Stirrings Of Change

In almost 2.5 million years, hominids had moved only slightly beyond the most primitive level of Stone Age life, adding control of fire and gradually more sophisticated stone tools to their meager repertoire of skills (table 6.1). But once our species appeared, 150,000 to 100,000 years ago, the pace of change quickened, at least by comparison to the nearly undetectable tempo of previous times. By 50,000 years ago, we see the first real evidence of human creative potential emerging in the artistic and aesthetic expression of people who were like us both in physical respects and mental capabilities. They painted amazingly lifelike portrayals of animals on the walls of caves and rock shelters, and they made small statues of human and animal figures. They created jewelry by stringing together shells. They buried their dead with food and possessions for use in a future life. In these changes, we can recognize the early origins of a human "culture" similar to ours today, and we can begin to relate to these people as "us."

Also appearing for the first time are much more sophisticated stone tools designed for specific functions, made using standardized (easily repeatable) methods. And for the first time people began to use bone, a substance much more workable than stone, yet hard enough for many uses. The first needles, awls (hole-punchers), and engraving tools appeared. Needles made possible sewn clothing that fit closely, rather than loosely draped animal hides. With greater protection from the elements, people pushed north into higher and colder near-Arctic latitudes. There they built dome-shaped houses using large mastodon bones for the superstructure and draping the roof with hides of animals for protection from rain and snow.

People also learned how to make rope from naturally available fibers and used it to make snares, lines, and nets to catch small animals, birds, and fish. The hunting-gathering life became a hunting-gathering-fishing life. Bone was shaped into spear throwers to hold stone spear points, and rope was used to help bind spear points to shafts of bone or wood. This new technology produced a lethal and revolutionary new way of hunting, combining a weapon that could kill efficiently with a hand that could grasp it and an arm with the natural range of motion to throw it. Hunters could now bring down larger game from a safer distance, and even huge mammoths could be killed, as cave paintings show.

Inevitably, improvements in technology were combined with improved hunting strategies. People now anticipated the movement of game, used fire and other

Table 6.1 Major Stages in Human Evolution

Time of Appearance

Relationship to Us

Mode of Existence

New Tools and Implements


2,500,000,000 years ago

Our genus Homo

Primitive spear points Grinding and crushing food Digging roots, tubers


500,000 years ago

Our species Homo sapiens

Hunting, gathering

Stone Age Less primitive spear points Fire (food and security)


150,000 years ago

Fully modern humans ("us")

Hunting, gathering, fishing

Stone Age

Advanced spear points Bone needles and awls, rope snares, nets, lines Houses and clothing Primitive metal working

Ceremonial burials Care for sick, elderly Cave paintings Statues, figurines Jewelry



[see chapter 7]

years ago years ago planning to startle and drive the animals toward intended areas, and communicated with each other as the hunt proceeded. In time the impact of these skills came to be felt in ways entirely different from anything in the prior history of humans or their more distant predecessors. On several continents where humans had not previously been present, major extinctions of large mammals occurred at or very near the time humans first arrived. In Australia people first arrived nearly 50,000 years ago, having by then learned how to make boats that could carry them over the deep channels that still separated Australia from the Southeast Asian mainland during a time of lower glacial sea level. Coincident with the first arrival of humans (insofar as dating methods can tell), a large fraction of Australia's indigenous marsupial population disappeared forever, including a marsupial lion, three kinds of wombat, nine genera of kangaroo, and a range of nonmarsupials including a giant tortoise the size of a Volkswagon "bug" and several kinds of lizards and flightless birds. Every vertebrate species larger than humans disappeared. Nothing even close to this concentrated a pulse of extinction had occurred for millions of years on Australia, despite an ongoing drift of climate toward much drier conditions.

Even stronger evidence ties humans to more recent mass extinctions of large mammals in the Americas. In North America nearly 12,500 years ago, and within the range of accurate radiocarbon dating, many large mammals disappeared in a very short time, most of them within just a thousand years. In all, 33 genera of large mammals died out, leaving only 12 still extant. The list of those lost includes the wooly mammoth, the mastodon, two species of buffalo, four genera of giant ground sloths, one kind of bear, the saber-tooth tiger, the camel, the cheetah, two types of llama, the yak, a giant beaver, several kinds of horse and donkey, the tapir, the peccary, one type each of moose, antelope, and deer, and three kinds of ox. Most of these mammals weighed 100 pounds or more, and most were larger than any surviving counterparts today. One of the ground sloths was up to 20 feet tall and weighed several tons. The Imperial mammoths had curving tusks 12 feet long.

Some of these genera went totally extinct in a global sense, while others survived in parts of the Old World. When Europeans later reintroduced the horse and donkey to North America, they were bringing those creatures back home to the continent where they had first evolved. For tens of millions of years, the grassy plains of North American had been richer in large-mammal game than the modern-day plains of Africa, but now this great collection of animals was abruptly reduced to slightly over a quarter of its original variety.

Some scientists argue that climate change was the culprit. They point to environmental stresses 12,500 years ago tied to changes in Earth's orbit: strong solar radiation that caused hot summers and rapid melting of the great North American ice sheets, weak solar radiation that produced cold winters, and swift northward migration of forest and grassland vegetation into regions from which the ice had melted. One argument was that the grassland habitats favored by many grazing and browsing animals were squeezed out by these coincident and large-scale climatic changes, and that many species became extinct for lack of suitable habitat.

In the 1960s geoscientist Paul Martin first argued the case against a strictly climatic control of the mammal extinctions. He pointed out that more or less the same set of environmental variables had recurred at least four times in the last million years, and yet no mass extinction of mammals had occurred earlier. He concluded that the mass extinctions 12,000 years ago must have been the result of a new factor that had not been present during any of the earlier intervals: predation by humans.

I am certainly no expert on either mammals or cultural anthropology, but I am an expert on climate cycles, and I find Martin's criticism of the climatic explanation absolutely convincing. In fact, it is even more convincing now than when he first presented it. When Martin first argued that humans were the primary cause of the mass extinctions, climate scientists still thought that only four glaciations had occurred. But over the last two decades it has become clear that 8 or 9 glacial cycles occurred in the last 900,000 years, and some 40 or 50 cycles in the full 2.75 million years of the Northern Hemisphere ice ages (chapter 4).

With all of these cycles of ice-sheet growth and melting, the same basic configuration of climatic factors that occurred 12,500 years ago had already happened dozens of times in the past: strong summer radiation, weak winter radiation, rapidly melting ice sheets, and swiftly migrating vegetation. In fact, the extremes of summer and winter radiation had often been larger in earlier glacial cycles than they were 12,500 years ago, with even faster rates of ice-sheet melting and geographic shifts in vegetation, yet no concentrated interval of extinctions had occurred during any of those earlier cycles. Indeed, the number of extinctions near 12,500 years ago in North America rivaled or exceeded the total from all of the previous ice-age cycles over 2.75 million years. Why would all of these different types of animals suddenly have become vulnerable to a combination of factors that had not affected them for several million years? To me, the answer is clear. Climate cannot be the primary reason for this unique and highly concentrated pulse of extinctions.

The only possible conclusion left seems to be the one Martin proposed: some entirely different factor must have been at work during the most recent deglaciation. And the most obvious "new" factor was the presence of humans in the Americas. Some scientists believe that humans first arrived almost simultaneously with the pulse of mammal extinctions, while others think they may have come somewhat earlier. Either way, the link in time remains highly suggestive. Part of the answer may also involve an innovation in human technology that appears at the time of the extinctions. A spear point called a "Clovis point" (for the site in Clovis, New Mexico, where it was recovered) was found embedded in the ribs of now-extinct mammals, including mammoths. This evidence tells us that humans were present and were armed with new weapons when the extinctions occurred.

North America is only one of several examples of sudden extinctions that have occurred when humans arrived. An even worse fate befell the mammals of South America, where 80% of the genera (46 out of a total of 58) have disappeared since 15,000 years ago, most during the early part of the interval of human occupation. Again, no such extinction pulse had occurred during the previous millions of years. The same story has been repeated on a smaller scale on islands like Madagascar, Hawaii, and New Zealand. Wherever humans have previously been absent and then appeared, massive extinctions have followed.

In Africa and Eurasia, where humans and animals had co-evolved over very long spans of time, no comparable pulse of extinctions occurred. This observation makes sense because the game hunted by humans had sufficient time in which to evolve strategies for surviving the pressures of human hunting, such as solitary habits and erratic unpredictable migration patterns.

The conclusion that humans caused these massive extinctions in the Americas and on Australia met (and still meets) with strong resistance from many respected scientists. The critics argue that the small number of people present in North and South America 12,500 years ago could not possibly have driven all of those animals to extinction, even using spears armed with Clovis points.

But those early Americans did not have to kill every last member of a species one-by-one with spears to cause the extinctions. Humans hunted in organized groups, using verbal communication and fire to drive animal herds into canyons or arroyos or other topographically constrained features. Some of these drives led to cliffs over which the herds were driven by yells and fire. Some ended in deadend traps where the confused creatures could easily be killed. Skeletal remains of animal herds dating to this interval have been found piled at the bottom of cliff edges, and often only the topmost skeletons show evidence of butchering for food. The hunting strategies worked so effectively that too many animals were killed to make use of them all.

Recent work by population ecologists shows that large-mammal species can be brought to the point of extinction surprisingly quickly by culling just a few "extra" percent of their population per year. Because most large mammals reproduce slowly, with long gestation periods and few young, they are vulnerable to very small increases in normal rates of mortality. Models that simulate population changes by calculating the long-term effect of rates of birth and death show that increases of just a few percent in normal mortality can cause extinction within a few centuries. Once a species is reduced below the level (density) needed for survival, it is doomed to extinction, even without heavy predation. If these early hunters culled just a few percent more individuals than were normally taken by disease, weather, and old age, extinction would have been inevitable.

Critics also raise the reasonable objection that the fossil record does not allow us to say that all of the mammal extinctions in the Americas happened at exactly the time humans first appeared. It does not. But think back to those volunteers sent looking through sediments for scarce dinosaur remains dating near the time of the asteroid impact (chapter 2). The closer they looked, the tighter (closer in time) the connection with the abrupt meteorite impact became.

Both the remains that mark the first arrival of humans in the Americas and those that record the extinctions of the mammal species are relatively scarce, but the general coincidence of these two phenomena in time is remarkable. I will place my bet that the more scientists look, the closer in time the mammal extinctions will lie to the appearance either of humans or of new hunting technologies. In summary, I reject climatic changes as a primary factor in these extinctions, and I find good reason to think that the cause is tied to humans.

People may have a partly unconscious reason for rejecting human predation as the explanation for the extinctions. Two centuries ago Jean-Jacques Rousseau introduced the concept of the "noble savage": a native people who once lived in total harmony with the environment, killing and eating only what they needed for survival, and nothing more. In a graceful phrase often used in recent years, these people were seen as having "lived lightly on the land." Because Rousseau never traveled to see first-hand the primitive cultures described to him by others, his views were second-hand, simplistic, and romanticized, but they remain influential today. It is unpleasant or even unimaginable to think of the pretechnologi-cal peoples of the Americas, Australia, Madagascar, New Zealand, or Hawaii as capable of killing on a large enough scale to cause several of the major extinction episodes in Earth's history. That large an assault on nature is viewed as uniquely a sin of modern society employing modern technology. But the facts say otherwise.

I don't put those early Americans in the company of a man like "Buffalo Bill" Cody, who personally slaughtered tens of thousands of bison from horseback and paraded his achievements in Wild West shows. Nor were they at the level of the passengers who shot buffalo for sport from the windows of moving trains. Like all of their human predecessors, the earliest Americans killed animals primarily for food, for clothing, and for bone tools. The massive extinctions were mainly the unintended consequence of new hunting methods that worked all too well. Nevertheless, the loss of much of this astoundingly diverse range of mammals and marsupials ranks as a tragic chapter in human history. Long before organized human civilization first appeared on the planet, we had greatly impoverished the natural fauna, leaving behind small remnants of a once-rich array of life.

All of this evidence from the last 50,000 years—the cave art, the jewelry, the burial rituals, and even the mammal extinctions—reveals new sets of skills, new kinds of imagination, and new abilities to express and communicate thoughts, feelings, and plans. And yet more than 99.5 percent of the way from the appearance of "man" to the modern era, everyone still lived a nomadic hunting-gathering-fishing life, and everyone still made tools out of stone or bone. Progress toward the modern era had still been incredibly modest, even allowing for the more recent emergence of a range of new capabilities.

But then, about 12,000 years ago, human ingenuity in Eurasia reached a turning point and began to produce an advance so momentous that it put humanity on a gradually accelerating path toward the modern era. This turning point, the discovery of agriculture, took place in the "fertile crescent" of the eastern Mediterranean, an arc-shaped region stretching from modern-day Turkey in the north down through Iraq and into Syria and Jordan in the south. At about the same time, similar changes began in the Yellow River Valley region of northern China.


Humans Begin to Take Control

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