Spontaneous Generation vs. Biogenesis Research Biology

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

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

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

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

Disproving Spontaneous Generation

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

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.

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

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.

Did Redi Use the Scientific Method?

John Needham By 1745 an English scientist used microscopic observations to support the theory of abiogenesis. To test the theory he boiled meat broth for several minutes in a loosely sealed flask. ( allow to cool down the flask) Immediately after boiling he saw under the microscope that the broth had no living things. After a few days he examined the flask and found microrganism.

John Needham Abiogenesis Microorganisms

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.

Spallanzani’s Results

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

Needham 1745 Spallanzani 1765

Louis Pasteur (1822-1895)

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

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

Pasteur's Experiment - Step 2 Flasks boiled Microbes Killed

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

Pasteur's Experimental Results

The Theory of Biogenesis 1864 Pasteur’s S-shaped flask kept microbes out but let air in. Proved microbes only come from other microbes (life from life) – biogenesis Life come from life

Review Mr. Velázquez Biology

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?

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?

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