The Stone Age world, we now know, was a rough place. Ten thousand years ago, if someone decided to use force to settle an argument, there were far fewer constraints on him - or, occasionally, her - than citizens of functioning modern states are used to. Most of the killing was on a small scale, in murders, vendettas and raids, but because populations were also small, the steady drip of low-level killing took an appalling toll. By many estimates, 10 to 20 per cent of all the people who lived in Stone Age societies died at the hands of other humans.

This puts the past 100 years in perspective. Since 1914 we have endured two world wars. There have been genocides and government-sponsored famines, not to mention civil strife, riots and murders. Altogether we have killed a staggering 100 to 200 million of our own kind. But over the past century as a whole, 10 billion or so lives were lived - which means that just 1 to 2 per cent of the world's population died violently in these years. If you were lucky enough to be born in the 20th century, you were on average 10 times less likely to come to a grisly end than if you had been born in the Stone Age. And since 2000, the United Nations tells us, the risk of violent death has fallen still further, to 0.7 per cent.

These are astonishing statistics. Thanks to books such as Steven Pinker's , they are increasingly widely recognised. There is less agreement, though, on why violence has declined. In my new book, , I suggest the explanation is even more astonishing than the pattern itself. In perhaps the greatest paradox in human history, the force that has made the world so much safer is war itself.

This is an uncomfortable claim to make. After all, war is mass murder. What sort of person goes around saying that thousands of years of mass murder have had positive consequences? The answer: the sort of person who has been very surprised by the results of his own research. The philosopher Thomas Hobbes reasoned his way to a similar conclusion in the 1640s, as the English civil war raged around him, but only now do we have the evidence to prove the case.

Archaeology shows that humans have been killing each other for at least 50,000 years, ever since we evolved into our modern form. Life, as Hobbes speculated, was "nasty, brutish, poor and short". But around 10,000BC, at the end of the last spasm of the ice age, the story of violence took a surprising turn. As the world thawed out, plants and animals, including our ancestors, took advantage of the extra warmth and multiplied madly.

For many species the story ended badly: they outran the resources and starvation brought their numbers back down. But in a band of what we might call "lucky latitudes" running from China to the Mediterranean in the Old World and Peru to Mexico in the New World plants and animals had evolved that humans were able to domesticate, vastly increasing humanity's food supply.

This set off a slow-motion population explosion. Before 10,000BC, all humans had been hunter-gatherers, and their numbers had doubled roughly every 12,000 years.

After 10,000BC, some humans became farmers, and the doubling time fell to roughly 2,000 years. Over a few thousand years, the lucky latitudes filled up with people.

Crowding changed everything. When prehistoric hunter-gatherers fought, the surviving losers usually had the option of moving away to a different part of the largely empty landscape. But as the lucky latitudes got more congested, farmers who lost fights found running away increasingly difficult. There was nowhere for them to go.

Starting in what we now call the Middle East and then spreading out across the planet, something very new happened: the winners of wars began incorporating the losers into larger societies. The first ruler we know by name, King Narmer of Egypt, finished forcing much of the Nile valley into a single society around 3100BC. At about the same time, the rulers of Uruk were apparently doing something rather similar in what we now call Iraq. A millennium later, similar processes seem to have unfolded in the Indus Valley, in what is now Pakistan.

Another millennium after that, around 1100BC, the Shang dynasty was bringing the banks of China's Yellow River under its control. Still another millennium later, a band of great empires spanned the Old World, from Rome in the west to Han China in the east, and the people of Teotihuacan were busy conquering the Basin of Mexico while the Moche were taking over the coast of northern Peru.

Wherever they lived, the winners of these wars quickly found that the only way to make larger societies work was by developing stronger governments. And one of the first things these governments had to do, if they wanted to stay in power, was suppress violence among their subjects.

The men who ran these governments hardly ever pursued policies of peacemaking purely out of the goodness of their hearts. They cracked down on killing because well-behaved subjects were easier to govern and tax than angry, murderous ones. The unintended consequence, though, was that they kickstarted the process through which rates of violent death fell by 90 per cent between the Stone Age and the 20th century.

Because there are no really reliable official statistics before modern times, the best evidence for rates of violence comes from telltale wounds on the skeletons excavated by archaeologists, but even this evidence is currently only available in patches. Decades of research will be required before we can build a comprehensive database. Even so, if we put together all the pieces of information already available - from literature, art and excavations - we can get a rough sense of how the story played out.

By 2,000 years ago, in the age of Eurasia's great ancient empires, rates of violent death had fallen far from the Stone Age's 10 to 20 per cent. By my calculations, they had probably tumbled into the 2 to 5 per cent range. After about AD200, however, the trend was thrown into reverse. The steppe nomads of central Asia learned to use cavalry with devastating effectiveness and, for the next 1,000-plus years, empires found no way to cope with them.

From Attila the Hun in the fifth century to Genghis Khan in the 13th, steppe raiders smashed one large, peaceful society after another, driving rates of violent death up into the 5 to 10 per cent range - not as bad as the Stone Age, but much worse than the ancient empires. Not until the 17th century did large, settled states find a solution, in the form of guns that delivered enough firepower to stop horsemen in their tracks. Combining these guns with new, ocean-going ships, Europeans were by then exporting war around the world, creating the largest societies yet seen, and ultimately driving rates of violent death lower than ever before.

The pacification process which delivered these low death rates was not pretty. In particular times and places, violent death could even spike back up to Stone Age levels. Between 1914 and 1918, for instance, nearly one Serb in six died as a result of violence, disease or starvation. And, of course, not all governments are equally good at delivering peace. Democracies may have their faults, but on the plus side they rarely devour their citizens; dictatorships may get things done, but they tend to shoot, starve and gas a lot of people. And yet despite the Hitlers, Stalins, Maos and Idi Amins, in the 10,000-year-long run, war made states and states made peace.

The unpleasant truth seems to be that while war is probably the worst way imaginable to create larger, more peaceful societies, it is pretty much the only way humans have found. If only the Roman Empire could have been created without killing millions of Gauls and Greeks; if only the US could have been built without killing millions of Native Americans.

In these and countless other cases, if conflicts could have been resolved by discussion instead of force, humanity would have had the benefits of larger societies without paying such a high cost. But that did not happen. It is a depressing thought, but the evidence again seems clear. People almost never give up their freedom, including their "rights" to kill and impoverish each other, unless forced to do so; and virtually the only force strong enough to bring this about has been defeat in war or fear that such a defeat is imminent.

If I am right that governments have made us safer and that war is pretty much the only way we have discovered to make governments, then it seems that war, ultimately, has an upside. Another lesson of long-term history, though, goes beyond survival. As well as making people safer, the larger societies created by war have also - over the 10,000-year long run - made us richer.

Peace creates the conditions for economic growth and rising living standards. But again, getting there is often messy and uneven: the winners of wars regularly go on rampages of rape and plunder, selling survivors into slavery and stealing their land. The surviving losers may be left impoverished for generations.

It is a terrible, ugly business. And yet, with the passage of time - maybe decades, maybe centuries - the creation of a bigger society starts making everyone, the descendants of victors and vanquished alike, better off. The long-term pattern is again unmistakable. By creating larger societies, stronger governments and greater security, war has ultimately enriched the world.

Pulling these various strands together, only one conclusion is possible. War has been good for something: it has produced bigger societies, ruled by stronger governments, which have imposed peace and created the preconditions for prosperity.

Ten thousand years ago, there were only about six million people on Earth. On average they lived about 30 years and supported themselves by hunting and gathering, on the equivalent of less than US$2 per day in today's terms. Now there are more than a thousand times as many of us (seven billion), living more than twice as long (the global average is 67 years) and earning more than a dozen times as much (last year the global average was US$25 dollars per day).

In fact, war may be so good at delivering peace and wealth in the long run that it finally seems to be putting itself out of business. Not only that, in the modern age we have become so good at fighting - our weapons so destructive, our organisations so efficient - that further widespread war is looking nigh-on impossible. Had someone actually pushed the big red button back in the 1980s, when the world's nuclear stockpile was at its peak, a billion of us would soon have died, pushing the 20th century's rate of violent death back to the levels of the Stone Age. And had the legacy of all those warheads been as toxic as some scientists feared, by now there might have been none of us left at all.

The good news is not just that this didn't happen, but also that it was frankly never very likely to happen. The biggest lesson of long-term history is that we humans are remarkably good at adapting to our changing environment. Violence, after all, is an evolved adaptation. Almost every species of animal has evolved to use violence. Each species has its own particular way of doing this and, as their environments change - in terms of the climate, food sources and rivals' ability to retaliate, for example - they evolve to use violence in different ways.

We humans are exactly the same but, in addition to evolving biologically, our big brains allow us to evolve culturally. Consequently, when our environment changed at the end of the Ice Age, we learned not only how to farm but also how to live in bigger societies and kill each other less. That is why, although we are still basically the same animals as we were 10,000 years ago, our rate of violent death has fallen by 90 per cent.

It is also why, despite having invented nuclear arms 70 years ago, our rate of violent death keeps falling. We are the most adaptable animals to have ever walked the Earth.

As we grieve for the fallen on the anniversary of the outbreak of the first world war, we should bear this overarching story in mind. We fought countless wars in the past because fighting paid off; but in the 20th century, as the returns of violence declined, we found ways to solve our problems without bringing on Armageddon.

There are no guarantees, of course, but there are nevertheless grounds for hope that we will continue to avoid this outcome. The 21st century is going to see astounding changes in everything, including the role of violence.

The age-old dream of a world without war may yet come to pass.

New Scientist

The absurdity of war

Precisely 100 years after the trigger was pulled on Archduke Franz Ferdinand of Austria - the June 28, 1914 assassination that ignited the first world war - the Hong Kong Players will be nearing the end of a Saturday matinee performance of Oh, What a Lovely War!, in Wan Chai.

When Joan Littlewood's musical (right) debuted in London in 1963, Hong Kong was suffering a severe drought, the Beatles had released their first LP and just enough time had passed for critical reflection on the conflict that claimed 16 million lives between July 1914 and November 1918.

Deliberately provocative, the play packed no patriotic punches. Incompetent generals, brave men's lives squandered in futile battles and disillusioned soldiers - all commonplace ideas today - were flippantly articulated by Littlewood's agitprop, paving the way for later British television comedies such as Dad's Army and Blackadder Goes Forth.

By telling the story in jolly pantomime fashion, the absurdity of war was exposed. A news ticker overhead flashed the number of casualties from the bloodshed, ramming home the reality of war.

Jodi Gilchrist, director of the Hong Kong Players' production, says: "With all the famous songs it's meant to be fun, but as the casualties rise by the end of Act 2, the mood darkens.

"There's one scene where the wounded come back from the battlefield and the nurse says, 'Don't worry, we'll have you back on the frontline in no time.' These soldiers were so bewildered, they didn't have any idea of the scale of battle, and so of course they became disillusioned."

Littlewood's point was that lessons could be learned from the past.

Perhaps unsurprisingly, the establishment wasn't initially enamoured. The British censor only gave it the green light for London's West End after a ringing endorsement from Princess Margaret, who, after an early performance, told a member of the royal household: "What [was] said here tonight should have been said long ago. Don't you agree?"

Jenni Marsh

Why the West Rules - For Now. War! What is it Good For?