Waves of creation
Ellen Ruppel Shell
The aging Volvo cuts through the drizzly New Haven night, darting expertly through the rush-hour traffic. The driver, Elisabeth Vrba, keeps a nervous eye on the rearview mirror.
* “The last time I got stopped, the policeman kept me waiting 15 minutes while he lectured me on the hazards of speeding,” she says. “He was terribly nice, but it was such a waste of time.”
* Time is never far from Vrba’s mind; she is acutely, almost painfully aware of its passage. She talks as fast as she drives, the ideas pouring forth so rapidly that they sometimes collide. Vrba is aware of this and says that her students complain of it, but she doesn’t apologize. In the heady and competitive scientific circles she travels in, waiting patiently for less nimble minds to catch up can be counterproductive.
Vrba’s anxiety about time stretches easily from minutes to millennia–the time it takes for evolution to work. She is a biologist and paleontologist by training, and an evolutionary theorist by inclination. She works in an area in which evidence is scarce and hypotheses are tightly held, where one bold, well-supported idea can launch a scientist out of obscurity and into the limelight. Vrba, who will turn 51 this month and holds a tenured chair in the department of geology and geophysics at Yale, has had several such ideas.
Best known among them is the “turn-over pulse hypothesis,” a notion that may very well explain the curious evolutionary bursts that first separated our lineage from the apes and later pushed our forebears toward bigger brains, dexterous hands, tool making, and other attributes of modern humanity. Those bursts, Vrba argues, caught other species at the same time, and the consequences can be read in the fossil record. They were bursts not just of creativity but also of destruction, killing off some species while playing midwife to new ones.
Sudden pulses of evolution are not what Charles Darwin had in mind 134 years ago when he argued in his theory of natural selection that evolution is mainly a steady, unceasing process of one species beating out another. “Each new variety, and ultimately each new species,” he wrote in the Origin of Species, “is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of the less favored forms almost inevitably follows.” A new type of organism replaced an older one because it was better suited to exploit its surroundings; the new breed slowly, painfully pushed the inferior breed to the fringes of a habitat and, finally, off the stage of life completely.
Darwin’s notion of a long-drawn-out war of attrition, however, didn’t quite match up with the fossil record, which showed that some species remained the same for millions of years, then were suddenly transformed into new creatures. In the 1970s paleontologists Stephen Jay Gould of Harvard and Niles Eldredge of the American Museum of Natural History gave this pattern of evolutionary fits and starts a name: punctuated equilibrium. The slow grind of creatures elbowing one another for living space over millions of years, they proposed, would at times suddenly rev into high gear, perhaps when some crucial threshold of physical variation was reached–when, say, some isolated creatures developed a slightly better mouth than their brethren and started eating better, having more babies, and shouldering their poor-mouthed relatives aside. In these relatively brief moments of evolutionary high drama, new species were born.
Like orthodox Darwinism, though, punctuated equilibrium sees change as coming from within the species. But Vrba has a different perspective: a species’ evolution, she maintains, is driven by the changing world around it. “Evolution is conservative,” she says. “Making a new species requires a physical event to force nature off the pedestal of equilibrium.”
That fall, Vrba believes, is triggered by a radical shift in global climate. Earth’s climate is not stable: any number of things, from a shift in the tectonic plates making up Earth’s outer shell to a sharp drop in sea levels to slight but regular changes in our planet’s orbit and orientation to the sun, can cause a plunge or rise in world temperatures. This, Vrba says, sets off a chain reaction that races around the globe, annihilating some species while anointing others as world inheritors. As far as evolutionary change goes, she continues, without these climate pulses, pretty much nothing happens.
Vrba has neatly tied geologic evidence of worldwide climate change to three sudden accelerations in the rate of species creation and destruction, at approximately 5 million, 2.5 million, and possibly 900,000 years ago. But her idea–that climate, rather than some dynamic among animals and plants themselves, is the dominant force behind evolution–dropped like a bomb when Vrba first proposed it at a meeting in France about a decade ago. “Many of my colleagues were extremely skeptical, bordering on rude,” she says, forcing a smile. “Some of them were clearly hostile.” Others, though, including original “punc eekers” Gould and Eldredge, were intrigued.
OVER DINNER AT A RESTAURANT NEAR HER home, Vrba explains her turn-over pulse hypothesis with a grand show of gestures and visual aids. Silverware, napkins, and table settings take on the role of Africa and the ecological regions that covered various parts of that continent millions of years ago. “Most past ecosystems were constantly and repeatedly forced by climatic changes to alter, dismantle, and reconstitute,” Vrba says. Prolonged cool periods dried the climate and caused the lush African rain forests to break up into patches of open woodland. These patches of forest were isolated from one another by extensive grasslands, like islands in a vast green sea.
This change in circumstances could have posed problems for any animal that had evolved in, and was therefore well suited for, a wide-ranging, dense forest. Such a species, Vrba explains, had three ways of coping with the change. “The Hindus believe in a triad with three deities–Brahma the creator, Vishnu the preserver, and Siva the destroyer,” she says, falling back on one of her favorite analogies. “Species can follow one of these three paths.” Some groups, taking the creative path of Brahma, split off into new, better-adapted species: they develop the ability to live in the grass, for example. Other species migrate, as if following Vishnu not to greener pastures but to more forested ones. And species that turn away from those paths have but one alternative left: to fall into the arms of Siva the destroyer and flicker into extinction.
Whatever path a group of organisms takes has an effect on other organisms whose lives are enmeshed with theirs, and this is where the pulse comes in. A change in local plant life brought on by a sudden cool period might result in the migration of one or more species of insects that feed on the plants, which might lead to the extinction of the reptiles that feed on the insects. This might then lead to the creation of a new species–one that can exploit another food source–from among the small mammals that used to feed on the reptiles, and so on to the large mammals that feed on the small mammals.
“Punctuated-equilibrium theorists never matched these changes in different lineages against an absolute time scale,” Vrba says, “so they never noticed these episodes kept bunching together, which indicates a common cause. These theorists were also focused on the creation of new species, and they missed the significance of the extinctions that were occurring right alongside. What the pulse theory suggests is that death is the flip side of creation. Nature is pushing these creatures onto a knife edge, and it’s a close call as to whether they will die out or become something new and different.”
Perhaps it is because Vrba is self-taught as an evolutionary theorist that she was able to conceive an idea as simple and yet as startlingly fresh as turn-over pulse. Like the rapidly evolving species she describes, Vrba has lived much of her professional life poised on the edge, isolated from the mainstream. She has the defensive posture of someone who has not always been treated with kindness, and she gives details about her upbringing only hesitantly. She was born in Hamburg, Germany, and moved to Namibia with her mother at the age of two after her father, a professor of law, died. Vrba’s mother remarried, to a sheep farmer who, Vrba says, gave her no encouragement whatsoever to pursue science or, for that matter, any career.
“He thought it was useless to educate women,” she says. “But I wanted to think out problems. I’ve always been an academic by nature. I wanted an education to liberate me.”
Vrba inquired at a number of girls’ high schools and, using her inheritance to pay the tuition, enrolled in the one that had a picture of students working in a laboratory on the cover of its brochure. “When I got there I discovered that picture was a total farce,” she says, still angry. “There was some math taught but no science. There were courses in makeup and posture, and I walked around with a book on my head.”
She managed to make it through high school playing field hockey and tennis and reading Ovid and Tolstoy on the sly. She did well in school, but on entering the University of Cape Town the sketchiness of her academic preparation became painfully obvious.
“There were mostly men in my classes, and they had all had math and science in school,” she says. “But I hadn’t, and I was at a terrible disadvantage. I knew at once that I would have to work awfully hard to make it up.”
Vrba did work hard. She majored in zoology and statistics, graduated with honors, and decided, given Cape Town’s proximity to the sea and her love of it, to become a marine biologist. But that decision was short-lived.
“By then my second father was bankrupt and I had to support myself,” she says. There were no grants to be had in Cape Town. “I went where the money was. I went inland, to Pretoria.” That was in 1967, the same year Vrba married her husband, George. She got a job teaching high school in Pretoria, while George built up his civil-engineering business. The two of them roughed it in a one-room studio furnished with a single chair. “We fought over that chair,” Vrba recalls.
One day in 1968, looking for a more challenging job, Vrba went to the Transvaal Museum and met its director, paleontologist C.K. “Bob” Brain, who had devoted his career to the excavation and analysis of South African cave fossils. Those fossils are what first got Vrba thinking about the relationship of one species to another.
“Bob had this great big pile of rocks containing antelope fossils that needed cleaning and sorting,” she says. “They had been found in a cave, all piled together in one solid rock deposit. He pointed to the pile and said, ‘We can’t pay you, but you’re welcome to it.’ So I decided to take the job, clean the bones, and have a good look at them. I gave myself six months.”
Removing the rock surrounding the delicate fossils was a tedious business. “At first I thought, ‘What the hell am I doing this for?'” Vrba says. “But then I learned that sometimes you can approach very interesting, very profound questions by looking at very humble materials.”
Vrba became particularly intrigued when she found that, unlike most animals, antelopes leave a telltale sign of their species affiliation in the fossil record. There is, in fact, much dispute about what constitutes a species. But scientists have come to a rough agreement that members of the same species reproduce with each other and share a mate-recognition system–that is, some feature or group of features that allows them to distinguish members of their own kind. Unfortunately, most mate-recognition systems involve features like a bird’s plumage or a monkey’s fur color, which do not fossilize. So generally it is very difficult to tell one species from another by their fossil remains. But the mate-recognition system of the antelope is the horn, and antelope horn cores, which are bony, fossilize nicely. Since no two species of antelope share the same horn configuration, sorting out antelope species is a relatively simple matter.
Through her sorting of antelope species, Vrba hit upon her first big idea, a revision of the notion of evolutionary “success.” At the time, the standard assumption among evolutionary biologists was that the most successful animals were those that managed to split into many viable species. And when the scientists looked at a group of related species–say the wildebeests, a type of African antelope–and noticed that those species all shared some unusual feature, like an oddly shaped mouth or head, they concluded that the repeated appearance of that feature in different species meant the feature was instrumental in the animal’s success. Otherwise, the scientists asked, why would the feature appear so often? It all seemed logical enough.
But Vrba noticed there were other differences between “successful” animals such as wildebeests, which had split into some 40 different species during the previous 6 million years, and “unsuccessful” related animals, such as the impala, which stayed relatively unchanged through time. First of all, wildebeest species didn’t last very long. They died out within a million years, while the one or at most two species in the impala line lived for more than 4 million years. In the second place, today there are a lot more individual impalas than there are wildebeests. Vrba conducted a survey with colleague Michael Greenacre and found that fully 72 percent of all antelopes in the Kruger National Park game preserve in South Africa were impalas, more than all the wildebeests and other antelopes combined. Vrba began to wonder just how “successful” the wildebeest, despite its large number of species, could really be.
Moreover, she noted, the two animals differ in how they go about making their living. Impalas thrive on almost any kind of vegetation and can acclimate to a number of different habitats, ranging from savanna to woodland. They migrate very little, preferring instead to tough it out under a wide range of conditions from southern to eastern Africa. Wildebeests, by contrast, are specialists: they prefer grazing in dry, wide-open spaces and are willing to migrate long distances in search of a cozy niche.
The result of their eating habits is that specialists are more sensitive to environmental change and are more prone to evolutionary pressures than are generalists, which are better equipped for survival during evolution’s long haul. “Six million years ago natural selection made grazing wildebeests,” Vrba says. “Let’s say it gave them a certain-shaped nose and mouth. But frequently Africa gets covered with bush and is thus cut into tiny islands of grazing territory. So wildebeest populations are cut off from their parent populations. As a consequence of that strange mouth the creature can’t go across all Africa. It will speciate like mad but not because that mouth gave it some advantage.” Rather, it will produce more species because isolated populations of wildebeests will get trapped in those tiny islands for which they are adapted. So that mouth was one reason behind the many wildebeest species, but it was a handicap, not an advantage. The large number of wildebeest species was an unintended effect of that disadvantageous mouth, and Vrba called her idea the “effects hypothesis.”
This simple but elegant idea hurled Vrba out of obscurity and into the thick of international attention. Requests for her 24-page paper, published in 1980, poured in by the hundreds from all over the world, and Vrba was invited to speak at, among many other places, Harvard, Oxford, and Cambridge universities. By this time she had risen from volunteer fossil washer to deputy director of the Transvaal Museum. And it was in that job that she began to realize that her studies of the interaction between antelopes, evolution, and the environment might have implications for many other species–including the hominids, the forerunners of modern human beings.
One of her projects at the museum had been to begin renewed excavation of the famous Kromdraai site, a cave on the south side of the Bloubank River where in 1938 paleontologist Robert Broom had discovered the first evidence of robust australopithecines (early hominids). Vrba’s interest in hominids was piqued by this experience, and shortly thereafter Brain put her in charge of the museum’s world-famous collection of hominid fossils.
“The fossils were kept in a very unattractive room,” Vrba recalls. “The walls were rough brick, and there was little light. I figured the best way to improve it was to create a kind of crazy beatnik quality, so I painted it blood red. I put in a soft carpet and a lamp and a comfortable chair and beautiful mahogany cabinets. It was a lovely room, and completely quiet, a place to be alone with ancient bones.”
As keeper of what became known as the “red cave,” and at various excavation sites, Vrba came in contact with a steady stream of anthropologists and paleontologists from all over the world, people like Mary Leakey, Desmond Clark, and Donald Johanson. And she made friends with all of them.
“Part of my job was to manage hominid studies. To do this I had to understand what the problems were, which was difficult because paleoanthropologists fight so much,” she says. “In fact, it seemed to me that there were more paleoanthropologists than hominid bones. Yet there were many more antelope fossils than hominid fossils, so I though that by looking at antelopes maybe we could learn more about hominids.”
The key issues facing paleoanthropologists were: What initially separated the apes from the first hominids, and when did this happen? What were the relationships between the early, apelike australopithecines, of which there were three or four species? Why did one of them make the leap to a bigger-brained tool-using creature called Homo habilis? And why did that individual take a further stride down the road to humanity and become Homo erectus? What Vrba noticed was that some of these crucial events in the development of humans coincided with dramatic episodes of evolutionary change in her antelopes–new species being formed, old ones dying out–at about 5 million and again at about 2.5 million years ago, and she started looking for a common cause behind the changes in both four-footed and two-footed lineages. The answer she came up with was the climate.
“If I had seen this in just one little antelope, I wouldn’t have come to any conclusions from it,” she says. “But I had a whole bunch of material, and the evidence was clear. And it told me something about hominids: the antelopes associated with the early australopithecines were more bush adapted, and the antelopes associated with the last australopithecines–and Homo–were more arid adapted.” Vrba speculated that hominids simply “rode along” on the waves of general evolutionary activity seen in antelopes and other mammals.
She and others have squared this theory with what is known about ancient climate. “We paleontologists saw evidence long before the climatologists found it of massive climatic change 2.5 million years ago by looking in the fossil record,” she says proudly. These climatic changes have since been confirmed in studies of oxygen molecules in foraminifera, tiny animals whose shells are preserved in layers of the ocean floor. There are two forms of oxygen in the ocean, oxygen 16 and the heavier oxygen 18. Oxygen 16 tends to evaporate more readily into the atmosphere, to be returned later in rain. But during cold periods, more oxygen 16 falls on growing polar ice caps and gets locked up there, so the foraminifera contain a higher-than-normal concentration of the heavier oxygen isotope.
The story told by the isotopes and the fossils is this: During much of the Miocene Epoch, which began about 23 million years ago, dense forest carpeted virtually all of Africa. The environment was fairly stable and predictable, and dozens of species of apelike creatures thrived under the tropical canopy, feasting on fruit and other vegetation. When the Miocene came to its end 5 million years ago, a severe cold spell dried the climate and broke up the forest into patches of woodland and grassland, forcing many of the forest-dwelling apelike species to follow Siva and die out, some to go the way of Vishnu and move to an oasis of forest resembling their native habitat, where they began to evolve into modern apes and chimpanzees, and at least one, taking the path of Brahma, to come down from the trees and split off into a new species. This species developed the ability to walk on two legs, making it, in the minds of some paleontologists, a more efficient food gatherer than its predecessors, able to stride through the grass and open woodland in search of ever scarcer food supplies. This was the gracile australopithecine called Australopithecus afarensis, the first hominid.
The climate then went through a warm period that lasted 2.5 million years, until a second, even more severe cooling set in. Polar ice caps grew, and pollen samples from Ethiopia show a vegetation shift from woody plants to grasses and shrubs. Vrba’s own antelope fossils show a profusion of new hartebeest, wildebeest, springbok, and gazelle species, still around today, that are best adapted to drier savanna conditions. Along the hominid line, gracile australopithecines gave way to a series of other, larger hominids, one of which turned into a larger-brained creature, the first member of the genus Homo. It is right at this time, 2.5 million years ago, that paleontologists find evidence for the first stone tools–none are known from any earlier time.
VRBA IS MORE TENTATIVE IN HER SUGGEStion that a third cooling period, at 900,000 years ago, killed off the other australopithecines, but isotopes from the ocean bottom point toward such a period, as does a massive die-off and creation episode among European mammals at this time. This is also about the time when Homo erectus, a humanlike creature with the biggest brain and most advanced tools yet, made its appearance outside Africa. It is the first human relative found in Asia, India, Australia, and southern Europe. If Vrba is right, climate is what brought it to all those places.
Desire for a change in academic climate took Vrba and her husband and daughter away from Pretoria in 1986, when the paleontologist joined the faculty at Yale. Her laboratory (which she also had painted red “to make it look less like a hospital room”) is not yet complete, and she is having trouble getting used to the American way of doing science, which sometimes entails as much time writing grant proposals as it does doing research.
“Just think how much more American scientists could accomplish if they didn’t have to waste so much time asking for money,” she says, bustling around her office, collecting papers for a seminar she has to dash off to. “It’s really something of a disgrace, don’t you think?”
In addition to teaching, lecturing, and doing her own research, Vrba is working on a book tentatively entitled “Habitat Theory,” a sort of unifying theory of large-scale and small-scale evolution. “I’m interested in pushing out the frontiers of science, not sailing my boat through tranquil seas,” she says. “If you’re worried about storms, you really shouldn’t be in the boat in the first place. Frankly, I find storms quite thrilling.” And with that, Vrba bolts for the door.
“You know, I really need to get a good deal of work done tonight,” she says, shoving her hand out for a quick shake. “I have so much to do, and there’s so very little time.”
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