Lecture 45: Human History and the Biosphere
to manipulate our surroundings, so collective learning necessarily implies an
increasing impact on the environment.In the Paleolithic era, the impact of this constant exploration of the
environment was limited. As “foragers,” Paleolithic humans consumed
natural products largely in their natural form. Populations were small and
communities were scattered, so their ecological “footprint” was small.Yet even in the Paleolithic era, the environmental impact of our species
was remarkable. Our Paleolithic ancestors learned how to exploit natural
environments throughout the world. As they did so, they developed new
techniques for dealing with different environments, from the tropics to the
tundra. Each new migration was a signi¿ cant technological achievement.
Some of the techniques developed in the Paleolithic era had a signi¿ cant
environmental impact. Foragers throughout the world ¿ red the land regularly
to increase plant growth and attract prey species. Over thousands of years,
such practices could change the mix of plants and animals over entire
continents. As their hunting techniques improved, our ancestors may also
have helped drive many large mammal species to extinction, particularly in
newly colonized lands such as Australia and the Americas, where local fauna
had no experience of dealing with humans. Evidence on the “megafaunal”
extinctions remains ambiguous, but the fact that these extinctions appear
to coincide roughly with the arrival of humans makes it likely that humans
played a signi¿ cant role.In the Agrarian era, humans began to transform their environments more
systematically. Agriculture requires systematic, large-scale manipulation of
the natural environment. Farmers transform environments so as to discourage
species they don’t need (which they call “weeds” or “pests” or “rodents”)
and encourage species they do need (which they call “domesticates”).
This may mean plowing (removing weeds and exposing fertile sub-soils)
or the deliberate elimination of pests such as wolves. It may require more
elaborate changes such as diverting entire rivers into arti¿ cial channels to
water crops in arid regions. Used badly, such methods could ruin the fertility
of soils over large areas, as seems to have happened in Sumer 4,000 years
ago. Swidden agriculture led to widespread deforestation. Farmers also
manipulated species of plants and animals through domestication, turning