Connie Barlow. 2000. Basic Books, New York. 291 pp., hardbound.
My
husband and I grow grapes in our urban garden, just enough
so that a few times each summer I have the pleasure of announcing
desert to our dinner guests with a casual, "John, will you
harvest some grapes to serve with the coffee?" We’d have
more grapes, however, if we weren’t in fierce competition
for each ripe orb. All day, cardinals, catbirds, mockingbirds,
blue jays, and starlings stop by to pluck the fruits; all
night, a raccoon chows down. This underscores a simple fact:
Ripe fruit is meant to be eaten. Tasty, nutritious fruit is
the reward many plants bestow on the animals that disperse
their seeds.
So when tropical ecologist Dan Janzen noticed that the forest floor in his Costa Rican field station was littered with large, uneaten fruits, he wondered why. Why would plants waste their energy producing big fruits that nobody eats? Ah, but somebody did sometimes eat them. Set loose in the forest, domestic cattle scarfed them up, although clearly these New World plants hadn’t evolved fruits for Old World cows to eat. In a flash, Janzen had an idea—and a likely solution to the mystery. The animals these fruits evolved to entice are extinct. Paleontologist Paul Martin assembled a list of suspects: species like the mastodont elephants and giant ground sloths that once grazed in these forests but disappeared at the end of the Pleistocene about 13,000 years ago.
Janzen and Martin outlined their hypothesis in a paper published in the prestigious journal Science in 1982. Although not without controversy, the article launched a new line of inquiry in ecology and evolutionary biology that has explored dozens of "evolutionary anachronisms." Or, as science writer Connie Barlow calls them in her new book, "ghosts of evolution." Barlow tells wonderful stories as she reviews the scientific literature of this burgeoning field.
It turns out there are many anachronistic fruits. "Grocery stores are excellent places to encounter ghosts," Barlow writes, as are city streets and parking lots. Papayas, avocados, and gingkos are just a few of the familiar fruits that once attracted giant mammals. Barlow also combed the literature and conducted her own admittedly crude experiments to assess the likelihood that four North American natives—desert gourd, pawpaw, persimmon, and honey locust—all evolved fruits with dispersers other than their current ones in mind.
Neither are fruits the only anachronistic plant parts. Some plants bear thorns and spines far beyond the height that any modern browsers can reach. Devil’s walking stick, for instance, grows a nine–foot-tall prickly stem before it branches out. If the growing tip of the stem is bitten off, the plant loses precious time in its race to reach the sunlight and branch out, so it makes sense to defend the stem with prickles. But the defense is overkill. No browser exists today in the walking stick’s native eastern deciduous forest habitat that could bite off the tip of a nine-foot-tall plant. The big herbivores of the Pleistocene, however, could have, and this probably accounts for the walking stick’s profligate prickles.
North America’s pronghorn antelope is a probable animal anachronism, an idea developed by mammalogist John A. Byers in his 1997 book: American Pronghorn: Social Adaptations and the Ghosts of Predators Past. Why, Byers asked, do these animals run so fast? Pronghorns can reach speeds of about 60 miles per hour and run 40 to 45 miles per hour for nearly two miles without tiring. No existing predators, such as wolves, coyotes, and pumas, come close to achieving such speed. But extinct American cheetahs likely did at one time, and pronghorns may still be running from their ghosts.
People seem to act as substitutes for the seed dispersers of some of the fruit ghosts, reports Barlow. In some cases, people may do an even better job. She cites work by ecologist Gary Paul Nabhan, who found that of 112 species of New World tropical fruits likely eaten by extinct herbivores, at least 98 survive because people use them. Barlow writes, "In the Western Hemisphere, four out of five tropical and subtropical species ecologically stranded by the loss of their mutualists have been rescued, in part, by human attention. In the case of Curcurbita [gourds, squashes, pumpkins, and similar plants]...we nurtured the lineage to newfound splendor."
But loss of dispersers may have dire consequences for some plants. Many African trees rely on highly endangered elephants for dispersal, for instance. Barlow quotes ecologist Richard Corlett, who wrote, "Loss of seed dispersal agents may, in the long term, be as serious a threat to tropical plant diversity as deforestation."
The Ghosts of Evolution is a fascinating, highly readable account of the natural history of the relationships between plants and animals. While sometimes her ideas are highly speculative, Barlow clearly separates hypotheses with scientific support from those that are pure conjecture. After reading her stories, you’ll never again eat fruit without wondering how useful you might be in spreading its seeds—or without looking over your shoulder for ghosts.
—Susan Lumpkin
Zoogoer 30(5) 2001. Copyright 2001 Friends of the National Zoo. All rights reserved.
