seas as a key supporting argument
for evolution. Wallace noted, for
example, the ocean straits that
produced a sharp division between
the flora and fauna of Australasia
and Southeast Asia.
With a better understanding of
Earth’s biogeography, 20th-century
ecologists divided the planet into
biomes—broad communities of flora
and fauna that interact in different
habitats, such as tropical rain forests
or tundra. Botanist Leslie Holdridge
refined the concept in 1947 with his
life zone classification, in which he
mapped zones based on the two
crucial influences on vegetation:
temperature and rainfall.A “whole Earth” approach
The word “biosphere” was coined
by Austrian geologist Edward
Suess in 1875 to signify all theareas at or near the surface of the
Earth where organic life can exist.
In 1926, the Russian geochemist
Vladimir Vernadsky explained
the biosphere’s close interaction
with the planet’s rock (lithosphere),
water (hydrosphere) and air
(atmosphere). This in turn led
American biologist Eugene Odum
to advocate a holistic approach to
ecology. Odum argued that it was
not possible to understand a single
organism, or a group of organisms,
without studying the ecosystem in
which they live. He described this
view as “the new ecology.”
In 1974, British scientist James
Lovelock advanced the Gaia
hypothesis that the interaction
of living and nonliving elements
in the biosphere reveal Earth to be a
complex, self-regulating system that
perpetuates the conditions for life.Almost two centuries earlier Hutton
had articulated a similar idea—that
biological and geological processes
are interlinked and that Earth could
be viewed as a superorganism. In
Hutton’s words, “The globe of this
earth is not just a machine but also
an organized body as it has a
regenerative power.”Heading for extinction?
Life has survived on Earth for
billions of years, despite the ravages
of five mass extinctions. However,
environmentalists now question
whether it will survive another.
Indeed, some contend that a sixth
mass extinction has already started,
as a result of human activity. Yet,
if Lovelock’s Gaia theory is correct,
it seems likely that the planet will
endure—even if humans and many
other current life forms do not. ■THE LIVING EARTH
1980
Luis and Walter
Alvarez suggest that the
mass extinction of the
dinosaurs was caused
by a meteor strike.197
1947
Lesley Holdridge’s
biogeographic classification
maps 38 life zones where
vegetation is determined
by rain and heat.1939
In their book Bioecology,
Frederic Clements and
Victor Shelford popularize
the idea of the biome.1953
Eugene and Howard Odum
describe the living planet
as a global collection of
interlocking systems.1974
James Lovelock’s Gaia
hypothesis presents
Earth as a single,
self-regulating system.1970
The first Earth Day is
celebrated, and the idea
that humans should view
Earth holistically
grows in popularity.US_196-197_Ch_7_The_Living_Earth_Intro.indd 197 22/11/2018 17:50