Spontaneous Generation
vs.
Biogenesis
Honors Biology
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
Early Science
• 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
Example of Observations
• Observation: Every year in the spring,
the Nile River flooded areas of Egypt
along the river, leaving behind nutrientrich 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
Example cont…
Conclusion: It was
perfectly obvious to people
back then that muddy soil
gave rise to the frogs
John Needham
By 1745 an English scientist used
microscopic observations to support the
theory of abiogenesis.
To test the theory he boiled meat broth
for several minutes in a loosely sealed
flask. ( allow to cool down the flask)
Immediately after boiling he saw under
the microscope that the broth had no
living things.
After a few days he examined the flask
and found microrganism.
John Needham
Abiogenesis
Microorganisms
Disproving
Spontaneous
Generation
Francesco Redi (1668)
• In 1668,
Francesco Redi,
an Italian
physician, did an
experiment with
flies and widemouth jars
containing meat
Redi’s Experiment
• Redi used open & closed flasks
which contained meat.
• His hypothesis was that rotten
meat does not turn into flies.
• He observed these flasks to see
in which one(s) maggots would
develop.
Redi’s (1626-1697) Experiments
Evidence against spontaneous generation:
1. Unsealed – maggots on meat
2. Sealed – no maggots on meat
3. Gauze – few maggots on gauze, none on meat
Redi’s Findings
• He found that if a flask was closed
with a lid so adult flies could not
get in, no maggots developed on the
rotting meat within.
• In a flask without a lid, maggots
soon were seen in the meat because
adult flies had laid eggs and more
adult flies soon appeared.
• Did Redi Use the
Scientific Method?
Lazzaro Spallanzani’s (1765)
• Boiled soups for almost
an hour and sealed
containers by melting
the slender necks closed.
• The soups remained
clear.
• Later, he broke the
seals & the soups
became cloudy with
microbes.
Spallanzani’s Results
Conclusion
• Critics said sealed vials did not
allow enough air for organisms
to survive and that prolonged
heating destroyed “vital force”
• “Vital force” needed to life to
form.
• Therefore, spontaneous
generation remained the theory
of the time
Needham 1745
Spallanzani 1765
Louis Pasteur (1822-1895)
Pasteur's Experiment
• Hypothesis: Microbes come from
cells of organisms on dust particles
in the air; not the air itself.
• Pasteur put broth into several
special S-shaped flasks
• Each flask was boiled and placed
at various locations
Pasteur's Experiment - Step 1
S-shaped Flask
Filled with broth
The special shaped was
intended to trap any dust
particles containing
• bacteria
•
•
•
•
Pasteur's Experiment
- Step 2
• Flasks boiled
• Microbes Killed
•
•
•
•
Pasteur's Experiment Step 3
Flask left at various locations
Did not turn cloudy
Microbes not found
Notice the dust that collected in
the neck of the flask
Pasteur's Experimental Results
The Theory of Biogenesis
1864
• Pasteur’s S-shaped flask kept
microbes out but let air in.
• Proved microbes only come from
other microbes (life from life) -
biogenesis
Review
Mr. Velázquez
Biology
Evidence Pro and Con
• 1668: Francisco Redi filled six jars
with decaying meat.
Conditions: Results:
3 jars covered with fine net - No maggots
3 open jars - Maggots appeared
From where did the maggots come?
What was the purpose of the sealed jars?
Spontaneous generation or biogenesis?
Evidence Pro and Con
• 1765: Lazzaro Spallanzani boiled nutrient
solutions in flasks.
Conditions:
Results:
Nutrient broth
No microbial
placed in flask,
growth
heated, then
sealed
Spontaneous generation or biogenesis?
Evidence Pro and Con
• 1861: Louis Pasteur demonstrated
that microorganisms are present in
the air.
Conditions:
Results:
Nutrient broth placed
Microbial growth
in flask, heated, not
sealed
Nutrient broth placed
No microbial growth
in flask, heated, then
sealed
Spontaneous generation or biogenesis?