, for example—can also be prone to sliding, regardless of the slope's angle. But there's no way to tell by eyeballing it, so consult a U.S. Geological Survey hazard map, which identifies landslide danger zones, before your next scramble up a talus face. A last resort, says Lynn Highland, a geographer with the USGS, is to use your ears: "If you hear a loud rumble coming down the canyon, it's time to get out of there."

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