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Aristotle (384 –322 BC)
• Proposed the theory of spontaneous generation
• Also called abiogenesis
• Idea that living things can arise from nonliving
matter
• Idea lasted almost 2000 years
Spontaneous Generation
• For centuries, people based their beliefs on their
interpretations of what they saw going on in the
world around them without testing their ideas
• They didn’t use the scientific method to arrive at
answers to their questions
• Their
conclusions
were
based
on
untested
observations
“Evidence” for Spontaneous Generation
Example 1
• Observation: Every year in the spring, the Nile River
flooded areas of Egypt along the river, leaving
behind nutrient-rich mud that enabled the people to
grow that year’s crop of food. However, along with
the muddy soil, large numbers of frogs appeared
that weren’t around in drier times
• Conclusion: It was perfectly obvious to people back
then that muddy soil gave rise to the frogs
Example 2
• Observation: In the cities centuries ago, there were
no sewers, no garbage trucks, no electricity, and no
refrigeration. Sewage flowed down the streets, and
chamber pots and left over food were thrown out
into the streets each morning. Many cities also had
major rat problems and a disease called Bubonic
plague.
• Conclusion: Obviously, all the sewage and garbage
turned into the rats.
Example
Observation: Since there were no refrigerators, the
mandatory, daily trip to the butcher shop, especially in
summer, meant battling the flies around the carcasses.
Typically, carcasses were “hung by their heels,” and
customers selected which chunk the butcher would carve
off for them.
• Conclusion: Obviously, the rotting meat that had been
hanging in the sun all day was the source of the flies.
Meat &
Maggots
He boiled broth in the flask to sterilize the
broth. He boiled it much longer than
Needham had done.
While the broth was still very hot he sealed
the flask.
No bacteria grew in the
sterilized broth.
Critics said that boiling it
too long had destroyed
the invisible “life source”
in the broth. They said
that cool fresh air was
needed to supply the
necessary magic life
force.
1862 - French
Scientist Louis Pasteur
was determined to
prove that even
microscopic life forms
came from others of
their own kind.
He was certain that even
bacteria came only from
other bacteria of the
same kind.
Previous experiments
had shown him that
there were bacteria
floating in the air.
But how could he let air into a flask without also
allowing bacteria to enter?
First , he put broth into several flasks.
He boiled the broth to sterilize the broth and the
flasks.
With a flame he
heated the neck of
the flasks to soften
the glass.
He pulled the softened glass
into a long swan neck shape.
Then he let the broth and flask
cool.
Although the bacteria are very
small, they still have mass.
He thought they would collect in
the bottom curve of the neck,
and air would move too slowly
up the neck to carry the bacteria
into the broth.
No bacteria grew in the broth. They were trapped
here.
Then , he broke the neck off several of the flasks.
Now bacteria in the air could enter the flasks!
Bacteria grew in the broth.
He had proven that all living things come from their
own kind and not from spontaneous generation!
Hooray!