Origin of life
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Origin of Life. Aristotle (384 –322 BC). Proposed the theory of spontaneous generation Idea that living things can arise from nonliving matter Idea lasted almost 2000 years. Spontaneous Generation.

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Aristotle 384 322 bc
Aristotle (384 –322 BC)

Proposed the theory of spontaneous generation

  • Idea that living things can arise from nonliving matter

  • Idea lasted almost 2000 years

Spontaneous generation
Spontaneous Generation

  • 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 1
Example #1

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 11
Example #1

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

Example 2
Example #2

Observation:In many parts of Europe, medieval farmers stored grain in barns with thatched roofs. As a roof aged, it was not uncommon for it to start leaking. This could lead to spoiled or moldy grain, and of course there were lots of mice around.

Example 21
Example #2

Conclusion:It was obvious to them that the mice came from the moldy grain.

Example 3
Example #3

Observation:In the cities centuries ago, there were no sewers, no garbage trucks, no electricity, and no refrigeration. Sewage flowed down the streets. Chamber pots and left-over food were thrown out into the streets each morning. Many cities also had major rat problems and a disease called Bubonic plague.

Example 31
Example #3

Conclusion:Obviously, all the sewage and garbage turned into the rats.

Example 4
Example #4

Observation:Since there were no refrigerators, the mandatory, daily trip to the butcher shop, especially in summer, meant battling the flies around the carcasses (dead animals). Typically, carcasses were “hung by their heels,” and customers selected which chunk the butcher would carve off for them.

Example 41
Example #4

Conclusion:Obviously, the rotting meat that had been hanging in the sun all day was the source of the flies.

Francesco redi 1668
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’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
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

Results of redi s experiments
Results of Redi’s Experiments

The results of this experiment disproved the idea of spontaneous generation for larger organisms, but people still thought microscopic organisms like algae or bacteria could arise that way.

The scientific method
The Scientific Method

  • Observation

  • Hypothesis

  • Experiment

  • Accept, Reject, or Modify hypothesis

Step 1 observation
Step 1 - Observation

  • There were flies around meat carcasses at the Butcher shop.

  • Where do the flies come from?

  • Does rotting meat turn into or produce rotting flies?

Step 2 hypothesis
Step 2 - Hypothesis

If rotting meat is exposed to flies, then flies will lay eggs in rotting meat, because baby maggots will need the rotting meat for food.

Step 3 experimentation
Step 3 - Experimentation

  • Wide-mouth jars each containing a piece of meat were subjected to several variations of “openness” while all other variables were kept the same.

  • Control group — These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop.

  • Experimental group(s) — One group of jars were sealed with lids, and another group of jars had gauze placed over them.

Step 4 data
Step 4 - Data

  • Presence or absence of flies and maggots observed in each jar was recorded.

  • Control group – flies entered, laid eggs, & maggots emerged

  • Gauze covered – flies on gauze, but not in jar

  • Sealed jars – No maggots or flies on the meat

Step 5 conclusion
Step 5 - Conclusion

Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms.

Anton van leeuwenhoek 1674
Anton van Leeuwenhoek (1674)

  • Leeuwenhoek began making and looking through simple microscopes

  • He examined pond water and observed tiny animals, fungi, algae, and single celled protozoa; “animalcules”

  • By end of 19th century, these organisms were called microbes

John needham 1745
John Needham (1745)

  • He showed that microorganisms flourished in various soups that had been exposed to the air

  • He claimed that there was a “life force” present in the molecules of all inorganic matter, including air and the oxygen in it, that could cause spontaneous generation to occur.

Needham s results
Needham’s Results

  • Needham’s experiments seemed to support the idea of spontaneous generation.

  • People didn’t realize bacteria were already present in Needham’s soups.

  • Needham didn’t boil long enough to kill the microbes.

Lazzaro spallanzani s 1765
LazzaroSpallanzani’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.


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

  • Therefore, spontaneous generation remained the theory of the time.

How do microbes arise
How Do Microbes Arise?

  • By 1860, the debate had become so heated that the Paris Academy of Sciences offered a prize for any experiments that would help resolve this conflict.

  • The prize was claimed in 1864 by Louis Pasteur, as he published the results of an experiment he did to disproved spontaneous generation in microscopic organisms.

Pasteur s problem
Pasteur's Problem

  • Hypothesis: If the dust in the air is not allowed into broth, then microbes will not exist in broth, because microbes live and feed on dust particles in the

  • Pasteur put broth into several special S-shaped flasks to trap dust particles.

  • Each flask was boiled and placed at various locations.

Pasteur s experiment step 1
Pasteur's Experiment - Step 1

  • Create S-shaped Flask - The special shape was intended to trap any dust particles containing bacteria.

  • Fill with broth

Pasteur s experiment step 2
Pasteur's Experiment - Step 2

  • Flasks boiled

  • Microbes Killed

Pasteur s experiment step 3
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

The theory of biogenesis
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

Origin of life

1668: Francisco Redi

Origin of life

1745: John Needham

Origin of life

1765: LazzaroSpallanzani

Origin of life

1861: Louis Pasteur