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

I've always been interested in a lot of things, and a lot of things at the same time, and I always tried to explain them to myself. I ask a lot of questions.

Not until the beginning of the 20th century did Europe's urban populations finally become self-sustaining: before then, constant immigration of healthy peasants from the countryside was necessary to make up for the constant deaths of city dwellers from crowd diseases.

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.

We study the injustices of history for the same reason that we study genocide, and for the same reason that psychologists study the minds of murderers and rapists... to understand how those evil things came about.

Put another way, the chimpanzees' closest relative is not the gorilla but humans.

Forced to choose between limiting population or trying to increase food production, we chose the latter and ended up with starvation, warfare, and tyranny.