1. Biogenesis vs.
Spontaneous Generation
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How do organisms come into being?

2. The History
• The principle of biogenesis, which states
that all living things come from other living
things, seems very reasonable to us
today.
• Before the 17th century, however, it was
widely thought that living things could also
arise from nonliving things in a process
called spontaneous generation.
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3. The History
• For centuries, people accepted spontaneous
generation as the explanation for the
sudden appearance of some organisms. For
example, it was believed that maggots arose
from the meat, mice from the grain, and
beetles from the dung. When fish appeared
in ponds that had been dry the previous
season, people thought mud might have
given rise to the fish.
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4. The Experiments
• In attempting to learn more about the
process of spontaneous generation,
scientists performed controlled experiments.
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5. Redi’s Experiment
• In the middle of the 17th century, the Italian scientist
Francesco Redi noticed and described the different
developmental forms of flies.
• Redi observed that tiny wormlike maggots turned into
sturdy oval cases, from which flies eventually emerge.
• He also observed that maggots seemed to appear where
adult flies had previously landed.
• In 1668, Redi proposed a different hypothesis: that
maggots came from eggs that flies laid on meat.
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6. Redi’s Experiment
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7. Redi’s Experiment
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8. New Discovery
• At about the same time that Redi carried out his
experiment, other scientists began using a new
tool—the microscope.
– Their observations revealed that the world is teeming
with tiny creatures.
– They discovered that microorganisms are simple in
structure and amazingly numerous and widespread.
– Many investigators at the time thus concluded that
microorganisms arise spontaneously from a “vital force”
or “life force” in the air.
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9. New Discovery
– Anton van Leeuwenhoek of the
Netherlands discovered a world of tiny
moving objects in rainwater, pond water,
and dust. Inferring that these objects
were alive, he called them “animalcules,”
or tiny animals. He made drawings of his
observations and shared them with other
scientists.
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10. Needham’s Experiment
• In the mid-1700s, John Needham, an English scientist, used
an experiment involving animalcules to attack Redi’s work.
• Needham claimed that spontaneous generation could occur
under the right conditions.
• To prove his claim, he sealed a bottle of gravy and heated it.
• He claimed that the heat had killed any living things that
might be in the gravy.
• After several days, he examined the contents of the bottle
and found it swarming with activity.
• He inferred that “these little animals can only have come
from juice of the gravy.”
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11. Spallanzani’s Experiment
• In the 1700s, another Italian scientist, Lazzaro Spallanzani,
designed an experiment to test the hypothesis of
spontaneous generation of microorganisms.
• He thought that Needham had not heated his samples
enough and decided to improve upon Needham’s
experiment.
• Spallanzani hypothesized that microorganisms formed not
from air but from other microorganisms.
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12. Spallanzani’s Experiment
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13. Spallanzani’s Experiment
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14. Spallanzani’s Experiment
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15. Spallanzani’s Experiment
• Spallanzani concluded that nonliving gravy did not produce
living things. The microorganisms in the unsealed jar were
offspring of microorganisms that had entered the jar through
the air.
• This experiment and Redi’s work supported the hypothesis
that new organisms are produced only by existing
organisms.
– Spallanzani’s opponents, however, objected to his method and
disagreed with his conclusions.
– They claimed that Spallanzani had heated the experimental flasks
too long, destroying the “vital force” or “life force” in the air inside
them. Air lacking this “vital force” or “life force,” they claimed, could
not generate life.
– Thus, those who believed in spontaneous generation of
microorganisms kept the idea alive for another century. 15

16. Pasteur’s Experiment
• By the mid-1800s, the controversy over
spontaneous generation had grown fierce.
The Paris Academy of Science offered a
prize to anyone who could clear up the
issue once and for all. The winner of the
prize was the French scientist Louis
Pasteur.
• In 1864, Pasteur found a way to finally
disprove the hypothesis of spontaneous
generation. He designed a flask that had a
long curved neck. The flask remained
open to the air, but microorganisms from
the air did not make their way through the
neck into the flask.
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17. Pasteur’s Experiment
• Pasteur boiled the flask thoroughly to kill any
microorganisms it might contain.
• Pasteur waited an entire year. In that time, no
microorganisms could be found in the flask.
• About a year after the experiment began, Pasteur
broke the neck of the flask, allowing air dust and
other particles to enter the broth.
• In just one day, the flask was clouded from the
growth of microorganisms.
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18. Pasteur’s Experiment
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19. Pasteur’s Experiment
• Pasteur had clearly shown that microorganisms had entered
the flask with particles from the air.
• His work convinced other scientists that the hypothesis of
spontaneous generation was not correct.
• With Pasteur’s experiment, the principle of biogenesis (all
living things come from other living things) became a
cornerstone of biology.
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20. The Impact of Pasteur’s Work
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• Pasteur made many discoveries related to
microorganisms. His research had an impact on
society, as well as, on scientific thought.
− He saved the French wine industry, which was troubled
by unexplained souring of wine, and the silk industry,
which was endangered by a silkworm disease.
− Moreover, he began to uncover the very nature of
infectious diseases, showing that they were the result of
microorganisms entering the bodies of the victims.
• Very short biographical video clip of Pasteur’s work:
http://www.biography.com/people/louis-pasteur-9434402