(On the beginning of the mid-1990s' genocidal war in Rwanda:)

Within six weeks, an estimated 800,000 Tutsi, representing about three-quarters of the Tutsi then remaining in Rwanda, or 11% of Rwanda's total population, had been killed.

Genocide, after all, is an exercise in community building.

The history of interactions among disparate peoples is what shaped the modern world through conquest, epidemics and genocide. Those collisions created reverberations that have still not died down after many centuries, and that are actively continuing in some of the world's most troubled areas.

Livestock adopted in Africa were Eurasian species that came in from the north. Africa's long axis, like that of the Americas, is north/south rather than east/west. Those Eurasian domestic mammals spread southward very slowly in Africa, because they had to adapt to different climate zones and different animal diseases.

The southward advance of native African farmers with Central African crops halted in Natal, beyond which Central African crops couldn't grow - with enormous consequences for the recent history of South Africa.

Easter Island collapsed in not just an epidemic of civil war but cannibalism. Of all the collapses of the past, I find the one that grabs people the most is that of Easter Island.