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Historical documents contain “recipes” for creating creatures such as: bees, frogs, mice, etc.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. 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.

4. Spontaneous Generation is put to the test
Francesco Redi (1698):
The first scientist to design and carry out a controlled experiment to test the idea of spontaneous generation.
Redi’s hypothesis:
Flies appear near rotting meat, not because the meat gives rise to flies, but because flies lay eggs on rotting meat.

5. Redi’s Experiment Two sets of flasks. Formal Hypothesis:
Both contain rotting meat.
One set is open to the air (and flies) = control group
The other set has a covering over the meat, preventing flies from landing on it = experimental group
Formal Hypothesis:
If flies only come from eggs laid by other flies, then no flies will appear on rotting meat kept in a sealed container.

6. Redi’s Experiment Control Group Experimental Groups

7. Identify the Variables of Redi’s Experiment
Independent Variable:
Remember, it’s what differs between the control and experimental groups.
Covering on Jar
Dependent Variable:
Remember, it’s 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.

8. Redi’s Conclusion
The results of Redi’s experiment supported his hypothesis. If the eggs of existing flies were not able to be laid on rotting meat, no flies appeared on the meat.
People were able to accept the idea that “large” organisms, such as flies, mice, and frogs could not arise spontaneously.
Some still wondered about the spontaneous generation of microbes (microscopic organisms).
Score:
Spontaneous Generation 0
Biogenesis 1

9. 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, 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 days later it will be teeming with them.

10. 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.
Spallanzani’s hypothesis:
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.

11. Spallanzani’s Experiment
Two sets of Flasks:
Boil beef broth in both sets. (Kill all microorganisms initially present.)
Leave one set of boiled broth open to the air = control group
Seal the flasks of the other set of boiled broth to prevent air from entering = experimental group
Formal Hypothesis:
If bacteria do not arise spontaneously from air, then the broth in the sealed flasks will remain sterile and clear.

12. Spallanzani’s Experiment

13. Identify the Variables of Spallanzani’s Experiment
Independent Variable:
Seal of flask
Dependent Variable:
Appearance of broth (clear or cloudy)
Controlled Variables:
Broth boiled for same amount of time, type and size of flask, type and amount of broth, location of flasks, time period, etc.

14. Spallanzani’s Conclusion
The result of Spallanzani’s experiment supported his hypothesis. The beef broth only became contaminated with bacteria 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 wasn’t 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.

16. Pasteur’s Experiment Designed a modified flask. Formal Hypothesis:
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)
Formal Hypothesis:
If air does not contain a “vital force”, then the broth in the curved-neck flask will remain clear of microorganisms.

17. Pasteur’s Experiment

18. Identify the Variables of Pasteur’s Experiment
Independent Variable:
Broken neck of flask
Dependent Variable:
Appearance of broth (clear or cloudy)
Controlled Variables:
Broth boiled for same amount of time, air can enter flask, type and size of flask, type and amount of broth, location of flasks, time period, etc.

19. Pasteur’s Conclusion
The results of his experiment supported Pasteur’s 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.