300
SYMBIOSIS IS
EVERYWHERE
LYNN MARGULIS (1938 –2011)
C
harles Darwin’s theory of
evolution coincided with a
cellular theory of life that
emerged in the 1850s, asserting
that all organisms were made of
cells, and new cells could only
come from existing ones by a
process of division. Some of their
internal components, such as food-
making chloroplasts, apparently
reproduced by division too.
This last discovery led Russian
botanist Konstantin Mereschkowsky
to the idea that chloroplasts may
once have been independent life
forms. Evolutionary and cellular
biologists asked: how did complexcells arise? The answer lay in
endosymbiosis—a theory that was
first proposed by Mereschkowsky
in 1905, but was only accepted
after an American biologist called
Lynn Sagan (later Margulis)
furnished the evidence in 1967.
Complex cells with internal
structures called organelles—the
nucleus (which controls the cell),
mitochondria (which release
energy), and chloroplasts (which
conduct photosynthesis)—are
found in animals, plants, and many
microbes. These cells, now called
eukaryotic, evolved from simpler
bacterial cells, which lack
organelles and are now called
prokaryotic. Mereschkowsky
imagined primordial communities
of the simpler cells—some making
food by photosynthesis, others
preying on their neighbors and
engulfing them whole. Sometimes
the engulfed cells were left
undigested and, he suggested,
became chloroplasts—but without
proof, this theory of endosymbiosis
(living together and within) faded.New evidence
The invention of the electron
microscope in the 1930s, combined
with advances in biochemistry,Mitochondria are organelles that
make the energy-carrying chemical
adenosine triphosphate (ATP) inside
a eukaryotic cell. This mitochondrion
has been artificially colored blue.IN CONTEXT
BRANCH
BiologyBEFORE
1858 German doctor Rudolf
Virchow proposes that cells
arise only from other cells, and
are not formed spontaneously.1873 German microbiologist
Anton de Bary coins the term
“symbiosis” for different kinds
of organisms living together.1905 According to Konstantin
Mereschkowsky, chloroplasts
and nuclei originated by a
process of symbiosis, but his
theory lacks evidence.1937 French biologist Edouard
Chatton divides life forms by
cell structure, into eukaryote
(complex) and prokaryote
(simple). His theory is
rediscovered in 1962.AFTER
1970–75 US microbiologist
Carl Woese discovers that
chloroplast DNA is similar
to that of bacteria.