1. Spontaneous Generation vs. Biogenesis

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

4. Early Science
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

5. Example of Observations
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

6. Example cont…
Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs

7. Disproving
Spontaneous
Generation

8. Francesco Redi (1668)
In 1668, Francesco Redi, an Italian physician, did an experiment with flies and wide-mouth jars containing meat

9. 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.

10. 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.

11. 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

12. Francesco Redi

13. Did Redi Use the Scientific Method?

14. Disproving Spontaneous Generation of Microbes

15. 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.

16. Spallanzani’s Results

17. Conclusion
Critics said sealed vials did not allow enough air for organisms to survive and that prolonged heating destroyed “vital force”
“Vital force” needed to life to form.
Therefore, spontaneous generation remained the theory of the time

18. Louis Pasteur ( )

19. Pasteur's Experiment
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

20. Pasteur's Experiment - Step 1
S-shaped Flask
Filled with broth
The special shaped was
intended to trap any dust particles containing
bacteria

21. Pasteur's Experiment - Step 2
Flasks boiled
Microbes Killed

22. 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

23. Pasteur's Experimental Results

24. 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

25. Review

26. Evidence Pro and Con
1668: Francisco Redi filled six jars with decaying meat.
Conditions: Results:
3 jars covered with fine net - No maggots
3 open jars - Maggots appeared
From where did the maggots come?
What was the purpose of the sealed jars?
Spontaneous generation or biogenesis?

27. Evidence Pro and Con Conditions: Results:
1765: Lazzaro Spallanzani boiled nutrient solutions in flasks.
Conditions:
Results:
Nutrient broth placed in flask, heated, then sealed
No microbial growth
Spontaneous generation or biogenesis?

28. Evidence Pro and Con
1861: Louis Pasteur demonstrated that microorganisms are present in the air.
Conditions:
Results:
Nutrient broth placed in flask, heated, not sealed
Microbial growth
Nutrient broth placed in flask, heated, then sealed
No microbial growth
Spontaneous generation or biogenesis?