301
See also: Charles Darwin 142–49 ■ James Watson and Francis Crick 276–83 ■
James Lovelock 315
FUNDAMENTAL BUILDING BLOCKS
helped biologists to unlock the
inner working of cells. By the 1950s,
scientists knew that DNA provided
genetic instructions for carrying
out life processes and was relayed
from generation to generation. In
eukaryotic cells, DNA is packaged
in the nucleus, but it is also found in
chloroplasts and mitochondria.
In 1967, Margulis used this
discovery as evidence to revive and
substantiate the endosymbiosis
theory. She included the suggestion
that there had been an oxygen
“holocaust” in the early history of
life on Earth. About two billion
years ago, as photosynthesizers
flourished, they saturated the world
with oxygen, which poisoned many
of the microbes around at the time.
Predatory microbes survived by
engulfing others that could “soak
up” the oxygen in their energy-
releasing processes. These became
mitochondria: the “power packs” of
cells today. At first, this appeared
farfetched to most biologists, but
the evidence for Margulis’s theory
gradually became persuasive,
and it has now been widely
accepted. For example, the DNA of
mitochondria and chloroplasts are
made from circular molecules—just
like the DNA of living bacteria.
Evolution by cooperation was
not something new: Darwin himself
had conceived the idea to explain
the mutually beneficial interplay
between nectar-giving plants and
pollinating insects. But few had
thought it could happen so
intimately—and fundamentally—
as when cells merged together at
the very dawn of life. ■Lynn Margulis
Lynn Alexander (later Sagan,
then Margulis) entered
Chicago University at just 14,
before earning a PhD at
the University of California,
Berkeley. Her interests in the
cellular diversity of organisms
led her to revive and champion
the evolutionary theory
of endosymbiosis, which
biologist Richard Dawkins has
described as “one of the great
achievements of 20th-century
evolutionary biology.”
For Margulis, cooperative
interactions were as important
as competition in driving
evolution—and she viewed
living things as self-organizing
systems. She later supported
James Lovelock’s Gaia
hypothesis that Earth, too,
could be viewed as a self-
regulating organism. In
recognition of her work, she
was made a member of the
US National Academy of
Science and received the
National Medal of Science.Key works1967 On the Origin of
Mitosing Cells
1970 Origin of Eukaryotic Cells
1982 Five Kingdoms: An
Illustrated Guide to the Phyla
of Life on EarthSymbiosis is everywhere.
The complex cells of
animals and plants
contain organelles, which
are lacking in the simpler
cells of bacteria.The organelles—
nucleus, mitochondria,
and chloroplasts—
duplicate by division
of preexisting organelles.The DNA of chloroplasts
and mitochondria
is similar to that
of bacteria.These organelles
lived independent lives
before coming
together in the process
of endosymbiosis.