Its size is both a strength and a weakness. It is a strength because the sheer size and multiple redundancies of our civilisation make total destruction unlikely. We have no Library of Alexandria that could be destroyed in a single fire, no priestly class in whose minds all important knowledge is stored. Our billions mean that the awfulness of the cataclysm needed to wipe us out needs to be that much greater than the plagues and earthquakes of history.

And yet our civilisation's size and complexity could also be its downfall. We are immersed in a new kind of ecology, the technosphere, in which complex interactions of economies and machines are needed to feed and water us and provide shelter and jobs. Twenty thousand years ago, most people alive understood and actively took part in all the basics of survival. Today, equipped still with Stone Age brains, we modern humans live in a hi-tech world oiled by technologies and complex economic relationships few of us understand, which might as well have been put there for us by the gods. No wonder so many of us fear it will all fall apart, perhaps as punishment for our profligacy.

And some of those engines are indeed fragile. Turn over all commerce to the internet and what happens if it is killed by a super-virus? Most large cities are a week's worth of food deliveries away from anarchy. Most of all there is the thing no one wants to talk about, or even think about - the extra three billion who will have to be catered for over the coming half-­century. One undramatic (but depressing) future consists of a slow descent into global chaos and malnutrition, with wealthy hi?tech enclaves surrounded by a growing sea of ­poverty.

But we must remain optimistic. The biologists say all species become extinct after a few million years. Maybe that is our fate, but perhaps the rules don't apply to a species in possession of both antibiotics and the Bomb. As to our civilisation, it is proving durable and popular, spread at the point of commerce and a TV screen as much as the sword. Rome fell, but perhaps we can win the barbarians over this time. The future is within our grasp as it has never been before.