Man: the killing machine

COLIN TUDGE takes the long view. The last Ice Age? An heartbeat ago: the ancient Egyptians could remember nearly that far back; Australian aborigines tell stories of landscapes that have been lost under the sea for 8,000 years. The earth's magnetic poles casually flip their polarities. Ice sheets leave boulders the size of hotels on mountain tops. The Straits of Gibraltar close, drying out the Mediterranean, and then open, letting the sea surge back in.

That's just the prologue. The central event of the modern era, in this book, is India crashing into Asia some 40 million years ago, throwing up the Tibetan plateau. This sucks excess carbon dioxide out of the atmosphere, cooling the planet to a temperature in which humanity can flourish. Viewed at this breakneck pace, the survival of life on earth looks fragile and accidental.

The thread of Tudge's unashamedly narrative history of our species is how humans developed into the successfully destructive species we are. Our ancestors had long arms and well-developed shoulder muscles for swinging n the trees. When the forests covering the earth contracted, and sparser woodlands and grassy plains took their place, hominids used these arms and muscles for other purposes. Hands allowed the making of tools and the brain their effective use. As Tudge points out, there is a powerful positive feedback loop that operates between brain and hands: complexity theory in action. Either on its own would be far less useful - "Under most natural circumstances, a big, contemplative brain is totally useless. What, for example, would a codfish do with the brain of Jane Austen? Fit a duck-billed platypus with the hands of Alfred Brendel, come back in a million years and you would find them re-evolved into paddles" - but the combination brings evolutionary advantages astonishingly quickly.

Coupled with this is the power of speech; not just communication (which most animals can manage, to a certain extent), but syntax that allows the expression of complex ideas: "All human beings may in principle partake of the thoughts of all other human beings through all time." Moreover, our varied diet means we can harass some species to extinction without running out of food. This is no trivial matter, as Tudge points out: red squirrels have lost out to greys because the red ones eat mostly the seeds of conifers, while greys have a more varied diet.

As a consequence, man is the most efficient predator the world has seen. This ground has been covered before, for example in Richard Leakey's Origins Revisited. But Tudge differs from Leakey in particular in arguing that the history of humanity is a history of deliberate aggression at the level of the ecosystem. It is not only in modern times that Homo sapiens has transformed the world. In the late Pleistocene, mammoths and other species dwindled to extinction as man moved into the areas they occupied. No coincidence, says Tudge, who depicts Palaeolithic game managers driving mammoths and giant sloths to extinction in order to starve the sabre-toothed tigers and running bears which ate them and were rival predators to humans.

But it was the growth of agriculture that spelled doom for the rest of the planet. Hunter-gatherers are inherently opportunist, whereas farming is a full-time job. Agriculture, though, has a slight edge over hunting, which meant that husbandry began to win out. As it did, some animals became domesticated and the areas available for non-domesticated wildlife shrank, another feedback loop which made life increasingly hard for hunters. The increasing populations which agriculture allowed made it the only feasible option. But now we are close out-farming the earth. Agriculture looks like an evolutionary cul-de-sac.

And our actions make things worse. Two large sources of greenhouse gases are the belches of cattle and the flatulence of termites. Both are increasing, the cattle because we cut down forests to grow grass for them, the termites because they too prefer grass to forest.

The inevitable sequel to an explanation of the past is a prescription for the future. Tudge modestly suggests that we should confine our attention to the next million years; in 13 million years, after all, the earth is due for another minor mass-extinction-by-asteroid. His priority is to limit human population. Humanity, he thinks, would be sustainable with a world population no greater than a billion, all enjoying the relatively modest living standards of a Mediterranean village. This is the lifestyle, insists Tudge, which produced people ranging from Jesus Christ to El Greco, and can hardly be said to be culturally limiting. Yet his whole book has argued the case for its impossibility.

Tudge loves provocative thought- experiments (such as the Austen-brained fish and the pianist platypus), and is a stranger to Occam's Razor: when he sees an opportunity to plump for the more surprising explanation rather than the less, he leaps at it. "Time after time, the most extraordinary ideas have turned out to be right, and ... conservative notions ... inadequate."

As to whether his recommendations will be followed, the last few million years give little cause for optimism. Evolution has favoured man as an aggressive edger-out of other species. But homo sapiens has adapted before, when it has needed to. Can we still?