The following is taken from Rick Potts' Humanity's Descent:

Evidence from the polar regions indicates that something very important happened to the global environment of this period. Antarctic ice cores reveal a major expansion in the southern ice cap around 2.4 to 2.5 million years ago. The first major ice rafts are also recorded in the Northern Hemisphere at this time. The foram thermometer, mirroring the oxygen isotope signal of global ice, shows a critical drop at about 2.5 million years, a brief time after a reversal in Earth's magnetic field, known as the Gauss/Matuyama boundary. The abrupt shift in the ice curve resulted from an increase in the heavier form of oxygen, 18o, as massive quantities of the lighter 16o were stolen from the oceans. This occurred at the same time that vast ice sheets began to grow. On a global scale, Earth experienced a stepwise cooling, a dramatic prelude to a future frigid age.

This distant warning reverberated over the continents. About 2.4 million years ago, an extensive series of windblown deposits, called loess, began to form over much of China. Loess deposits meant that winds were able to carry particles of sand and silt over open expanses of low-lying vegetation. Great depths of windswept dust appeared for the first time in central Europe and western Asia. In the Andean highlands of South America, studies by Henry Hooghiemstra of the University of Amsterdam record a formidable shift in fossilized pollen, signaling a grassland expansion at the expense of forest. The interval 2.4 to 2.5 million years ago is the marker for all of these events. The tandem of cooling and drying was felt far and wide.

In a very real sense, this worldwide episode was a single distillation of the kind of environmental change commonly thought to have spurred critical developments in human evolution. The shift to open savanna seems to have occurred in one major upheaval, so the savanna hypothesis may be salvaged after all. Perhaps the expansion of savanna around 2.5 million years ago was the crucial nudge that set the human lineage on its course. An influential hypothesis advanced by Elisabeth Vrba of Yale University takes its cue from this important event.

Vrba's turnover-pulse hypothesis has stimulated a tremendous amount of research, spanning the disciplines of geology, paleontology, climatology, and evolutionary biology. A turnover pulse refers to any synchronized set of species extinctions and origins in many groups of animals over a limited period of time. Such bursts of biological change, according to this hypothesis, are initiated by shifts in global climate. Climatic events not only alter the conditions of life, they cause habitats to become more fragmented. As a consequence, animal populations also become more divided, and new species emerge as separated populations respond locally to their changing habitats. Vrba's hypothesis is that the origins of new species and new adaptive capabilities are ultimately linked to a major directional shift in global climate. ;

Vrba states that the change in the environmental settings of early hominids around 2.5 million years ago was exactly the kind expected by her hypothesis. Worldwide cooling and drying were sharply mirrored in African settings. Arid and open habitats infected eastern and southern Africa. Woodlands and forests ebbed. The Cenozoic decline was writ large in one wrenching leap, and as the wave of grass moved over the African landscape, natural selection favored creatures who could thrive in it.

Africa was an epicenter of a changing globe, and a significant measure of it was the fossil antelopes. New species of the wildebeests and gazelles, the dominant grazers of the African plains, made their appearance in the fossil record between 2.5 and 2 million years ago. According to Vrba, the antelopes of eastern and southern Africa underwent a major episode of extinction and new species origins at this time.

Change was not confined to the antelopes. In the Omo basin of Ethiopia, about 2.4 million years ago, rodents from dry grasslands replaced others typical of wetter, forested environments. As already noted, several groups of large mammals evolved higher-crowned teeth and longer limbs, indicative of an open habitat in the late Pliocene. These changes are also said to correspond to an extension of grassland around 2.4 million years ago. Shifts in fossil pollen in the Omo-Turkana region and in the highlands of Ethiopia also hover around this date. The challenge of a widening savanna left its mark.

Vrba suggests that our own intimate phylogeny succumbed to this same directional change. As wooded habitats fragmented and dissolved into open vegetation, bipeds spawned new and independent examples of the hominid condition. Behaviors crucial to the human enterprise evolved with the spread of a cooler, drier habitat. Our own genus, Homo, was a founding member of the new savanna biota. Stone toolmaking and the dental machinery of the robust australopiths evolved as adaptations oriented to the resources of the drying, opening landscape, evolutionary events that centered around the global climatic change 2.4 to 2.5 million years ago.

The turnover-pulse idea proposes, first, that global temperature fell precipitously. Second, that this event caused the spread of arid grasslands within the savanna patchwork of Africa. And, third, that the growth of these grasslands prompted synchronized change in hominids and other animal populations. The regional division of populations led new species to arise, while the force of natural selection caused new adaptations to evolve. The hominids were converted to live in open terrain.

Vrba's proposal leans heavily on the savanna hypothesis: Directional change to an open habitat caused profound alterations in human origin. The idea of a turnover pulse is appealing for other reasons. It appears to account for the era's dynamism. Evolutionary events between 3 and 2 million years ago, according to Vrba, were focused on a single climatic alteration, and included all the events described earlier in this chapter the formation of new species, parallelisms across many lineages, and vital developments in the ecological history of our own lineage. The concept of a turnover pulse during the late Pliocene has, moreover received resounding support from climatologists, who have discovered ir local and global archives evidence of significant change about 2.5 million years ago, just after the Gauss/Matuyama magnetic boundary was imprinted in sediments around the world. The concentration of climatic and biotic change at this special point in time is the core of this hypothesis.

Vrba has done a remarkable job of scrutinizing her own hypothesis, but three critical questions test its relevance to human origin: What was the nature of this climatic episode 2.5 million years before the present? Were global change, local shifts in habitat, and developments in hominids precisely correlated? Does directional change provide a stronger explanation of hominid evolution during this period than environmental fluctuation? Let us put the turnover-pulse hypothesis and the thesis we have begun to develop here to these tests.

A curious pattern can be seen in our investigationwhenever Cenozoic environments became cooler or drier, they manifested greater instability at about the same time, or immediately before the main shift. This happened with the cooling event of 2.5 million years ago.

In a paper widely cited by advocates of the turnover-pulse idea, Nicholas Shackleton and his colleagues showed that large icebergs formed in the northern oceans for the first time about 2.4 million years ago. Because of recent redating of the Gauss/Matuyama magnetic reversal, the time of this change was probably closer to 2.5 million years ago. Before this date, the ice system of the planet was largely confined to Antarctica, and this single ice pole controlled the heat gradient between the equator and the poles. After 2.5 million years ago, however, the North Pole also developed large ice sheets and began to play a major role in governing Earth's climate.

Overshadowed by this dip in the oxygen isotope curve is a series of intense oscillations. According to Shackleton's paper, global climate varied a great deal before 2.5 million years ago, affecting the signal of ice fluctuation in the North Atlantic. Indeed, a wider range of fluctuation in the isotope curve began at least 300,000 years before the critical date. With the approach of the 2.5-million-year mark, the range of variation rose even more.

The curve below, from Shackleton and colleagues' original 1984 paper, shows that the dip at 2.4 to 2.5 million was not a permanent change, but part of a longer period of increased instability. On occasion, the isotope readings rebounded to levels that existed prior to the dip.

Isotope measures in other ocean cores portray a slightly different trend and pattern of oscillation. According to a Pacific Ocean core published in I98I, climatic variation began to widen as early as 3. 1 million years ago. The widened pattern of oscillation occurred up to 2.1 I million years ago, when even larger variations began to occur.

In 1984, geologists R. Stein and M. Sarnthein examined several other ocean cores drawn from the Atlantic Ocean, and inferred that deviations in the climatic curve increased in both frequency and intensity between 3.5 and I.9 million years ago. They concluded that much of this fluctuation occurred 500,000 years on either side of the 2.5-million-year markthat is, the entire span between 3 and 2 million years before the present.

The volume of terrestrial dust blown into the ocean is another indication of continental drying. Measuring sediments buried off the west coast of Africa, William Ruddiman and his colleagues found that the amount of terrestrial dust in the ocean increased around 2.5 million years ago, supporting the idea of increased African aridity at that time. However, many large-scale oscillations occurred in the quantity of dust that fell to the ocean floor. Ruddiman attributes the large, repeated shifts in the dust curve after 4 million years ago to "an increase in the amplitude of arid/humid cycles" over geologic time, with a major increase in fluctuation, including greater aridity, at about 2.5 million years ago.

Further clues come from a long terrestrial record often cited by the turnover-pulse proponents. Henry Hooghiemstra's record of fossil pollen in the high plains of Bogota, South America, displays a significant drying at the 2.5-million-year Rubicon. Around the same time, a c hange also occurred in the oscillation of climate and vegetation. The pollen record indicates many small-scale variations in temperature and flora after 3.5 million years ago. Around 2.5 million, these variations became less frequent but of far greater magnitude. The key event of 2.5 million years ago was actually part of a lengthy spell of deepening fluctuation in global climate.

Besides climatic factors, volcanic and tectonic forces controlled the habitats occupied by hominids. There is no evidence to suggest that these irregular, more catastrophic events were unusually potent at the 2.5-million-year mark. Rather, they exaggerated the degree of habitat change that hominids faced time and again.

In the Omo-Turkana region of East Africa, volcanic eruptions periodically sent enormous clouds of ash and debris over the landscape, 4.2 and 1.4 million years ago. At least nine such events blanketed an area of more than ten thousand square miles. In some cases, erupted ash was carried eight hundred miles away to the Gulf of Aden. Even the smaller eruptions repeatedly altered the vegetation and soil chemistry over a wide area.

The Omo-Turkana study shows that arid savanna increased over time, but the water budget of this vast region was controlled mainly by tectonic activity, changing the flow of the Omo River and causing lakes to appear and disappear over spans of 100,000 years. The forest and woodland area surrounding these water sources was susceptible to significant, recurrent alteration.

The environments of eastern and southern Africa became drier overall, but the key global event, shifts in local habitat, and changes in hominids were not precisely synchronized. A pollen study of the OmoTurkana region by Raymonde Bonnefille, for example, points to a shift to grassland habitats between 2.3 and 2.4 million years ago. In this same region, changes in antelopes, pigs, and other animals took place over a few hundred thousand years around the 2.3-million-year mark. Because of astronomical effects on Earth's climate, a difference of 200,000 years is a very important one, encompassing several reversions in the climatic trend.

The first appearance of robust australopiths, stone tools, and possibly the genus Homo precede the pollen shift in the Omo region. The oldest known find of a robust australopith is the Black Skull, Australopithecus aethiopicus, found in deposits several meters beneath a volcanic tuff dated 2.52 million years ago. Independent studies of the skull suggest that its peculiar combination of ancient and advanced traits must have branched from the hominid family tree independent of A. africanus, who was known in South Africa between 3 and 2.5 million years ago. Since aethiopicus has certain key similarities with A. afarensis, the former may have evolved from the latter. And since afarensis is unknown in the fossil record after about 2.8 million years ago, this date may provide a minimum age for the origin of aethiopicus The robust lineage of East Africa arose before the abrupt savanna expansion posited by the turn-over-pulse hypothesis.

The oldest stone tools known from the Omo-Turkana basin are dated about 2.4 million years. A few early archeological sites in this region and in Zaire and Malawi appear to be about 2.3 to 2.1 million years old. Yet the oldest definite stone implements, from Hadar, Ethiopia, are nearly 2.6 million years old. While these early implements correlate roughly with the proposed turnover pulse, they precede other signals of drying and grassland expansion in East Africa by at least 200,000 years.

New dates on a fossil fragment discovered in 1967 near Lake Baringo, Kenya, suggest that the oldest known member of the genus Homo may be 2.4 million years old. It has been argued that this appearance coincides approximately with the oldest stone tools and is related to the dramatic spread of open savanna. Other researchers have difficulty accepting the Baringo fragment's age and identity, and see definite signs of Homo no older than about 2 million years. But even if Homo and flaked tools first appeared around the 2.5-million-year date, they lie within the period of intensified oscillations that began prior to the abrupt cooling event. Was it habitat oscillation or a single megashift to cooler, drier habitat that figured more importantly in the turnover of species and adaptive innovation among the hominids?

Five distinct species of early human may have inhabited the OmoTurkana region between 2.6 and 1.8 million years ago. The turnover - pulse idea has remarkable appeal partly because it sees a dynamism in the hominids similar to that in other lineages of mammals. According to Vrba's hypothesis, the spread of savanna caused a phenomenon known as "environmental forcing"the division and extinction of lineages under pressures associated with a particular climatic trend.

It seems to me that vacillationreversals in the trendcreated a much more appropriate setting for this mutable era of human evolution. This, too, is a kind of "environmental forcing." But the critical factor was the increasing degree of habitat fluctuation, not a single directional change. Hominids and other organisms had to survive the spread of grassland beyond its previous limits; but our grasp of this vital period in human origin is not complete without also asking how these ancestors fared in subsequent moist and warm phases. The survival conditions of human forebears cannot be rendered by any one portrait, or any single type of biome. The change in climate at 2.5 million years ago may have been dramatic, but the longer interval of heightened instability between 2 and 3 million years ago also cast its influence on hominid evolution.

Robert Foley of Cambridge University has underlined the importance of habitat fluctuation in the turnover of late Pliocene species. In his scenario, cycles of change caused habitats to break up into smaller, more distant fragments. Populations became disjoined and isolated for varying lengths of time, adapting to different local conditions. Periodic dividing and coalescing of habitats seem more essential to a continental I radiation of species than any unidirectional habitat trend. Environmental variabilitythe intensity and rate of fluctuationmay well have held sway over the persistence, splitting, changing, and extinction of of lineages.

Two proponents of the savanna-pulse idea, George Denton and Michael Prentice, shed a different light on the timing of the climatic decline. According to their data, the main climatic event in the late Pliocene was 2.4 million years ago, during the buildup of ice at the North Pole. Again, this marker seems too late to have affected the earliest appearance of robust hominids and stone tools. Denton and Prentice suggest, however, that climatic deterioration began around 2.8 million years ago and culminated some 400,000 years later. Their broader "pulse" model is based on oxygen-isotope data and sea-level change. In recent papers, Vrba has embraced this revised time scale, but she still stresses the power of the linear trend, the rise of savanna, as the critical source of change in hominid evolution.

The environmental code of the era was a sequence of dots and dashes. Over this lengthy interval, numerous stops, starts, and reversals occurred in ice buildup, ocean depth, and planetary temperature. It was literally a tide with ebbs and flows. As water was taken up and released by polar ice, sea level fluctuated at least fifty and up to one hundred meters, constricting and reexposing huge areas of the continental shelf. The amount of moisture monsoon winds carried to the continental interior varied with distance from the ocean. Swings in sea level, which took place over tens of thousands of years, altered the atmospheric moisture blown into the continental basins occupied by hominids, affecting the rainfall and vegetation of these areas.

Prentice and Denton interpret the change in sea level between 2.8 and 2.4 million years ago "as primarily reflecting Antarctic Ice Sheet fluctuations with a minor but increasing component attributable to Arctic ice caps." It is reasonable, then, to suggest that fluctuations regulated the pattern of environmental change. Cooling and drying occurred as part of a widening spectrum of oscillation over at least several hundred thousand years.

How do toolmaking and other curious innovations in hominid behavior tie in with this picture of an unsteady environment? In trying to decide why early attempts at flaking and carrying stone persisted in the human repertoire, we are stuck with our old conflict. Were these behaviors adaptations to the open savanna? Or did they mainly provide certain hominids with a way of dealing with environmental variability? Let us consider what these novel behaviors were good for.

It has occurred to many of us who are curious about the oldest stone tools that the hominids who processed their food partially outside of their bodies were the most liberated of all bipeds from the demands of any single type of environment. The new dental opportunities made possible by stone tools meant that the toolmakers could transcend the status quo of any single habitat or slice of time.

In any particular environment, there is a recognizable pattern to how foods are distributed, and certain foods occur more abundantly than others. Therefore "optimal" foraging routes and "best" food sources can be defined in any given habitat. But even in the brevity of an annual season, the savanna offered a changing buffet. The ultimate test of the toolmaker way of life came as environmental extremes were felt over longer spans of time. I believe that lithic toolmaking persisted as a useful strategy precisely because it enabled a hominid to switch to different resources when the old ones were gone. By chipping rocks, certain hominids discovered a new form of versatility. A heavy stone i id a sharp-edged flake meant that a tremendous variety of items could be opened, cut, or crushed. Changes in food supply were handled by making implements capable of processing whatever kinds of food happened to be available.

For all their simplicity, fractured rocks offered a kind of buffer against natural shifts in the resources affecting survival and reproduction. A chipped stone first became valuable when it performed some specific taskcutting a tough plant stem, sharpening a stick, or slicing an animal hide. I believe that stone flaking endured not because it encouraged this specific task, or because the original environment of toolmaking continued to influence the hominids, but because cutting edges and pounding stones allowed potential differences between an arid grassland and a moist woodland to be reconciled. Stone flaking afforded a resilient means of obtaining needed resources in the full range of environments.

Similarly, carrying food and stones to the same sites buffered potential conflicts between one environmental state and another. The first time lithic tools and animal bones were brought together took place in some specific environmental setting. But the act of transporting items persisted and developed because of the benefits it provided in a changing environment. Once there was transport, the fact that stones and a particular food resource changed from being fifty feet apart to being two miles apart did not prevent the toolmakers from bringing these two critical resources together. Transport, much like stone toolmaking itself, enabled hominids to survive unexpected changes in the distribution and abundance of natural resources.

Although the earliest Oldowan flaking is more than 2.5 million years old, the deeper implications of this primal technology may not have been discovered until 2.2 to 1.8 million years ago. Sites in this time range preserve the oldest evidence of wide stone transport and tool cut marks on animal bones. Deep-sea cores, drilled from the North Atlantic and the tropical Pacific, reveal two unusual times of oxygen-isotope fluctuation between 2.2 and 2 million years ago. Two ocean cores from the western Atlantic indicate an intense oscillation at about 2.2 million years ago, and another just before 2 million years ago. On a global scale, relatively wide climatic oscillations coincided with the advent of distant transport and dietary changes in human toolmakers.

On land, this period is less well known. The best environmental record we have from this era, from the Olduvai Gorge, is 1.8 to 1.7 million years old. At the outset of this interval, Olduvai was dominated by moist woodland vegetation and high lake stands, an environmental state that was maintained for about 40,000 years. Within the next 10,000 years, extremely arid conditions developed. Lake level dropped significantly; fossil animals indicate semi-desert conditions; the pollen record shows a shift from river forests to steppe. Shortly before 1.75 million years ago, lake levels were again on the rise, and closed vegetation and humid climate prevailed, until the trend was reversed by another abrupt shift to aridity, which piled up windblown dust about 1.7 million years ago.

The vegetation was drier and more open at the end of this sequence than it was at the outset, but the trend was not smooth. Aridity was interrupted by moist periods; cooling, by warming. In the shadowed passages of time preserved in the gorge, we see that toolmakers practiced their craft over the entire range of habitats. Long-term and gradual shifts in rainfall and temperature, occasional deluges of volcanic ash, tilts in the landscape from rare and awesome fracturing of Earth's crust together must have caused impressive rearrangements in the resources of survival. Yet there is hardly a stratum in the most ancient depths of Olduvai Gorge where the imprint of stone toolmakers is absent.

To accept the anthropological dogma that the toolmakers flourished in drier, open habitats is to imply that human adaptation was molded primarily during periods of aridity. Alternatively, the environments of every time span were involved; natural selection produced relative success in all climates, wet and densely vegetated spans as well as periods of dry grassland. Knapping and stone gathering cut across the fluctuarions. The hominids were geared toward surviving the entire environmental panoply.

Although it invited the attention of other meat eaters, the inclusion of large animals in the diet also succeeded as a buffer to change. Any major climatic shift or geologic event would have disturbed the vegetation, altering the abundance and location of plant foods. Herds of large animals would also have been affected, but animals do not manifest anything like the dramatic variation in nutritional quality and toxins associated with different kinds of plants. To eat an open plain's zebra is much like eating a woodland's kudu. To digest an underground tuber is not the same as eating the soft pulp of a fruit. Eating large animals helped stabilize the diet when climate and vegetation changed.

To select from the mosaic's cornucopia, it was necessary to follow the dilation and contraction of edible items. Some foods were given up and others adopted as the occasion arose. Bringing resources together required the toolmakers to maintain a good mental map of their milieu, and also the capacity to change the template. The toolmakers had to respond to the opportunities of moister terrains as much as they did to growing aridity.

The search for mobile resources, cunning links between rocks and food, carrying things over a distance, avoidance of predatory carnivorescomprised the package of the earliest stonesmiths. The combination was both dynamic and critical to the future of their descendants. Each of these novelties became especially significant in the next round of human originthe rise of a large-bodied, large-brained, sweaty, long-distance nomad, who would later become the sole survivor, the stem from which all future humanity would emerge.

RETURN TO DISCUSSION OF FIRST TOOLS....