Carve human history into thousand-year units. Look at the last 10. Dr. Alfred Crosby does. What he sees is the human animal engaged in "ecological imperialism." Dr. Crosby is an historian curious about how people have become earth's keystone species and the dramatic impact we have had on the planet. Our defining moment? Embracing agriculture. Everything else follows.

      We're a pushy species, Dr. Crosby observes. Homo sapiens push each other around. And we have done the same with everything else as weâve migrated all over the planet. But humans did not travel alone. We had company, what Dr. Crosby calls our "portmanteau biota." (A portmanteau is a kind of suitcase.) We knew we were taking along wheat and rice, sheep and horses, pigs and dogs. But we didn't know that our ragged little assemblage included microbes which both keep us alive and can kill us, the microbes on which Dr. Crosby reminds us the rest of life surfs. Or the weeds. Or the rats and mongooses and lice. Or the unpredictable ways plants and animals and microbes and soil interact when new species invade a territory, population explosions of smallpox, feral horses, pigs, crabgrass or humans.

      Whether talking about New Zealand's radically altered flora and fauna or microbes' devastating effects on "immunologically naive" human populations, humans are part of the biological systems wherever we've gone. Tempted to think we're in charge? Dr. Crosby directs our gaze to New World cultures that fell to European diseases long before the first European fired a gun. Or more recently to the 25 million people around the world who died in the 1918-1919 influenza pandemic. Or to an all-too-likely new biological Armageddon.

      Were we any other species, Dr. Crosby would predict our astonishingly swollen human population doomed for a catastrophic population crash. But he insists we're not "any other species." We are intelligent. We are adaptable. And we can change. These attributes have catapulted us from our hunter-gatherer roots to every conceivable habitat on/under/above the planet. The question -- an urgent one -- is: Will we use our intelligence, adapt and change to limit population and restrict resource exploitation in time to avoid returning a tiny number of humans to a Paleolithic condition? We might....

Conversation 1

Dr. Alfred Crosby describes for Paula Gordon and Bill Russell the state of human beings 10,000 years ago -- the continental glaciers had retreated, we had spread out from Africa and cultural adaptations had equipped us to become the most wide-spread species on the planet.

Conversation 2

Homo sapiensâ transition from hunter-gatherer to farmer, Dr. Crosby assures us, was the most important thing ever to happen to our species. He compares it to the Roman Empire and Industrial Revolution. He explains why that transition was also a great moment in the history of the biosphere. He describes humans as the planetâs keystone species, with implications. He summarizes the results of all the other organisms we carried with us -- intentionally and, more importantly, unintentionally -- as we expanded over the planet with our ãportmanteau biotaä (portmanteau means suitcase or baggage). He gives examples of what happened when these various organisms, from diseases to horses to people, had exponential Malthusian population explosions.

Conversation 3

The relationship between humans and the other species in humanâs portmanteau biota is considered. Dr. Crosby explains why he chose to explore humansâ ãecological imperialismä instead of simply looking at the European expansion associated with Christopher Columbus and others. He explains why in the New World, Africans who came to the tropics were more important than Europeans ecologically. He offers New Zealand as an example of humans revolutionizing a land mass. He expands on the implications of human beingsâ two unique attributes: the ability to throw accurately and being the first creatures to control fire.

Conversation 4

Dr. Crosby gives vivid explanations of how diseases roll ahead of invading humans and have often been much more effective conquerors than the humans themselves. He gives examples of the devastating effects diseases have had on ãimmunologically naiveä indigenous human populations. ÊHe spotlights the Moundbuilders of the American Middle West. He compares their cities to European cities of the same era. Dr. Crosby explains the interaction between herd animals, plants and soil in explaining the role ãweedsä have played across the planet. He gives comparable examples of what happens when human populations are devastated. He reminds us of the many ways in which humans and their cultures are dynamic.

Conversation 5

With a nod to human cockiness, Dr. Crosby reminds us that the planet is really controlled by micro-organisms. Noting that human beings have transformed the global ecosystem, Dr. Crosby gives an idea of the enormous implications of that reality, citing the Great Influence Pandemic of 1918-1919 as a striking example. He wonders aloud why we have forgotten about it. He relates our experience with flu to how we reacted to the wide-spread appearance of AIDS. While we are inadvertently the planetâs keystone species, Dr. Crosby assures us we are not in command of whatâs going on. He elaborates, suggesting we get ready for a biological Armageddon. He assures us that we are still part of the ecosystem, albeit an intelligent part. ÊHe urges us to use that intelligence.

Conversation 6

Dr. Crosby believes we are more likely to advance using the power that culture gives us than to develop into a new species, especially because the move into agriculture effectively made us into Superman and Superwoman. If we were any other species, heâd predict we are doomed for an enormous population crash. But since we are intelligent, he hopes we can deal with our ecological problems, though he questions how much margin we have. He expresses confidence that we CAN change and address our vast overpopulation, but worries whether we can change fast enough to avoid returning humans to an essentially Paleolithic state.

Acknowledgements