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Historical documents contain “recipes” for creating creatures such as: bees, frogs, mice, etc.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
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.
Redi’s hypothesis (informal):
Flies appear near rotting meat, not because the meat gives rise to flies, but because flies lay eggs on rotting meat.

7. Redi’s Experiment Control Group Experimental Groups Control Group
Meat left in jars with no covering.
Experimental Group
Meat left in jars that were covered.
Control Group
Experimental Groups

8. 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.

9. Redi’s Results and Conclusion
The results of Redi’s 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 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.
Spallanzani’s 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.

13. Spallanzani’s Experiment
Control Group:
Flask with boiled broth left open
Experimental Group:
Flask with boiled broth sealed tightly.

14. Spallanzani’s Conclusion
The result of Spallanzani’s 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 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.
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. Pasteur’s Experiment

18. 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.