Frozen Earth: The Once and Future Story of Ice Ages (32 page)

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?

In 1979, an undergraduate student at the University of Massachusetts, Eileen O’Brien, made a fiberglass model of the Acheulean hand axe and discovered that it had such distinctive aerodynamics that, when thrown, it always oriented itself vertically part way through its flight.
The experiment has been repeated and confirmed.
One (or many) of these hand axes, thrown by
Homo erectus
into a herd of animals crowded together at a watering hole, would have landed, spinning vertically, and sliced into the back of one of the herd.
It wouldn’t even require very accurate throwing.
Critics claim that it also wouldn’t have caused a large animal much injury, but supporters of the idea claim that such attacks would have caused some animals to stumble, panicked the herd, and at least increased
Homo erectus
’s chances of securing a few meals.

So it is possible that even tools like the hand axe can be related to the ice age—the drought cycles of the fluctuating climate provided the
best opportunities for their use.
And through this tool, the link can be made to evolution.
Once our ancestors learned that thrown objects were useful for hunting and also kept them out of harm’s way, greater emphasis could be put on accuracy.
Hand axes would be fine for a herd but not very useful for small groups or single animals, which would need to be precisely targeted.
And the coordination required for accurate throwing went hand in hand with larger brain capacity.
In turn, a bigger, high-energy-consumption brain required a reliable food supply, reinforcing and extending the importance of the newly acquired skills.

Whatever the real purpose of the hand axes, they must have been very effective, because they are found in the fossil record over a span of more than a million years, and they spread from Africa to Europe and Asia—always with the same basic shape.
That is perhaps the strongest argument for their use as throwing weapons, because it is the basic form that gives hand axes their aerodynamic properties.

By the time the hand axe appears on the scene, humans had already migrated—or perhaps
spread
is a better word—out of Africa onto other continents.
The routes they took were most likely influenced by the ice age climate.
Europe was repeatedly being inundated by continental ice sheets at this time, its vegetation zones shifting back and forth as the glaciers advanced and retreated.
There is no evidence of humans there, despite much searching.
However, a fossil
Homo erectus
individual dated at 1.8 million years has been found in Georgia, at the eastern end of the Black Sea, and there are other hominid fossils in Indonesia and China of similar or slightly younger age.
All the evidence suggests that hominids turned right when they left Africa, avoiding the extremes of European ice age climate and opting for the more equable regions to the east and south.
Population fragmentation, and probably the emergence of new species, resulted from this dispersal, but none of the new species survived.
Our own species,
Homo sapiens,
appeared much later, not in Asia or Indonesia, but in Africa, some time around 150,000 years ago.
This age comes from the dating of fossils, and has
recently been corroborated by DNA evidence, which can be used, in principle, to pinpoint the time when one species branched off from another.

When
Homo sapiens
arose 150,000 years ago, the Earth was close to the maximum cold of the last but one glaciation of the Pleistocene Ice Age; some 30,000 years later, by about 120,000 years ago, the ice had receded and the climate was much like today’s, or perhaps slightly warmer.
However, the archeological evidence indicates that our species remained in Africa through the interglacial warm interval and did not spread out until much later—probably about 70,000 years ago.
By then the climate had again swung back into a another glacial period—northern Europe and North America were ice-covered, and the sea level was low, because large quantities of ocean water were frozen into the continental ice sheets.
As happened during earlier waves of hominid migration,
Homo sapiens
first ventured into the Middle East, and then to India and Asia.
Only later did our species enter Europe, and then from Asia, not directly from Africa.
Aided by the lowered sea level—which meant that there were land bridges or only short stretches of water to cross in South East Asia and Indonesia—
Homo sapiens
had reached Australia by about 65,000 years ago, and New Guinea by 45,000 years ago.

By the time our species reached Europe, there had already been spectacular changes in human behavior and culture, even compared to direct African ancestors.
Early cave paintings in France and Spain are rightly recognized as beautiful art, not crude petroglyphs.
The rapid evolution continued after their arrival, and many anthropologists have argued that the pace of evolution over the past 50,000 years or so has been nothing short of phenomenal.
But the details are still controversial, because in physical terms, including the size of their brains, humans were “modern” much earlier.
Sophisticated toolmaking, often entailing assembly from multiple components and probably requiring some degree of complex thinking, had emerged long before, nearly 150,000 years before
Homo sapiens
appeared on the scene.
Unfortunately, culture
and behavior and language are not fossilized; they have to be inferred from scattered artifacts.
Many workers have concluded, however, that the complex language and thought that characterize humans today have developed only over the past 50,000 years or so.
William Calvin and others believe that no additional increase in brain size was required, because the necessary mental activities simply took over parts of the brain that were already there, developed earlier for other functions, such as accurate throwing.
And while not everyone agrees, a case can be made that the abrupt, whiplash temperature fluctuations of the past 50,000 years of the Pleistocene Ice Age, dramatically revealed in the Greenland and Antarctic ice cores, played a part in forcing this evolution (figure 24).