The other hotly contested argument over Martin's blitzkrieg theory concerns the supposed over-hunting and extermination of big mammals. It seems hard to imagine how stone-age hunters could kill a mammoth at all, let alone hunt all mammoths to extinction. Even if the hunters could slaughter mammoths, why would they want to? And where are all the skeletons now? Certainly, when we stand under a mammoth skeleton in a museum, the thought of using a stone-tipped spear to attack such a gigantic tusked beast feels utterly suicidal. Yet modern Africans and Asians with equally simple weapons do succeed in killing elephants, often hunting as a group relying on ambush or fire, but sometimes stalking an elephant as a single hunter armed with a spear or poisoned arrow. These modern elephant hunters still rate as amateur dabblers, compared to the mammoth hunters of Clovis times, heirs to hundreds of thousands of years of hunting experience with stone tools. Museum artists like to depict late stone-age hunters as naked brutes risking their lives to hurl boulders at an enraged charging mammoth, with one or two hunters already lying trampled to death on the ground. That is absurd. If any hunters had died in a typical mammoth hunt, mammoths would have exterminated hunters, rather than vice versa. Instead, a more realistic picture is of warmly-clad professionals safely spearing a terrified mammoth ambushed in a narrow stream bed.

Recall also that the big mammals of the New World had probably never seen humans before Clovis hunters, if the hunters indeed were the first people to reach the New World. We know from Antarctica and the Galapagos how tame and unafraid are animals that evolved in the absence of humans. When I visited New Guinea's isolated Foja mountains, which lack any human population, I found the large tree kangaroos so tame that I could walk up to within a few yards of them. Probably the New World's large mammals were equally naive and were killed off before they could have time to evolve a fear of man.

Could Clovis hunters have killed mammoths fast enough to exterminate them? Assume again that an average square mile supports one hunter-gatherer and (by comparison with elephants in Africa today) one mammoth, and that one-quarter of the Clovis population consisted of adult male hunters who each killed a mammoth every two months. That means six mammoths killed per four square miles per year, so the mammoths would have to reproduce their numbers in less than a year to keep up with the killing. Yet modern elephants are slow breeders that take about twenty years to reproduce their numbers, and few other large mammal species breed fast enough to reproduce their numbers in less than three years. It could plausibly have taken Clovis hunters only a few years to exterminate the large mammals locally and to move on to the next area. Archaeologists trying to document the slaughter today are searching for needles in a fossil haystack: a few years' worth of butchered mammoth bones among the bones of all the mammoths that died naturally over hundreds of thousands of years. It is no wonder that so few mammoth carcasses with Clovis points among the ribs have been found.

Why would a Clovis hunter even want to kill a mammoth every two months, when a 5,000-pound mammoth yielding 2,500 pounds of meat would provide ten pounds of meat per day for two months for the hunter, his wife, and two children? Ten pounds may sound like gross gluttony, but it actually approaches the daily meat ration per person on the US frontier in the last century. That is assuming that Clovis hunters really ate all 2,500 pounds of mammoth meat. But to keep the meat for two months would require drying it: would you go to the work of drying a ton of meat, when you could instead just go and kill a fresh mammoth? As Vance Haynes noted, Clovis mammoth kills prove to be only partly butchered, suggesting very wasteful and selective utilization of meat by people living amidst an abundance of game. Some hunting probably was not for meat at all but for ivory, hides, or just machismo. Seals and whales have similarly been hunted in modern times for oil or fur, leaving the meat to rot. In New Guinea fishing villages I often see the discarded carcasses of large sharks, killed only for their fins to make the delicacy of shark's fin soup.

We are all too familiar with the blitzkriegs by which modern European hunters nearly exterminated bison, whales, seals, and many other large animals. Recent archaeological discoveries on islands have shown that such blitzkriegs were an outcome whenever earlier hunters reached a land with animals unused to humans (Chapter Seventeen). New Zealand's giant flightless birds, the moas, were all exterminated by Maori colonists within a few centuries. The Indonesians and Africans who colonized Madagascar 1,500 years ago exterminated other giant flightless birds (the elephant birds), along with a dozen species of primates (the lemurs) ranging up to the size of a gorilla. Polynesian colonists of Hawaii exterminated numerous species of big flightless geese. Since the collision between humans and large naive animals has always ended in an extermination spasm, how could it have been otherwise when Clovis hunters entered a naive New World?

This end, though, would hardly have been foreseen by the first hunters to arrive at Edmonton. It must have been a dramatic moment when, after entering the ice-free corridor from an overpopulated, overhunted Alaska, they emerged to see herds of tame mammoths, camels, and other beasts. In front of them stretched the Great Plains to the horizon. As they began to explore, they must soon have realized (unlike Christopher Columbus and the Plymouth Pilgrims) that there were no people at all in front of them, and that they had truly arrived first at a fertile land. Those Edmonton Pilgrims, too, had cause to celebrate a Thanksgiving Day.

NINETEEN

THE SECOND CLOUD

Human society is now at risk of destroying itself by stumbling into either a nuclear holocaust or an environmental holocaust. The latter could arise from mass extinction of species. This chapter estimates how many species we have already exterminated, how many more we are likely to exterminate within the next century, and how mass extinction would affect us. Until our own generation, no one had grounds to worry whether the next human generation would survive or enjoy a planet worth living on. Ours is the first generation to be confronted with these questions about its children's future. We devote much of our lives to training our children to support themselves and to get along with other people. Increasingly, we are asking ourselves whether all those efforts of ours might be wasted.

These concerns arise because of two clouds hanging over us—clouds that would have similar consequences, but that we view very differently. One, the risk of a nuclear holocaust, first revealed itself in the cloud over Hiroshima. Everyone agrees that the risk is real, since there are huge stockpiles of nuclear weapons and since politicians throughout history have occasionally made dumb miscalculations. Everyone agrees that, if a nuclear holocaust does happen, it will be bad for us and might even kill us all. This risk shapes much of current world diplomacy. The only thing about which we disagree is how best to handle it—for instance, whether we should aim for complete or partial nuclear disarmament, nuclear balance, or nuclear superiority.

The other cloud is the risk of an environmental holocaust, of which one often discussed potential cause is the gradual extinction of most of the World's species. In contrast to the case with nuclear holocaust, there is almost complete disagreement about whether the risk of a mass extinction is real and about whether it would really do us much harm if it happened. For instance, one of the most frequently cited estimates is that humans have caused about one per cent of the world's bird species to be, come extinct within the last few centuries. At one extreme, many thoughtful people—especially economists and industrial leaders, but also some biologists and many laypeople—think that that loss of one per cent would have been inconsequential, even if it had really happened. In fact, such people reason that one per cent is a gross overestimate, that most species are superfluous to us, and that it would do us no harm to lose ten times more species. At the opposite extreme, many other thoughtful people—especially conservation biologists and a growing number of laypeople belonging to environmentalist movements—think that the one per cent figure is a gross underestimate, and that mass extinction would undermine the quality or possibility of human life. Obviously, it will make a big difference to our children which of these two extreme views is closer to the truth.