Something like the punctuated equilibrium model, shaped by climate changes of the Pleistocene Ice Age, may apply to human evolution. Unfortunately, the fossil record of hominids is very far from complete, which makes piecing together the details quite difficult. In contrast to ocean-dwelling organisms, those that live on land are rarely well preserved. On the seafloor, the steady rain of sediment buries and protects fossils; on land, bones get scattered by predators or scavengers, carried away by floods, or even blown about in windstorms. In spite of intensive collection efforts, there may be entire hominid species not yet represented in our fossil collections. Most of the specimens that exist come from special environments—caves, lakeshore settings where fluctuating water levels periodically buried skeletons in a protective blanket of mud, and regions where frequent volcanic eruptions laid down ash layers that buried and preserved hominid remains. And yet, in spite of the fragmentary evidence, the history of hominid evolution looks very much as though it follows the isolation-then-sudden-change model. The australopithecines, who, as we have seen, seemed to be comfortably successful in Africa, disappeared quite abruptly, with only a short overlap with our own genus, Homo. Another Australopithecus-like hominid, called Paranthropus, coexisted for more than a million years, then became extinct. Several species of Homo that we know about appeared and disappeared over the past 2.5 million years before Homo sapiens finally arrived on the scene, probably between 100,000 and 200,000 years ago.
For those attempting to establish a link between human evolution and the Pleistocene Ice Age, it is the overall climate and its effect on the environment, not just temperature, that is the key. The oxygen isotope data from deep-sea cores, discussed in the previous chapter, show that seawater temperatures, and by implication surface temperatures in general, have been on a downward trend since about sixty million years ago. Especially rapid decreases occur around thirty-five million years ago, when glaciers started to accumulate on the Antarctic continent, and also beginning about three or four million years ago. By then, ice caps were forming in northern Europe, Greenland, and North America. By two and a half million years ago, the Pleistocene Ice Age was in full swing in the Northern Hemisphere, with ice sheets advancing and retreating over the continents at regular intervals. Both ice and sediment cores exhibit high dust contents during the glacial periods of the Pleistocene Ice Age, indicating that the cold periods were not only windy but also dry. As global cooling proceeded, the main result in tropical Africa was that forests, the natural habitat of the hominids, shrank, primarily due to aridity, not the cooler temperatures. The forests probably grew back a bit during the warmer and wetter interglacial periods, but the overall trend was toward much less tree cover; in place of the forests, grasslands expanded. It is this change in vegetation, not the actual temperature variations, that seems to be the key to human evolution.
Before the Pleistocene Ice Age began, the widespread grasslands of central and eastern Africa did not exist. There were no great herds of antelope and other ungulates that migrated with the seasons, following the rains to greener pastures. There were similar animals—fossils show, for example, that there were precursors of the modern antelope. However, they were not adapted to the grass of the savanna that their descendents feed on today; instead, they ate the tender leaves of shrubs and small trees in open woodlands. It is quite clear that the environmental transformation engendered by the ice age forced evolutionary change among the antelope. The question is, How did it affect the hominids?
We saw from the previous chapter that temperature changes during the ice age are sometimes very abrupt, especially the so-called rapid terminations that characterize the switch from cold to warmer temperatures as recorded in the ice-core data. As we have also seen, the glacial-interglacial cycles of temperature change occur regularly on the 100,000-year timescale of the eccentricity of the Earth’s orbit. But before about 800,000 years ago, for reasons not fully understood, the cycles of the ice age were shorter—closer to 40,000 years—and the glacial intervals may not have been quite as severe. (Note that 40,000 years is the approximate cycle time for the tilt of the Earth’s rotation axis, the orbital parameter that Milankovitch thought would be especially important. In spite of the different cycle length, it seems that there was still an astronomical control of glaciation and deglaciation.) Earth was thus seesawing between cold and warm, dry and less dry, more than twice as fast during this earlier phase of the ice age. Such rapid climate switches may have engendered what William Calvin refers to as “boom and bust” cycles among early hominid populations (and later ones, too). In this scenario, the cool and dry periods were stressful. Only those populations with specific adaptive characteristics were successful in the changed environment, and in the boom times, when life was much easier, these surviving populations flourished and expanded rapidly. They also produced offspring with a whole new range of biological variations. Because the living was (relatively) easy in the boom times, these variations could persist even if they were neutral or even slightly negative as far as survival was concerned. Occasionally, these characteristics might be quite unexpectedly helpful when the going suddenly got tough as the next cold and dry glacial cycle began.
Our australopithecine ancestors were predominantly woodland dwellers. Probably they frequented the open woodlands on the margins of denser forests, like the predecessors of the present-day antelope mentioned above. Lacking natural protection and unable to travel quickly through the forest canopy like their close relatives the chimpanzees, they were relatively easy targets for the large carnivorous predators that abounded at that time. They did climb trees for safety and probably slept in them at night to avoid danger on the ground, but the shrinking and fragmenting woodlands of the building ice age meant that their population began to shrink and fragment too. It was a classic scenario for the punctuated equilibrium model of evolution. Steven Stanley refers to the events that led to the human lineage as the “terrestrial imperative.” Our ancestors were literally forced to come down out of the trees to survive. They did not suddenly become bipedal because of the climate; they already were. But the vagaries of the ice age climate eventually forced them to abandon the arboreal part of their existence and become full-time ground-dwellers.
Because the hominid fossil record is quite sparse, it is not currently possible—and may never be possible—to trace out exactly what happened among the hominids between two and three million years ago. But we do know that Australopithecus had a small brain, and the very first species of Homo that we know about had a significantly larger one. All who deal with human evolution agree that this has to do with the slowdown of the rate at which Homo matured, a process often referred to as juvenilization. Juvenilization is by no means restricted to humans. Simply put, it is a kind of backing down of development, so that adults end up having features that, far back in the history of the species—or in a predecessor species—were juvenile characteristics. One result in humans is that newborn infants are helpless; another is that, unlike those of other primates, our brains continue to grow at a rapid rate after birth, approximately doubling in size during the first year. Steven Stanley has argued that juvenilization, and especially the resulting bigger brain of Homo compared to Australopithecus, was a direct result of the cool, dry ice age climate of Africa that forced our ancestors to abandon tree-climbing and become firmly terrestrial. He suggests that australopithecine infants must have been fully capable of clinging onto their mothers as they clambered up trees for protection; in contrast, ground-dwelling Homo youngsters were much less tactile, and their mothers needed to have hands and arms free to carry them. The species would have had the luxury of slow development, no longer requiring the infant survival skills of its predecessors. And when they weren’t occupied with babies, hands freed full-time from their tree-climbing duties could also be employed for communication, tool-making, and throwing—all activities that would have required new developments in the brain, and an increase in its size.
Thus there are
plausible arguments that link the beginnings of our genus with the onset of the Pleistocene Ice Age. But the story does not end with the “terrestrial imperative” and the advent of Homo. The entire history of humankind has been played out against the backdrop of the seesaw advances and retreats of massive continental ice sheets, and the worldwide climate effects that accompanied them. The same kinds of evolutionary principles that may have led to Homo—isolation of populations due to environmental changes, boom-and-bust cycles, and adaptation to new environments—almost certainly continued to play a role in our evolution, virtually up to the present time. Again the sparseness of the fossil record limits our ability to know all of the details of the climate-evolution links. But again there are many coincidences between the available fossil evidence and the details of the global climate record deduced from ice and sediment cores. In aggregate, they suggest a significant connection between ice age climate change and our evolution as a species.
Most of our information about climate during the early part of Homo’s existence comes from deep-sea sediment cores; the ice cores, which have the advantage that they provide climate details at higher resolution, are so far restricted to about the past 500,000 years. But one of the most important results of the ice-core research is the discovery that the well-documented 100,000-year cycles of the ice age themselves comprise a series of regular climate variations of shorter durations: there are cycles within cycles within cycles. Science reporters writing about this research have described abrupt temperature changes of these cycles as “flip-flops” and have characterized the ice age climate as “chattering” or “jittering.” In spite of those adjectives, however, the changes are not entirely chaotic. There seems to be a rough regularity on a variety of timescales that extends all the way down to cycles of only a few years.
The realization that climate swings in the past have often been very rapid has spawned a whole new subdiscipline in climate research: abrupt climate change. It has also generated a lot of concern, because of the implications of unexpected climate shifts for society—in 2002, the National Research Council of the National Academy of Sciences in the United States issued a detailed report on the subject that was subtitled Inevitable Surprises. One of the most discussed of those surprises is a relatively cold period called the “Younger Dryas.” Geologists and climatologists had known about this interval for some time, but the speed with which it began and ended were only realized when high-resolution data from ice cores became available. The Younger Dryas started quite suddenly 12,800 years ago, lasted for about 1,200 years, and then ended quite abruptly (there is an “Older Dryas” too, a much shorter cold period that occurred about one thousand years earlier). The name given to this cold interval comes from a species of Dryas, a small flowering plant that thrives today in the arctic tundra and alpine regions (figure 22); its leaves and pollen, along with those of other arctic plants, suddenly appear abundantly in European sediments dated near 12,800 years. Work in other parts of the world shows convincingly that the Younger Dryas event was not confined to Europe, however—it was a global phenomenon. The Younger Dryas cold interval was a sudden reversal of the generally rising temperatures that followed the peak of the most recent glacial episode some 20,000 years ago. The Greenland ice cores indicate that temperatures dropped abruptly, within a few decades, at the beginning of the Younger Dryas, then rose even more quickly—apparently in less than a decade—at its end (figure 23). Depending on location, temperatures seem to have been anywhere from 2–3° to 7–8°C cooler during the Younger Dryas than they were immediately before it began. The ice cores also contain much more dust during this interval, suggesting widespread arid and windy conditions and bolstering evidence from other localities that this was not just a local cooling in Greenland.
Figure 22.Dryas flowers brighten the summer tundra in the Canadian arctic. About 12,800 years ago, Dryas suddenly appeared in Europe after a long absence, marking the sudden drop in temperatures that led to the cold interval called the Younger Dryas.
The Younger Dryas lasted for thirty or forty generations of our Homo sapiens ancestors living twelve thousand years ago. Both its inception and its end occurred within a single generation. Because of their rapidity, both of these changes would have caused sudden disruptions in the ecosystems upon which our ancestors relied for food and shelter, and those generations having to deal with the abrupt changes would have been severely stressed—especially those living at high latitudes. They would have had to deal not only with a large shift in average temperature, but also with changes in the pattern of precipitation, and the availability of familiar plant and animal species. It is plausible to infer that events like the Younger Dryas caused population fragmentation and the kind of boom-and-bust scenarios that William Calvin and others believe have affected human evolution.
The Younger Dryas occurred relatively recently in the history of the genus Homo, and there is no obvious aspect of human evolution with which it can be associated. It is much more likely that prolonged series of such rapid climate shifts were influential in our evolution. We know from ocean sediments that 40,000-year climate cycles were affecting the Earth for roughly the first two million years of Homo’s existence, and by analogy with the more recent past documented in the ice cores, there were probably also shorter, rapid cycles superimposed on those longer periods. For the most part, these shorter cycles do not show up in the deep-sea cores. Sediments typically accumulate too slowly and are churned up sufficiently by worms and other bottom-dwelling organisms that even thousand-year events like the Younger Dryas are usually not discernible. Records of much shorter events, such as the decade-long temperature changes that can be resolved in the annual layers of an ice core, are completely indecipherable. Nevertheless, there are a few unique records, such as ancient lake sediments, that suggest abrupt climate changes like that of the Younger Dryas have occurred throughout the history of our genus. Rapid climate change and its “inevitable surprises,” including population fragmentation, forced migration, and boom-and-bust cycles, have probably been a fact of life since humans appeared on planet Earth.
Figure 23.Temperature in central Greenland, deduced from isotopes in Greenland ice cores, dropped suddenly 12,800 years ago at the beginning of the Younger Dryas interval, and stayed low for about 1,200 years. At the end of this period, the temperature increased by about 8°C in a decade or less. The vertical scale in this figure, and in figures 24 and 25, is highly exaggerated. All three graphs are based on data from a paper by P.M. Grootes and M. Stuiver, which appeared in the Journal of Geophysical Research 102 (1997): 26455.
At about the same time that species of Homo appear in the fossil record, so do crude stone tools. Toolmaking is one of the traits that distinguish us from our australopithecine ancestors, and it requires a brain that can make connections among events and is capable at some level of advance planning—the tools were made for a purpose. The earliest tools date to about 2.4 million years ago and are thought to have been made by an early human species called Homo rudolfensis, which already had a brain almost twice as large as that of Australopithecus and (based on studies of its teeth) a much more varied diet. The tools were probably used for hunting and/or butchering animals. How and why did this species learn to make and use tools? Most of the ideas about human evolution from the time of Homo rudolfensis up almost to the present are in the realm of working hypotheses—they are based on available fossils and climate records, and they are designed to be tested as new evidence is discovered, and, when necessary, discarded and supplanted by a new hypothesis. One of the current working hypotheses is that as the ice age climate of Africa became progressively cooler and drier, but also cycled rapidly between wetter and more arid intervals, early humans learned to be effective hunters. Food of the shrinking forests became less bountiful; they were forced to turn to creatures of the grasslands to survive. They became meat eaters, and in the process they had to acquire a whole range of new skills that would have required new mental capabilities. Planning,
cooperation, and throwing skills, the latter requiring a high degree of brain-body coordination, would have been essential. Think of a skilled pitcher throwing a fastball precisely to a catcher’s glove, or of Wayne Gretsky blasting an accurate slap-shot past a goalie. None of our ape relatives is capable of such exquisite coordination; it requires a larger, human, brain.
One rather enigmatic stone tool that has been directly connected (in another working hypothesis) to ice age climate is an object called the Acheulean hand axe. It first appeared about 1.8 million years ago and was probably designed and made by Homo erectus, the dominant human species at that time. The tool is roughly oval, usually with one end more pointed than the other, and—this is the enigmatic part—it is carefully sharpened around its entire perimeter. Although called a hand axe, it would actually lacerate any hand that used it to carve, slice, or pound. How might these peculiar implements be linked to ice age climate? The progressively more arid climate in Africa, especially during glacial periods, would have made lakes and watering holes magnets for large numbers of game animals—a concentrated food source for early humans. But it would not have been easy to approach these herds or single out individuals for attack. In East Africa, thousands of Acheulean hand axes have been found littering the ground around the shores of lakes and in ancient, now-dry lake beds. Could they have been hunting weapons?
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