2. 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
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4. 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
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6. 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
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7. Example #1
Conclusion: It was perfectly
obvious to people back then
that muddy soil gave rise to
the frogs
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8. Example #2
Observation: In many parts of
Europe, medieval farmers stored
grain in barns with thatched
roofs (like Shakespeare’s house).
As a roof aged, it was not
uncommon for it to start leaking.
This could lead to spoiled or
moldy grain, and of course there
were lots of mice around.
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9. Example #2
Conclusion: It was obvious to
them that the mice came
from the moldy grain.
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10. Example #3
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.
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12. Example #4
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.
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13. Example #4
Conclusion: Obviously, the
rotting meat that had been
hanging in the sun all day was
the source of the flies.
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14. Abiogenesis Recipes
Recipe for bees:
Kill a young bull, and bury it
in an upright position so
that its horns protrude from
the ground. After a month,
a swarm of bees will fly out
of the corpse.
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15. Abiogenesis Recipes
Recipe for mice:
Place a dirty shirt or some rags in
an open pot or barrel containing a
few grains of wheat or some wheat
bran, and in 21 days, mice will
appear. There will be adult males
and females present, and they will
be capable of mating and
reproducing more mice.
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17. Francesco Redi (1668)
In 1668, Francesco Redi, an
Italian physician, did an
experiment with flies and wide-
mouth jars containing meat
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18. 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.
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19. 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.
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20. 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
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21. Results of Redi’s Experiments
The results of this
experiment disproved the idea
of spontaneous generation for
larger organisms, but people
still thought microscopic
organisms like algae or
bacteria could arise that way.
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25. Step 1 - Observation
There were flies around meat
carcasses at the Butcher
shop.
Where do the flies come
from?
Does rotting meat turn into
or produce rotting flies?
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26. Step 2 - Hypothesis
Rotten meat does not turn
into flies. Only flies can
make more flies.
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27. Step 3 - Testing
Wide-mouth jars each containing a piece of
meat were subjected to several variations of
“openness” while all other variables were
kept the same.
Control group — These jars of meat were
set out without lids so the meat would be
exposed to whatever it might be in the
butcher shop.
Experimental group(s) — One group of jars
were sealed with lids, and another group of
jars had gauze placed over them.
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28. Step 4 - Data
Presence or absence of flies
and maggots observed in each
jar was recorded.
Control group – flies entered,
laid eggs, & maggots emerged
Gauze covered – flies on gauze,
but not in jar
Sealed jars – No maggots or
flies on the meat
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29. Step 5 - Conclusion
Only flies can make more flies. In the
uncovered jars, flies entered and laid eggs
on the meat. Maggots hatched from these
eggs and grew into more adult flies. Adult
flies laid eggs on the gauze on the gauze-
covered jars. These eggs or the maggots
from them dropped through the gauze onto
the meat. In the sealed jars, no flies,
maggots, nor eggs could enter, thus none
were seen in those jars. Maggots arose only
where flies were able to lay eggs. This
experiment disproved the idea of
spontaneous generation for larger organisms.
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31. Anton van Leeuwenhoek (1674)
Leeuwenhoek began making and
looking through simple microscopes
He often made a new microscope
for each specimen
He examined water and visualized
tiny animals, fungi, algae, and
single celled protozoa;
“animalcules”
By end of 19th century, these
organisms were called microbes
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34. John Needham (1745)
Showed that microorganisms
flourished in various soups that
had been exposed to the air
Claimed that there was a “life
force” present in the molecules
of all inorganic matter, including
air and the oxygen in it, that
could cause spontaneous
generation to occur
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35. Needham’s Results
Needham’s experiments
seemed to support the idea
of spontaneous generation
People didn’t realize bacteria
were already present in
Needham’s soups
Needham didn’t boil long
enough to kill the microbes
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37. 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.
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39. Conclusion
Critics said sealed vials did
not allow enough air for
organisms to survive and that
prolonged heating destroyed
“life force”
Therefore, spontaneous
generation remained the
theory of the time
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41. How Do Microbes Arise?
By 1860, the debate had become so
heated that the Paris Academy of
Sciences offered a prize for any
experiments that would help resolve
this conflict
The prize was claimed in 1864 by
Louis Pasteur, as he published the
results of an experiment he did to
disproved spontaneous generation in
microscopic organisms
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43. Pasteur's Problem
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
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44. Pasteur's Experiment - Step 1
S-shaped Flask
Filled with broth
The special shaped was
intended to trap any
dust particles
containing bacteria
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46. 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
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48. The Theory of Biogenesis
Pasteur’s S-shaped flask kept microbes
out but let air in.
Proved microbes only come from other
microbes (life from life) - biogenesis
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48
50. Evidence Pro and Con
1668: Francisco Redi filled six
jars with decaying meat.
Conditions: Results:
3 jars covered with fine No maggots
net
3 open jars Maggots appeared
From where did the maggots come?
What was the purpose of the sealed jars?
Spontaneous generation or biogenesis?
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51. Evidence Pro and Con
1745: John Needham put boiled
nutrient broth into covered flasks.
Conditions: Results:
Nutrient broth Microbial growth
heated, then placed
in sealed flask
From where did the microbes come?
Spontaneous generation or biogenesis?
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52. 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?
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53. Evidence Pro and Con
1861: Louis Pasteur demonstrated
that microorganisms are present in
the air.
Conditions: Results:
Nutrient broth placed in Microbial growth
flask, heated, not
sealed
Nutrient broth placed in No microbial growth
flask, heated, then
sealed
Spontaneous generation or biogenesis?
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