Spontaneous Generation vs. Biogenesis

Aristotle (384 - 322 BC)

• Proposed the theory of Proposed the theory of spontaneous generationspontaneous generation

• Also called Also called abiogenesisabiogenesis• Idea that Idea that living things can living things can

arise from nonlivingarise from nonliving matter matter• Idea lasted almost Idea lasted almost 2000 2000

yearsyears

Early Science• For centuriesFor centuries, , people basedpeople based

their their beliefsbeliefs onon their their interpretations of interpretations of what they what they sawsaw going on in the world going on in the world around them without testing around them without testing their ideastheir ideas

• They They didn’t use the scientific didn’t use the scientific methodmethod to arrive at answers to to arrive at answers to their questionstheir questions

• Their conclusions were based on Their conclusions were based on untested observationsuntested observations

Example of Observations

• Observation:Observation: Every year in the Every year in the spring, the spring, the Nile River floodedNile River flooded areas of Egypt along the river, areas of Egypt along the river, leaving behind nutrient-rich mud leaving behind nutrient-rich mud that enabled the people to grow that enabled the people to grow that year’s crop of food. However, that year’s crop of food. However, along with the muddy soil, along with the muddy soil, large large numbers of frogsnumbers of frogs appeared that appeared that weren’t around in drier timesweren’t around in drier times

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

Disproving Disproving

Spontaneous Spontaneous

GenerationGeneration

Francesco Redi (1668)Francesco Redi (1668)

• In 1668, Francesco In 1668, Francesco RediRedi, an , an Italian physician, did an Italian physician, did an experiment with experiment with fliesflies and and wide-mouth jars containing wide-mouth jars containing meatmeat

Redi’s ExperimentRedi’s Experiment

• Redi used Redi used open & closed open & closed flasksflasks which which contained meatcontained meat..

• His His hypothesishypothesis was that was that rotten meat does not turn rotten meat does not turn into flies.into flies.

• He He observedobserved these flasksthese flasks to to see in which one(s) maggots see in which one(s) maggots would develop.would develop.

Redi’s FindingsRedi’s Findings• He found that if a flask was He found that if a flask was

closedclosed with a lid so with a lid so adult flies adult flies could not get in, no maggotscould not get in, no maggots developed on the rotting meat developed on the rotting meat within.within.

• In a flask In a flask without a lidwithout a lid, , maggots soon were seenmaggots soon were seen in the in the meat because adult flies had meat because adult flies had laid eggs and more laid eggs and more adult flies adult flies soon appeared.soon appeared.

Redi’s (1626-1697) Experiments

Evidence against spontaneous Evidence against spontaneous generation:generation:

1. 1. Unsealed Unsealed – maggots on meat– maggots on meat2. 2. SealedSealed – no maggots on meat – no maggots on meat3. 3. GauzeGauze – few maggots on gauze, none on – few maggots on gauze, none on

meatmeat

Francesco RediFrancesco Redi

•Did Redi Use the Did Redi Use the Scientific Method?Scientific Method?

•Disproving Disproving Spontaneous Spontaneous Generation of MicrobesGeneration of Microbes

Lazzaro Spallanzani’s Lazzaro Spallanzani’s (1765)(1765)

• Boiled soups for almost an Boiled soups for almost an hourhour and sealed containers by and sealed containers by melting the slender necks melting the slender necks closedclosed.

• The The soups remained clearsoups remained clear..• Later, he Later, he broke the sealsbroke the seals & &

the the soups became cloudysoups became cloudy with with microbes. microbes.

Spallanzani’s ResultsSpallanzani’s Results

• CriticsCritics said sealed vials did said sealed vials did not allow enough air for not allow enough air for organisms to survive and that organisms to survive and that prolonged heating destroyed prolonged heating destroyed “vital force”“vital force”

• ““Vital force” needed to life to Vital force” needed to life to form.form.

• Therefore, Therefore, spontaneous spontaneous generation remainedgeneration remained the the theory of the timetheory of the time

ConclusionConclusion

Louis Pasteur (1822-Louis Pasteur (1822-1895)1895)

Pasteur's ExperimentPasteur's Experiment

•Hypothesis:Hypothesis: Microbes come Microbes come from cells of organisms on from cells of organisms on dust particles in the air; not dust particles in the air; not the air itself.the air itself.

• Pasteur put broth into several Pasteur put broth into several special special S-shaped flasksS-shaped flasks

• Each flask was Each flask was boiled and boiled and placed at various locationsplaced at various locations

Pasteur's Experiment - Pasteur's Experiment - Step Step 11

•S-shaped FlaskS-shaped Flask•Filled withFilled with broth broth •The special shaped was The special shaped was • intended to intended to trap any dust trap any dust

particles containing particles containing •bacteriabacteria

Pasteur's Experiment - Pasteur's Experiment - Step 2Step 2

•FlasksFlasks boiledboiled• Microbes KilledMicrobes Killed

Pasteur's Experiment - Pasteur's Experiment - Step 3Step 3

• Flask left at various Flask left at various locationslocations

• Did Did not turn cloudynot turn cloudy• Microbes not foundMicrobes not found• Notice the Notice the dustdust that that

collected in the neckcollected in the neck of the of the flaskflask

Pasteur's Experimental Pasteur's Experimental ResultsResults

The Theory of The Theory of BiogenesisBiogenesis

• Pasteur’s Pasteur’s S-shaped flask kept S-shaped flask kept microbes out but let air inmicrobes out but let air in..

• Proved microbes only come Proved microbes only come from other microbes (life from from other microbes (life from life) - life) - biogenesisbiogenesis

ReviewReview

Evidence Pro and ConEvidence Pro and Con• 1668: 1668: Francisco RediFrancisco Redi filled six jars filled six jars

with decaying meat.with decaying meat.Conditions:Conditions: Results:Results:

3 jars covered with fine net - 3 jars covered with fine net - No No maggotsmaggots

3 open jars - Maggots appeared3 open jars - Maggots appeared

From where did the maggots come?From where did the maggots come?What was the purpose of the sealed jars?What was the purpose of the sealed jars?SSpontaneous generation or biogenesis?pontaneous generation or biogenesis?

Evidence Pro and Con• 1765: 1765: Lazzaro SpallanzaniLazzaro Spallanzani boiled boiled

nutrient solutions in flasks.nutrient solutions in flasks.

Conditions:Conditions: Results:Results:Nutrient broth Nutrient broth placed in flask, placed in flask, heated, then heated, then sealedsealed

No microbial No microbial growthgrowth

SSpontaneous generation or pontaneous generation or biogenesis?biogenesis?

Evidence Pro and ConEvidence Pro and Con• 1861: 1861: Louis PasteurLouis Pasteur demonstrated that demonstrated that

microorganisms are present in the air.microorganisms are present in the air.

Conditions:Conditions: Results:Results:Nutrient broth Nutrient broth placed in flask, placed in flask, heated, not sealedheated, not sealed

Microbial growthMicrobial growth

Nutrient broth Nutrient broth placed in flask, placed in flask, heated, then sealedheated, then sealed

No microbial No microbial growthgrowth

SSpontaneous generation or biogenesis?pontaneous generation or biogenesis?