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Early Ideas About the Origin of Life

• The principle of “biogenesis” states that: All living things come from pre-existing life

• Prior to the 17th century, scientists believed in the idea of spontaneous generation

  • The idea that living things could arise from nonliving things

    • ex.: Maggots arise from dead, rotted meat. Mice arise from grain stored in a barn. Beetles arise from cow dung.

Francesco Redi - 1668

• Did not believe in spontaneous generation and conducted an experiment to disprove it

• It was believed at the time that maggots and flies arose from rotting meat. Redi wanted to disprove the idea that flies are produced spontaneously from rotted meat.

• Experiment: An open jar of meat, and a closed jar of meat were left to sit. (One with mesh on top was later added).

• Results: The cloth allowed air to enter, which was considered to be the “vital force”, but prevents the flies from landing on the meat (maggots appeared because eggs were laid on the mesh and fell through.) No flies or maggots appears in the other covered jar, only in the open.

Lazzaro Spallanzani (mid 1700’s)

• During Redi’s experiment, the microscope was invented, revealing the world is teeming with microorganisms. Many scientists believed that microorganisms appeared by spontaneous generation from the air.

• Spallanzani designed an experiment to test the hypothesis of the spontaneous generation of microorganisms.

  • Hypothesis: Microorganisms are not formed from air, but arise from other microorganisms.

  • Spallanzani reasoned that boiling broth would kill the microorganisms. He took broth in two flasks, boiled the brother, and sealed one by melting the neck of it. The broth in the sealed flask remained clear after several days, while the open flask became cloudy due to contamination with microorganisms.

  • Concluded that the boiled broth in the open flask became contaminated with microorganisms from the air. Since no microorganisms appeared in the closed flask, microorganisms were not produced by spontaneous generation.

  • The idea of spontaneous generation was kept alive for another century, due to people believed air was the “vital force” that allowed generation.

Louis Pasteur (mid 1800’s)

• Disproved spontaneous generation

  • Took a flask with broth, the flask having a long, curved neck. He boiled it.

  • He left it for a whole year, and no microorganisms appeared. When he broke off the curved neck, the broth became cloudly with microorganisms within one day.

  • Scientists who believed in spontaneous generation could not object since the “vital force” air was still allowed to enter the flask. The curved neck was open to the air, but the curved neck prevented solid particles containing microorganisms from entering the flask. Once the neck was taken off, microorganisms entered the flask and the broth immediately became cloudy with microorganisms.