We can't manipulate some stars while maintaining other stars as controls; we can't start and stop ice ages, and we can't experiment with designing and evolving dinosaurs.
Although native Africans domesticated some plants in the Sahel and in Ethiopia and in tropical West Africa, they acquired valuable domestic animals only later, from the north.
No government is here forever. And there are other forces - the most potent force in our society, in fact, big business - doing good for the environment.
Why did human development proceed at such different rates on different continents for the last 13,000 years?
We're uncomfortable about considering history as a science. It's classified as a social science, which is considered not quite scientific.
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.
Twenty years ago, you might have been pessimistic and said there's no hope. But these days, some of our very biggest companies are acting remarkably cleanly. And in some cases, although not all cases, the CEOs are the driving forces behind that.
Thousands of years ago, humans domesticated every possible large wild mammal species fulfilling all those criteria and worth domesticating, with the result that there have been no valuable additions of domestic animals in recent times, despite the efforts of modern science.
The United States has long thought of itself as the land of infinite plenty, and historically we did have abundant resources. But now we are gradually exhausting our fisheries, our topsoil, our water. On top of that, we're coming to the end of world resources.
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.
The main thing that gives me hope is the media. We have radio, TV, magazines, and books, so we have the possibility of learning from societies that are remote from us, like Somalia. We turn on the TV and see what blew up in Iraq or we see conditions in Afghanistan.
The broadest pattern of history - namely, the differences between human societies on different continents - seems to me to be attributable to differences among continental environments, and not to biological differences among peoples themselves.
Technology has to be invented or adopted.
Technology causes problems as well as solves problems. Nobody has figured out a way to ensure that, as of tomorrow, technology won't create problems. Technology simply means increased power, which is why we have the global problems we face today.
Tasmanian history is a study of human isolation unprecedented except in science fiction - namely, complete isolation from other humans for 10,000 years.