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with a microscope. He found it
to be teeming with the type of
microbes that had been linked
with the decay and spoilage of
food. It looked as though infection
was caused when microbes literally
fell out of the air. This was the
critical information Pasteur needed
to succeed in the next step, when
he took up a challenge laid down by
the French Academy of Sciences—
to disprove the idea of spontaneous
generation once and for all.
For his experiment, Pasteur
boiled nutrient-rich broth—just as
Needham and Spallanzani had done
a century before—but this time
made a critical modification to the
flask. He heated the flask’s neck to
soften the glass, then drew the glass
outward and downward to form a
tube in the shape of a swan’s neck.
When the setup had cooled, the tube
was part-way directed downward so
that microbes could not fall onto the
broth, even though the temperature
was now suitable for their growth
and there was plentiful oxygen
since the tube communicated with
the outside air. The only way
microbes could grow back in the
flask was spontaneously—and this
did not happen.
As final proof that microbes
needed to contaminate the broth
from the air, Pasteur repeated the
experiment, but snapped off the
swan-necked tube. The broth
became infected: he had finally
disproved spontaneous generation,
and had shown that all life came
from life. It was clear that microbes
could no more spontaneously
appear in a flask of broth than
mice could appear in a dirty jar.
Abiogenesis returns
In 1870, English biologist Thomas
Henry Huxley championed
Pasteur’s work in a lecture entitled
“biogenesis and abiogenesis.”
It was a crushing blow to the
last devotees of spontaneous
generation, and marked the birth
of a new biology solidly founded
on the disciplines of cell theory,
biochemistry, and genetics. By the
1880s, German physician Robert
Koch had shown that the disease
anthrax was transmitted by
infectious bacteria.
Nevertheless, nearly a century
after Huxley’s address, abiogenesis
would once again focus the minds
of a new generation of scientists as
they began to ask questions about
the origin of the very first life on
Earth. In 1953, American chemists
Stanley Miller and Harold Urey
sent electrical sparks through
a mix of water, ammonia, methane,
and hydrogen to simulate the
atmospheric conditions at the dawn
of life on Earth. Within weeks, they
had created amino acids—the
building blocks of proteins and key
chemical constituents of living
cells. Miller and Urey’s experiment
triggered a resurgence of work
directed at showing that living
organisms can emerge from
nonliving matter, but this time
scientists were equipped with
the tools of biochemistry and an
understanding of processes that
took place billions of years ago. ■A CENTURY OF PROGRESS
I observe facts alone;
I seek but the scientific
conditions under which
life manifests itself.
Louis PasteurLouis Pasteur
Born to a poor French family
in 1822, Louis Pasteur became
such a towering figure in the
world of science that, upon
his death, he was given a full
state funeral. After training
in chemistry and medicine,
his professional career
included academic positions
at the French universities
of Strasbourg and Lille.
His first research was on
chemical crystals, but he is
better known in the field of
microbiology. Pasteur showed
that microbes turned wine
into vinegar and soured milk,
and developed a heat-treating
process that killed them—
known as pasteurization. His
work on microbes helped to
develop modern germ theory:
the idea that some microbes
caused infectious disease.
Later in his career, he
developed several vaccines,
and established the Institut
Pasteur devoted to the study
of microbiology, which thrives
to this day.Key works1866 Studies on Wine
1868 Studies on Vinegar
1878 Microbes: Their Roles in
Fermentation, Putrefaction,
and Contagion