by Elizabeth S. Vrba
(Natural History, 5/93, pages 47 - 51)
Earth's crust and atmosphere change constantly, and wedged in between them like a sandwich filling, life evolves as well. To what extent are extinction and speciation - the evolution of new species out of ancestral populations - tied to geophysical changes? Did humans originate in response to some identifiable cosmic or climatic event?
Charles Darwin, in an 1838 notebook, noted that "man is a species produced like any other - lawfully." To learn more about our own origins, I have looked for evolutionary patterns in other organisms that lived at the dawn of humanity. Antelopes are good animals to examine for clues because they are reliable indicators of past environments and shared the African savanna with our early ancestors, the australopithecines, or man-apes. Conveniently, antelopes make up 60 to 80 percent of all large mammal fossils found in the same strata as hominids, the family of primates that includes australopithecines and humans.
About twenty years ago, as a South African paleontologist studying the Transvaal's limestone caves, I first noticed that somewhere in the fossil deposits between 2.5 million and 2 million years ago, antelopes and other animals changed strikingly. Anatomical features such as teeth indicated that antelopes living before 2.5 million years ago had occupied moist woodlands. Shortly afterward, however, these forest antelopes disappeared and were replaced by many new species that graze only in dry, open savannas.
But because the Transvaal cave strata were jumbles of washed-in bones, the sequence was not reliable and had to be dated by being compared with deposits elsewhere. Some years later, in East Africa, other researchers found similar fossils in deposits that could be dated by radiometric techniques. Several independent studies have since confirmed that the same kind of faunal change that took place in South Africa also occurred in Ethiopia, Kenya, and Tanzania close to 2.5 million years ago.
Such a dramatic change in large fauna, and by inference in vegetation, is one line of evidence that southern Africa became significantly drier about 2.5 million years ago. I wondered if this was a purely local phenomenon and whether the change was slow or rapid. A 1980s study of deep sea shores near Greenland by Nick Shackleton, of Cambridge University, contributed some answers. Shackleton discovered abundant evidence for an extensive arctic glaciation dated close to 2.5 million years ago.
During that period, global temperatures may have plummeted by as much as 10 to 20 degrees F, and the world became colder than at any time in the past 65 million years. The continents became much more arid, and the steppes advanced toward the equator. The earliest fossils of Oryx, the most arid-adapted antelopes of all, appeared along with other grazers of open savannas, such as the giant buffaloes Pelorovis and giant hartebeests Megalotragus. These species soon extended their ranges from southern and eastern Africa toward the equator, where we find fossil testimony of their presence. Steppe and grassland horses, Equus, from Europe moved east into India and Pakistan. They also migrated south into Africa, together with Asian forms like the Indian black buck, Antilope cervicapra, but while the horses evolved into several varieties of zebras, the Asian antelopes did not last long in Africa.
A similar, earlier cooling also seemed to affect all of sub-Saharan Africa about 5 million years ago, close to the Miocene-Pliocene boundary. Climatologists John Mercer and James Kennett and paleoanthropologist C. K. Brain have documented marked swings in the global climate at that time, with widespread continental cooling and the lowering and retreat of oceans. We also know that the Mediterranean dried up, leaving massive salt deposits, and that thousands of miles of savannas opened up at the expense of forests on several continents. This earlier cooling also produced waves of extinctions and radiations of species, giving rise to the seven major groups of antelopes that still dominate the African savannas today. Within these groups, however, many species went extinct at the later cooling 2.5 million years ago.
How did our primate ancestors fare during these climatic changes in South Africa? About 5 million years ago, the first proliferation of antelope species coincided with the occurrence of the first australopithecines in East Africa - perhaps their first appearance on the planet. Before that time, large primates were apish forest dwellers that shared their leafy refuges with ancestral antelopes. In between the 5-million-year-old deposits and those 2.5 million years old, the only known hominids are the lightly built australopithecines, five-foot-tall, bipedal primates with ape-sized brains and upright posture. After 2.5 million years ago, however, we see evidence of an explosive radiation among hominids. Apparently several different species appeared; paleoanthropologists are still arguing about how many. Clark Howell, of the University of California at Berkeley, for instance, thinks there were five or six. Among them were the large, vegetarian man-ape Paranthropus and various members of the human genus, Homo-who thrived with the further spread of dry grasslands and the continued shrinking of forest habitats. Other evidence of new kinds of hominids include the first-known robust australopithecine fossil, the Black Skull, found by Richard Leakey and Alan Walker near Kenya's Lake Turkana and the oldest-known fragment of a Homo species, from Lake Beringo, Kenya, dated by Andrew Hill's group at Yale to 2.5 million years ago.
The archeological record of artifacts offers additional corroboration of dramatic change. Stone tools from Ethiopia - the earliest yet discovered - date to between 2.4 and 2.6 million years ago. Using tools to obtain and prepare food was an important innovation for hominids and indicates increased brain size and function.
The British ecologist Evelyn Hutchinson once referred to the "evolutionary theater and the ecological play." Halfway through the hominid play that began 5 million years ago, something happened that pushed most of the old cast members (including the small-bodied australopithecines) off the stage and introduced new ones (such as Homo species) - a process I call turnover-pulse. I believe that when we better understand the events at the Miocene-Pliocene boundary, we will find that the appearance of the australopithecines was only one part of a global turnover of species, one of the recurrent pulses in the history of life. New species of primates, antelopes, birds, and carnivores may all have originated in response to several successive cooling plunges during the later Miocene. Many African lineages survived the climatic changes intact, but those that underwent extinctions and speciations appear to have done so in concert with others: plants, hominoids, antelopes, rodents, and such marine invertebrates as snails and foraminifera. All the evidence, as I see it, indicates that the lineage of upright primates known as australopithecines, the first hominids, was one of the founding groups of the great African savanna biota.
My hypothesis that the same climatic pulses dramatically stimulated both antelope and human evolution has increasingly gained support from specialists studying many disparate creatures. A decade ago, paleontologist Hank Wesselman discovered a rapid pulse of extinctions and speciations in African rodents at the 2.5-million-year mark. In addition, Raymonde Bonefille, of Marseille, has found that pollens in Ethiopia show that there was a simultaneous "turnover" of many plant species. In each case, the pattern is wholly consistent with global cooling, increased glaciation, a shrinking of wet, forested country, and the rise of vast, dry grasslands. Animal species everywhere either moved, bowed out, or evolved into new groups, as Italian paleontologist Augusto Azzaroli has demonstrated for many lineages of Eurasian mammals, including deer, cattle, various carnivores, and rodents.
According to the classic Darwinian view, the "engine" that drives the evolution of species is competition between organisms; extinctions occur when species out-compete others for the same resources. If one were to represent each species as a star that lights up in the sky, the Darwinian model would show some stars appearing and others going dark in a fairly random pattern. But the fossil record appears to tell a different tale. Species seem to arise and vanish in pulses of varying intensities, with many appearing and others disappearing at the same time. Some of these pulses would appear to be dim, while others would light up the heavens.
As a rain forest or grassland shifts or disappears, diminishing animal populations can remain unchanged within the shrinking habitats in which they evolved, perhaps expanding again when and if a favorable climate returns. On the other hand, if conditions are right, evolution within the threatened population may produce a new species. According to the scenario first proposed by Harvard biologist Ernst Mayr in the 1940s, an animal species may occupy an extensive geographic range until major changes in rainfall, temperature, or geographical boundaries break the population into smaller, isolated groups. These "island populations" cease to trade genes, and eventually some may diverge sufficiently to form the cores of new species. Without the impetus of environmental pulses, most species and ecosystems seem to remain in equilibrium most of the time and resist change until they are pushed.
I see the biosphere as a living layer, stretched thinly over the globe, responding rhythmically to the beat of the earth. During a climatic shift, species may either find some safety as generalists, able to tolerate a wide range of foods and conditions, or they may cling unchanged to the shrinking habitats in which they evolved. In the long run, however, both of these strategies may lead to dead ends. If a lineage's evolutionary success is measured by the production of many diverse species, then the successful gamblers on speciation are the big winners. Our ancestral species, the early australopithecines, entered a narrow genetic corridor about 2.5 million years ago. They disappeared, but as the progenitors of novelty - ultimately including the most ubiquitous large mammal species on earth - they hit the jackpot.