1. The History of Science
Unit 2 continued…

2. Think and Discuss
Over the years, can you think of any scientific theories or knowledge that has changed?
What does this tell you about science?

3. Where do living things come from?
Around 2,000 years ago, people believed that life came from non-living matter.
This is called “spontaneous generation”
Observations seemed to show that some living things could just suddenly appear.

4. Where do living things come from?
Maggots showed up on meat.
Mice were found in grain.
Beetles turned up in cow dung.

5. A Recipe for Making Bees:
Kill a bull during the first thaw of winter.
Build a shed.
Place the dead bull on branches and herbs inside the shed.
Wait for summer. The decaying body of the bull will produce bees.

6. Maggots appear on food that is left out in an open trash can during the summer.

7. Where do the maggots come from?
About 400 years ago, people began to question the idea of “spontaneous generation”.
In 1668, Francesco Redi proposed a different hypothesis for where the maggots came from.

8. Francesco Redi
Observed that maggots appeared on meat a few days after flies landed on the meat.
His hypothesis was that the flies laid eggs that were too small to see. The eggs hatched into maggots.

9. How would you test this hypothesis?

10. How to Design an Experiment
Variables are the factors in an experiment that can change.
Examples:
equipment used
type of material
amount of material
temperature
light

11. A Controlled Experiment
Only one variable is changed at a time.
All other variables are kept the same (they are controlled)
Manipulated (independent) variable: is changed on purpose
Responding (dependent) variable: changes in response to the manipulated variable
Control Group: the group that shows what would happen “normally” without treatment

12. Let’s talk about variables…
Manipulated
Responding
(Independent
(Dependent Variable)
Variable)

13. Francesco Redi’s Experiment
The Hypothesis: Keeping flies away from the meat will prevent maggots from appearing.
The Independent (Manipulated) Variable: Whether or not the jars were covered with gauze.
(diagram on page 9)

14. Francesco Redi’s Results
Maggots appeared on the meat in the uncovered jars (the control group jars)
No maggots appeared on the meat in the jars covered with gauze.
Francesco Redi’s Conclusion

15. Experiments must be Repeated!
Basic assumptions in science:
Nature is consistent.
Results can be reproduced.
Scientists test each other’s results.
Today, results must be published in a Scientific Journal and peer-reviewed to be considered valid (accepted).

16. John Needham’s experiment
Mid-1700s, John Needham attacked Redi’s work.
He claimed that spontaneous generation could happen in the right conditions.
He claimed that tiny “animalcules” came from gravy.

17. Needham’s gravy experiment
Experiment: He sealed a bottle of gravy, briefly heated it to kill any living things in the gravy, and then left it open in room temperature.
Results: After a few days, he examined the gravy with a microscope and found it swarming with activity.
His Conclusion: “These little animals can only have come from the juice of the gravy.”
Today, we know that the boiling time was insufficient to kill any endospores of microbes and the cooling of flasks left open to the air could cause microbial contamination. We would ascertain that Needham did not use proper sterile technique. His experiments were challenged and repeated by Lazzaro Spallanzani, an Italian scientist. Using a slightly different protocol (with a longer boiling time), Spallanzani did not have any microbes grow in his sealed flasks, contradicting Needham's findings.

18. Lazzaro Spallanzani’s Experiment
He repeated Needham’s gravy experiment.
BUT… Spallanzani boiled 2 flasks of gravy , one was sealed, the other was left open.
After a few days, the open flask had living microorganisms, but the sealed flask contained no microorganisms.

19. What about Spontaneous Generation?
During the 1800’s, some scientists continued to support the spontaneous generation hypothesis.
They argued that air contained the “life force” needed to produce new life.
They thought Spallanzani’s results weren’t accurate (should not be accepted) because air had been excluded from the sealed jar.

20. Pasteur’s test of Spontaneous Generation
During the 1800’s, some scientists argued that air contained the “life force” needed to produce new life.
In 1864, a French scientist named Louis Pasteur figured out how to solve the argument.
He designed a flask with a long, curved neck that would allow air into the flask, but would protect the flask from microorganisms in the air.

21. Pasteur’s discoveries were VERY important!
He saved the French wine industry from unexplained souring of wine.
He saved the silk industry, which was being harmed by a silkworm disease.
He began to uncover the nature of infectious diseases by showing that they were caused by microorganisms.

22. Robert Koch: Germ Theory of Disease
Previously, people thought that bacteria arose in sick animals AFTER infection.
However, Robert Koch ( ) proved that microscopic pathogens cause disease.
Steps of Experiment:
Isolated bacteria from sheep infected with anthrax
Placed bacteria on agar plates in Petri dishes so the colonies (groups of organisms) could reproduce.
Injected bacteria into healthy sheep
Result: Previously healthy sheep now had anthrax

23. Robert Koch: Germ Theory of Disease
After his experiments with anthrax, Koch came up with a series of steps that are needed to identify the micro-organism responsible for a particular disease.
These steps (Koch's postulates) can be used to identify the causative organism of an infectious disease.

24. Summarize:
Discuss and summarize how the work of Pasteur and Koch contributed to our understanding of the infectious nature of disease (germ theory of disease).
Discuss with a partner and write 5-7 sentences using your notes
(*to be turned in)