Spontaneous Generation vs. Biogenesis

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 People believed what they were told by “authorities” such as the Church, or the ancient Greek philosopher Aristotle Questioning Aristotle was like questioning the Church....

They didn’t use the scientific method to arrive at answers to their questions Their conclusions were based on untested observations

Aristotle (384 - 322 BC) Proposed the theory of spontaneous generation Idea that living things can arise from nonliving matter Idea lasted almost 2000 years

Examples of Spontaneous Generation

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

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

Example 2 Observation: In many parts of Europe, medieval farmers stored grain in barns with thatched roofs (like Shakespeare’s house). 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

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

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 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.

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

Francesco Redi

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.

Disproving Spontaneous Generation of Microbes

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

Louis Pasteur (1822-1895)

The Theory Finally Changes

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 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 Flasks boiled Microbes Killed

Pasteur's Experiment 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 Pasteur’s S-shaped flask kept microbes out but let air in. Proved microbes only come from other microbes (life from life) - biogenesis

Review

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?

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

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, not sealed Microbial growth Nutrient broth placed in flask, heated, then sealed No microbial growth

Origins of Life Oparin elaborated the first successful scientific approach to the problem of the origin of life, modernizing the old spontaneous generation controversy as a new hypothesis of biogenesis.

Oparin’s The Origin of Life To account for life on the Earth, several leading scientists even proposed that the first forms of life were delivered to the Earth from elsewhere but his basic premise was that living systems must have arisen from gases in the Earth’s early atmosphere combined to form more complex compounds.

Harold Urey & Stanley Miller In 1953, Harold C. Urey and his graduate student, Stanley L. Miller, conducted experiments that simulated hypothetical conditions present on the early Earth and test for the occurrence of chemical evolution

Life” Sparked in Test Tubes: The Miller-Urey Experiments Miller and Urey placed gases into the upper chamber thought to be present in Earth’s early reducing atmosphere, and applied a repeating spark to simulate lightning

Results Subsequent follow-up trials using various combinations of “primitive atmospheres,” produced even more complex organic compounds.