Rhino-mounted Bantu shock troops could have overthrown the Roman Empire. It never happened.

I personally am not conscious of my accent.

It invites a search for ultimate causes: why were Europeans, rather than Africans or Native Americans, the ones to end up with guns, the nastiest germs, and steel?

All recognized famous inventors had capable predecessors and successors and made their improvements at a time when society was capable of using their product.

Much of human history has consisted of unequal conflicts between the haves and the have-nots.

But this was the only way of life that humans knew for their first 6m years on the planet. In giving it up over the past few thousand years, we have lost our vulnerability to disease and cold and wild animals, but we have also lost good ways to bring up children, look after old people, stave off diabetes and heart disease and understand the real dangers of everyday life.

Take air quality in the United States today: It's about 30 percent better than it was 25 years ago, even though there are now more people driving more cars.