As leaves fall from a tree--presumably a symbol for Darwin's tree of life--University of Washington paleogeologist Peter D. Ward tells us: "Extinction is the termination of a species." At least 95 per cent of all species that have ever lived are now extinct. Extinction is normal, he says, and is happening all the time, at the rate of a few species per year.
We watch various animals foraging for food, and a lioness bringing down her prey. "The extinction of old species that can no longer adapt or compete creates opportunities for new species that can--in an endless cycle," the narrator says. "So evolution and extinction are in balance. But what happens when a planet-wide catastrophe strikes and a great dying begins?"
The scene changes dramatically--to lightning, volcanoes, and fire. Five times in the last half-billion years, we are told, mass extinctions wiped out most species alive at the time. As the smoke clears, we see Peter Ward driving through South Africa to investigate the greatest of these mass extinctions--the one that occurred at the end of the geological period known as the Permian. He stops at an old abandoned farmhouse, and sees from the tombstones in a nearby graveyard that the family that used to live there died within a five-year period about a century ago. "So a hundred years [ago], these people were just wiped off the face of the Earth, and we have no idea what killed them," says Ward. "And if that's the case, how am I going to figure out what killed animals that lived in those hills [gesturing], the fossils of which we have from 250 million years ago?"
In the rocks of those hills, Ward finds evidence that a great catastrophe occurred at the end of the Permian. "So catastrophic was that mass extinction," says Ward, "that even the small creatures have died out. It's not just the mighty, it's the meek." An animation shows us what might--or might not--have caused the Permian extinction. "When species died, they didn't die alone," says the narrator. "The collapse of one helped bring down the others."See . For more information on mass extinctions, see Peter D. Ward, Rivers in Time: The Search for Clues to Earth's Mass Extinctions (New York: Columbia University Press, 2001)
Ward explains: "You could almost analogize that to a house of cards. Each species props up another, in a sense." We watch as a huge house of playing-cards teeters in front of us. Ward continues: "Because the creature that you eat is that card that is sitting under you that gives you your energy. Now let's pretend that we start kicking out card after card after card--and that's what a mass extinction does, isn't it? It starts knocking out a species here, it knocks out a species there, but pretty soon lots of species are gone. And it's not just the disappearance of species now--the whole house of cards falls down."
Not everything died in the Permian extinction, however. Ward holds up the skull of a mammal-like reptile. He says that the few lineages that survived the extinction "start evolving, because the world is empty, and empty worlds really begat [a] tremendous amount of evolutionary diversifications."
But how do empty worlds beget new species, exactly? Mass extinction may be an important feature of the history of life; but the question is, how did living things diversify afterwards? That is the question Darwin's theory is supposed to answer, but the fact of extinction doesn't help us. Species go extinct, and new ones take their places. This may come as a surprise to people who believe that species never go extinct (if, in fact, there are such people); but how does it provide evidence for Darwinian evolution?
Ward takes us to visit a family that for four generations has been collecting fossils from the early Triassic. He explains that two important groups of land animals diversified after the Permian extinction: dinosaurs, some of which attained enormous size; and small mammal-like reptiles.See . Although not much is said about them here, mammal-like reptiles are often cited as good evidence for Darwinian evolution. Not surprisingly, however, the story is more complicated than promoters of Darwinian evolution make it out to be. For one review, see Ashby L. Camp, "Reappraising the Crown Jewel," at:
The scene shifts to the American Museum of Natural History in New York City, where mammal curator Michael J. Novacek tells us about his childhood fascination with stories of dinosaur fossils found by Roy Chapman Andrews in the Gobi Desert in China. Years later, Novacek went to the Gobi Desert himself--but to look for fossils of early mammals instead.
Novacek explains that early mammals were quite small, dwarfed by the dinosaurs who were their contemporaries. After millions of years another mass extinction occurred, at the end of the Cretaceous period, perhaps caused by a large asteroid that landed in what is now the Gulf of Mexico. This global catastrophe wiped out the dinosaurs--in a chain reaction dramatized once again by a falling house of cards.
The narrator tells us that the mammals survived because they were small and "could take refuge underground." When they re-emerged after the "K-T event" (K stands for Cretaceous and T for Tertiary, the geological period following the catastrophe), the dinosaurs were gone, and the world belonged to them. "Mass extinction made them evolution's big winners--by default."
As we are treated to more beautiful wildlife photography, the narrator tells us that mammals--freed from having to compete with the dinosaurs--"spread out to all parts of the world. They filled every empty niche. They competed, adapted, and diversified, until most of the world's largest animals were mammals. Then, around 5 million years ago, the first human precursors emerged in Africa--mammals that would play an unprecedented role in evolution's future." Bleached bones stick out of the dry soil, and we view a trail of footprints as the narrator explains: "They began to walk upright. They left their footprints in volcanic ash that hardened. They evolved into the genus Homo--humankind."
This makes an interesting story. But the K-T event, like the Permian extinction, does not help us to understand evolution. Instead of saying that mass extinction made the survivors "evolution's big winners," the narrator might just as well have said "history's big winners," or "life's big winners." Using the word evolution doesn't add a thing here--except perhaps to give us the impression that an explanation has been provided, when in fact none has.
The fossil skulls we saw certainly provide evidence that there were animals living in the past that are no longer with us, and they suggest that some of those animals had features intermediate between ancient reptiles and modern mammals. As we saw in Episode Two, however, fossils by themselves cannot provide evidence for ancestry and descent, much less evidence that the process occurred through natural selection. Life has a history. But was that history due to Darwinian evolution?
The scene shifts to Bangkok, Thailand. "Today," the narrator continues, "the world is bursting at the seams with people." Bangkok, we are told, has doubled in size in the last two decades, and now has more than 10 million inhabitants. In the past ten thousand years, we have out-competed--and thus driven extinct--many other species. The narrator concludes: "We may be the `asteroid' that brings on the sixth great mass extinction."
We fly over a mist-shrouded valley as the narrator describes a national park to the west of Bangkok, where the human population is nil. Wildlife Conservation Society scientist Alan Rabinowitz came here to study the ecosystem.
Rabinowitz says: "We're in grave danger of the `empty forest syndrome'--having a beautiful, seemingly intact forest on the surface, but inside that forest the natural components which maintain the flow of energy through the system--which has developed through millions of years of evolution--it's disrupted. Now people say, So what does it matter if one component's gone? What if you don't have the Sumatran rhino? What if the civet species are all gone--or other things? But each thing has evolved to play an incredibly important role within this complex puzzle."
The narrator explains that Rabinowitz is here to find out if the forest still has a balanced ecosystem in which evolution can continue without being affected by the increasing rates of extinction elsewhere. He is especially interested in large carnivores. Since they depend on species below them in the energy chain, they're the first to go when the ecological house of cards begins to collapse.
Rabinowitz uses remote cameras triggered by motion sensors to "catch" nocturnal animals on film that otherwise would be almost impossible to find. Finding that some of his cameras have been stolen or vandalized, Rabinowitz fears that humans are encroaching destructively on this remote forest.See . Alan Rabinowitz's work in Thailand and neighboring Myanmar was done under the auspices of the World Conservation Society, which is affiliated with the Bronx Zoo in New York. For more information, see:
"There's no doubt that the major cause of extinction on a global level is human-related," he says. "Everything from clear-cutting forests, to removing intact habitats, to just desecrating them, changing them." The narrator adds: "Habitat destruction is the number one cause of extinction."
The scene shifts to the Hawaiian Islands, as the narrator explains how they were formed from undersea volcanoes and then colonized by living things from distant lands. "Thousands of species made it by sea or by air. They evolved until they were found nowhere else on earth."
Fordham University paleoecologist David A. Burney and his son explore the Hawaiian Islands "to better understand what happened after the Polynesians arrived" centuries ago. We watch as Burney drains a sinkhole, "revealing ten thousand years of sediment and a story of evolution that's just as long."
Burney finds evidence that the Polynesians brought stowaway rats and a few other non-native species with them. But the Europeans brought many more. "We're now to the point," Burney says, "where there are about a thousand native species of plants in the Hawaiian Islands and over a thousand naturalized invasive species--things that have been introduced by people. Evolution has now entered a new mode. Something new altogether is happening, and it has to do with what humans do to the evolutionary process."
The scene shifts to various modes of air and sea transport, as the narrator explains how hitchhiking species colonize virgin territories and become pests. A Hawaiian agricultural inspector talks about the difficulty of keeping biological invaders out of the islands, and Burney predicts that biological invasions will be visible in the fossil record a million years from now.
"Scientists have a term for biological invaders," the narrator tells us. "They call them `weed species.' Like weeds, they survive and adapt almost anywhere, and push out the native species--sometimes to the point of extinction. They are the ultimate survivors."
After Novacek returns to speculate briefly on why biological invaders tend to be so successful, the narrator continues: "Of all the weed species on earth, we [human beings] are the most mobile, the most adaptive, and the most flexible--by far. The good news is, we'll probably be around for a long time. The bad news is, the world around us may be very different. Every species we drive towards extinction, no matter how inadvertently, is one less species that might help prop up the others."
Re-enter Peter Ward, who ties this thread back into the main theme of the episode: "So the question is, in our own modern world, with our own house of cards, How close are we to that whole edifice coming down? Have we reached that threshold?"
Curiously, there are several unanswered questions in what we have just been told. First, what, exactly, favors evolution? Does evolution proceed mainly when there is intense competition for survival, as Darwin proposed? Or does it proceed mainly when competition is eliminated through mass extinction, or through migration to an uninhabited island? Maybe it's both. Or maybe it's neither.
Second, is extinction good or bad? From an evolutionary perspective, it seems, extinction is a good thing, since it provides opportunities for surviving organisms to diversify. From a human perspective, however, our own extinction would be a bad thing, so we should interfere with evolution to preserve ourselves. Why, then, do we need an evolutionary perspective?
Third, What does evolution have to do with how some species replace other species over a span of ten thousand years? None of the species are changing. We saw more evolution in bacteria that develop resistance to antibiotics--and even that didn't really help Darwin's theory about the origin of species.
The scene shifts to the rolling grasslands of North Dakota. "It's where one of the battles against human-caused extinction is being fought," the narrator says. "The enemy here is a weed called `leafy spurge'--so adaptive and tenacious, it threatens to kill off all the native grasses. It's already spread across a million acres. Accidentally brought by pioneers in bags of seeds a century ago, the settlers' descendants are now faced with the consequences."
A rancher explains that leafy spurge limits the number of cattle he can put in a pasture, because the cattle won't eat the grass if it is infested with the weed. "I look at it as cancer to the land," the rancher says, "and it makes the land just totally useless." There is a chemical available that kills leafy spurge, but it is prohibitively expensive.
The narrator continues: "If the chemical won't stop it, what's left? How can the farmers fight back against a super-adaptive invader that threatens to drive native species to the brink of extinction, and take over their ecological niche? The solution may be another invader--discovered when scientists learned what kept leafy spurge in check in its native Russia. It's the flea beetle--a case of fighting evolutionary fire with fire." Flea beetles eat leafy spurge, and thereby help to keep it under control.
We watch as North Dakota ranchers spread flea beetles throughout their pasturelands in an effort to control leafy spurge. Then the narrator says: "So now we're in a race most of us don't even know we're running--to learn as much as we can about the meaning and message of extinction before it's too late."
But extinction has nothing to do with the leafy spurge story. Although the weed is out-competing native grasses in some areas, it is an exaggeration to say that it "threatens to drive native species to the brink of extinction." Furthermore, it does not render land "totally useless," except to cattle ranchers. Although cattle are repelled by leafy spurge, sheep and goats are not; the latter even seek it out.
The producers of Evolution want us to think that a "grounding in evolution is key to our understanding" of agriculture. Perhaps that's why they included the leafy spurge story in their PBS series. But the use of flea beetles to control leafy spurge is not "fighting evolutionary fire with fire"--it's just a variation on an ancient agricultural practice. Farmers have been using one organism to control another for centuries. For example, ants were used in ancient China and Yemen to control pests that would otherwise destroy citrus groves. The practice is known as "biological control," and it does not depend on a "grounding in evolution."See . Leafy spurge produces an irritating sap that is harmful and distasteful to cattle, so it renders grazing land unsuitable for cattle. As this episode mentions, flea beetles are one way to control leafy spurge. Other ways to control it without resorting to chemical pesticides include grazing with sheep or goats (goats actually seek it out), or planting wheat grass or wild rye. For general information about leafy spurge, go to:
The scene then shifts back to Alan Rabinowitz in Thailand. Happily, the other teams of scientists working with him had more success with their cameras than he had. Their films show that the forest is still populated with large carnivores, and thus still has a healthy ecosystem.
Rabinowitz reflects on the implications of this: "There are still places left where the natural evolutionary processes are going on. Most of my career involves documenting extinction, or species on the verge of extinction. But every now and then, you get a place like this. And you say, It's not lost yet. It's not gone yet. Knowledge is definitely our greatest tool against extinction--there is no, no doubt about it. Without knowledge, we continue in the dark. Many species are on a very quick downward slide, possibly to extinction, faster than they would be normally, because of human-related activities. But we're not at an end-point here, by any means. We're still in the middle of a completely complex, changing scenario. Evolution is going on around us."
The "evolution" that Rabinowitz sees going on around us, however, is not "evolution" in Darwin's sense. The "evolution" to which Rabinowitz refers involves the displacement of some species by others, but the "evolution" that matters in Darwin's theory is the origin of new species. And there is no evidence here for the latter.
No one would dispute Rabinowitz's claim that knowledge is important, and many people would approve of his efforts to preserve the unspoiled beauty of remote forests. But his comments about evolution are superfluous. Scientists do not need evolution to engage in conservation efforts, and it is questionable whether they even need it to understand ecology. According to evolutionary biologist Peter Grant, past president of the American Society of Naturalists, "an ecologist's world can make perfect sense, in the short term at least, in the absence of evolutionary considerations."See . Peter Grant's comment about ecology is from "What Does It Mean to Be A Naturalist at the End of the Twentieth Century?" The American Naturalist 155 (2000), 1-12, 9.
The episode closes with a reading from Darwin's The Origin of Species: "We need not marvel at extinction. If we must marvel, let it be at our own presumption in imagining for a moment that we understand the many complex contingencies on which the existence of each species depends. The appearance of new species and old species are bound together."
What does this last statement mean? Species go extinct, and new ones appear to take their places. But we still have seen no evidence of one species changing into another--much less through natural selection, as Darwin claimed. Extinction, as Peter Ward explained, is the termination of a species, not its transformation.
Based on new fossil discoveries, some scientists are now questioning whether the "Big Five" mass extinctions were really as big as previously believed. See Richard A. Kerr, "Mass Extinctions Facing Downsizing, Extinction," Science 293 (2001), 1037.
Although the use of flea beetles (as described in this episode) did not start until the 1980s, there is nothing new about biological control. In China and Yemen, farmers have been using ants to control crop pests for centuries. In India, intercropping (planting mixed crops to combat pests) has been used for centuries. Such ancient practices obviously owe nothing to Darwin or his theory. Here are some quotes from web sites of interest:
The Chinese were far ahead of the Western world in natural pest control. In the countryside frogs were always a forbidden food because they ate insects. Praying mantises were released in gardens among the chrysanthemums to drive away leaf-eating insects. The most remarkable and economically important of the ancient Chinese biological weapons was the yellow citrus killer-ant. Its use is described in Hsi Han's Records of the Plants and Trees of the Southern Regions, written in A.D. 340: "The Mandarin Orange is a kind of orange with an exceptionally sweet and delicious taste. . . . The people of Chiao-Chih sell in their markets [carnivorous] ants in bags of rush matting. The nests are like silk. The bags are all attached to twigs and leaves, which, with the ants inside the nests, are for sale. These ants do not eat the oranges, but attack and kill the insects which do. In the south, if the mandarin orange trees do not have this kind of ant, the fruits will be damaged by the many harmful insects, and not a single fruit will be perfect.
Yemenis were among the earlier nations that used biocontrol agents for the control of agricultural pests. . . . In traditional agriculture farmers in Tihama, Taiz and Hadramout used to collect predatory ants (qu'as) from mountains to control date palm pests which attack fruits. This has been described in a 13th century agricultural text by al-Malik al-Ashraf `Umar of Rasulid, Yemen. . . . When we asked old farmers about this practice they confirm that. They added that they have to put some sticks to make ants to travel from one tree to another.
Intercropping in India: Farmers in the developing world have been growing two or more crops together on the same piece of land for many centuries. In India, as many as 84 different crops are used in mixed cropping, but seldom do we find more than four at a time, and a relatively simple mixture of only two or three crops is most common. . . . Although research on intercropping may have started to provide an understanding of why the farmer used such mixtures, and to help improve his productivity in ways relevant to his practice, it has now been shown that intercropping may have several advantages over sole cropping. It appears to make better use of the natural resources of sunlight, land, and water. It may have some beneficial effects on pest and disease problems, although the overall results are somewhat inconclusive.