GHOSTS OF EVOLUTION: Nonsensical Fruit, Missing Partners and Other Ecological Anachronisms - Review

Washington Monthly, Sept, 2001 by Gregg Easterbrook

The weapons were never fired. Darwin found he could walk right up to Galapagos creatures, causing him to reason that local species had been exempt from predation so long, they had evolved away from the flight response. To this day, the same remains. When I was in the Galapagos a few years ago, I was able to approach an albatross or blue-footed booby without causing any reaction, other than tut-rutting from local naturalists. The passivity of Galapagos creatures made them ideal for study; the differing environments of the islands helped Darwin realize how species adapt to circumstances.

The Ghost in the Machine

Over the years, Darwin's premises have solved so many puzzles of biology that many now believe all questions have been answered. But they haven't been, not by a long shot. There are both small mysteries and great ones remaining.

One of the remaining small mysteries is the subject of The Ghosts of Evolution, a fine work by the science writer Connie Barlow. Her topic is the contemporary biologist Dan Janzen and his obsession with rotting fruit on the floors of Central American forests. Why do plants produce so much fruit that it ends up rotting? Why, Janzen long wondered, doesn't something eat the stuff? Janzen eventually came to theorize that the somethings that used to eat the fruit of Central American forests are now extinct, but the plants have not yet "realized" that.

Wild plants are thought to have evolved alluring fruits so that animals will munch the offerings, then walk somewhere else and excrete seeds, spreading the plant's DNA. The rotting fruits of Central American forest floors, Janzen thinks, once attracted large herbivores that no longer exist. This is in synch with what other researchers have found about the recent past, geologically speaking. Very large herbivores once were common in most of the world--mastodons, woolly mammoths, jumbo rhinos, glyptodonts (armadillos about the size of a car), ground sloths as big as elephants. In North America, giant camels, large native horses, and a beaver the size of a bear also roamed the landscape. But large land creatures started to fall extinct about 50,000 years ago, and every one mentioned in this paragraph was gone by about 13,000 years ago. Cows and bison are about all that's left to fill their vacated ecological niches. Barlow writes, "Janzen was beginning to suspect that the forests and savannas where livestock grazed were in some ways closer approximations to the human past."

Extinctions of the large land animals roughly coincided with the end of the Pleistocene ice age, but the fossil record suggests that these species had survived ice cycles before. What the large herbivores had never before had to deal with was Homo sapiens, which became anatomically and mentally "modern" about the time the extinctions began. It's not a wild guess that, as humans learned to make weapons and start fires, they hunted large game to extinction--if the creatures had never had predators, perhaps the humans could have walked right up to a mastodon or giant beaver and thrust a spear in. To ancient men and women, the presence of huge, dull-witted, appetizing mammals might have seemed providential--as if a higher power had placed them there for human sustenance.