The Science Book

158

flies, rather than spontaneously.
However, the significance of Redi’s
experiment was not appreciated,
and even Redi himself did not fully
reject abiogenesis, believing that it
did occur in certain circumstances.
Among the first makers and
users of the microscope for detailed
scientific study, Dutch scientist
Antonie van Leeuwenhoek showed
that some living things were so
small that they could not be seen
with the naked eye—and also that
the reproduction of larger creatures
depended upon similar microscopic
living entities, such as sperm.
Yet the idea of abiogenesis
was so deeply entrenched in the
minds of scientists that many still
thought that these microscopic
organisms were too small to
contain reproductive organs and
must therefore arise spontaneously.
In 1745, English naturalist John
Needham set out to prove it. He
knew that heat could kill microbes,
so he boiled some mutton gravy
in a flask—thereby killing its
microbes—and then allowed it to
cool. After observing the broth for a
time, he saw that the microbes had
come back. He concluded that they

had arisen spontaneously from the

sterilized broth. Two decades

later, Italian physiologist Lazzaro

Spallanzani repeated Needham’s

experiment, but showed that the

microbes did not grow back if

he removed air from the flask.

Spallanzani thought that the air

had “seeded” the broth, but his

critics proposed instead that air

was actually a “vital force” for the

new generation of microbes.

Viewed in the context of

modern biology, the results of

Needham’s and Spallanzani’s

LOUIS PASTEUR

experiments can be easily

explained. Although heat does

indeed kill most microbes, some

bacteria, for example, can survive

by turning into dormant, heat-

resistant spores. And most

microbes, as with most life, need

oxygen from the air in order to

derive energy from their nutrition.

Most importantly, however, these

sorts of experiments were always

vulnerable to contamination—

microscopic airborne microbes can

easily colonize a growth medium,

even after a brief exposure to the

atmosphere. So in fact, neither of

these experiments had addressed

conclusively the question of

abiogenisis, one way or another.

Conclusive proof

A century later, microscopes

and microbiology had advanced

far enough for it to became possible

to settle the matter. Louis Pasteur’s

experiment demonstrated that

there were microbes suspended

in air, ready to infect any exposed

surface. First, he filtered air through

cotton. Then he analyzed the

contaminated cotton filters

and examined the trapped dust

I intend to suggest that

no such thing as abiogenesis

has ever taken place in the

past, or ever will take place

in the future.

Thomas Henry Huxley

Pasteur’s swan-neck experiment

proved that a sterilized broth will remain

free of microorganisms as long as they

are prevented from falling into it from the air.

Air can get in

through tube

The broth is boiled to kill
any microoganisms in it.

When the broth cools

it remains free of

microorganisms.

Tilting the tube allows

microorganisms back

into the broth.

The microorganisms

quickly multiply again.

Microorganisms

get trapped in

the curve