The Birth of Experimental Biology

1
The Birth of Experimental Biology

• Spontaneous Generation
• vs.
• Biogenesis

2
Spontaneous generation

• Spontaneous generation was a widely-held idea by
  both scientists and non-scientists for a very
  long time.
• Historical documents contain recipes for
  creating creatures such as bees, frogs, mice,
  etc.
• These recipes were based on observations.
• When the Nile river flooded, soon many frogs
  appeared in the mud.
• If damp rags were left in a pile of wheat grain,
  many mice appeared 21 days later.

3
What did you see and how did it get there? Talk
to your partner and decide.
4
Spontaneous generation vs. Biogenesis

• Spontaneous Generation The belief that
  non-living material(s) give rise to living
  organisms.
• Biogenesis The principle that states that all
  living things come from other living things.

5
What do our experiments suggest?

• What conclusion could be supported by our data?

6
Spontaneous Generation is put to the test

• Francesco Redi (1668)
• The first scientist to design and carry out a
  controlled experiment to test the idea of
  spontaneous generation.
• Redis hypothesis (informal)
• Flies appear near rotting meat, not because the
  meat gives rise to flies, but because flies lay
  eggs on rotting meat.

7
Redis Experiment

• Control Group
• Meat left in jars with no covering.

• Experimental Group
• Meat left in jars that were covered.

8
Identify the Variables of Redis Experiment

• Independent Variable
• Remember, its what differs between the control
  and experimental groups.
• Covering on Jar
• Dependent Variable
• Remember, its what will be measured or observed
  to see if the hypothesis is supported or
  disproved.
• Appearance of flies inside the jar
• Controlled Variables
• Things that are kept the same in both groups.
• Type and size of jar, type and amount of meat,
  location of jars, time period, etc.

9
Redis Results and Conclusion

• The results of Redis experiment supported his
  hypothesis. No flies appeared on the meat in the
  covered jars if flies did not land on and lay
  eggs on the meat, no flies were produced.
• People were able to accept the idea that large
  organisms, could not arise spontaneously.
• such as flies, mice, and frogs
• Some still wondered about the spontaneous
  generation of microbes (microscopic organisms).
• Score
• Spontaneous Generation 0
• Biogenesis 1

10
Microbes

• Microbes are microscopic (very small), typically
  unicellular, organisms.
• Examples yeast, bacteria, etc.
• The invention of the microscope allowed
  scientists their first glimpse at a world of
  living creatures that surround us every day, yet
  go unnoticed.
• Many people, including scientists, thought that
  these simple organisms could arise from pond
  water, air, etc.
• In fact, if you place a bit of hay in water and
  look at the water under the microscope, initially
  it will be free of microorganims. 2-3 days later
  it will be teeming with them.

11
Another try, another hypothesis

• We will put hay in this water. When we learn to
  use microscopes, we will look to see what
  happens.
• What should be our control group?
• What should be our experimental group?
• What do you think we will find at the end of a
  couple of days?

12
The Spontaneous Generation debate heats up!

• Lazzaro Spallanzani (1767)
• a scientist that set up an experiment to test
  whether a vital force in the air could give
  rise to microorganisms.
• Spallanzanis hypothesis (informal)
• Beef broth becomes cloudy due to bacteria present
  in the air. The bacteria from the air enter the
  broth and contaminate it, rather than the
  bacteria arising from the air or broth itself.

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Spallanzanis Experiment

• Control Group
• Flask with boiled broth left open
• Experimental Group
• Flask with boiled broth sealed tightly.

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Spallanzanis Conclusion

• The result of Spallanzanis experiment supported
  his hypothesis. The beef broth only became
  contaminated with bacteria (cloudy)if the broth
  was exposed to air that contained living
  microorganisms.
• For many people, this experiment confirmed that
  simple organisms do not spontaneously arise
  either.
• But the debate wasnt settled yet! A fellow
  scientist, and others, said that the boiling
  killed the life force of the air in the flask.
  Supposedly the altered air inside the sealed
  flask lacked the life force and was not able to
  give rise to living bacteria.
• Score
• Spontaneous Generation 0
• Biogenesis 2

15
The Big Prize

• The Paris Academy of Science offered a prize to
  the person who could best resolve the spontaneous
  generation debate once and for all.
• And the winner was
• Louis Pasteur (1864)
• A scientist who designed an experiment using a
  flask that would allow air containing the vital
  force to enter the flask, but would keep
  microorganisms out.

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Pasteurs Experiment

• Designed a modified flask.
• The flask had a long neck that was curved like a
  side-ways S. (Control Group)
• The curved-neck flask allowed outside air to move
  into the flask containing boiled broth.
• The curve in the neck forced solid particles,
  such as microorganisms, to become trapped they
  could not enter the flask.
• More than a year later, for comparison, Pasteur
  broke the neck off of a flask. (Experimental
  Group)

17
Pasteurs Experiment
18
Pasteurs Conclusion

• The results of his experiment supported Pasteurs
  hypothesis. His curved-neck flask remained
  sterile for a year. This proved that air had no
  force with which to create life from non-living
  material. If the flask was opened, it quickly
  became contaminated by bacteria already present
  in the air.
• Score
• Spontaneous Generation 0
• Biogenesis 3
• The debate was finally settled! Biogenesis
  remains a key principle of Biology.