History 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 They didn’t use the scientific method to arrive at answers to their questions untested observations

Examples of Spontaneous Generation

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

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.

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.

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

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

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

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 (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 disproved spontaneous generation by demonstrating that maggots come from fly eggs and not from meat.

The debate over spontaneous generation continued

John Needham (1745) In 1745, John Needham, an English clergyman, proposed an experiment. Everyone knew that boiling killed microorganisms, so he proposed to test whether or not microorganisms appeared spontaneously after boiling. He boiled chicken broth, put it into a flask, corked it, and waited - sure enough, microorganisms grew. Needham claimed victory for spontaneous generation.

Needham’s Results It was believed that Needham’s experiment was flawed, he did not boil the broth for long enough.

Disproving Spontaneous Generation of Microbes

Lazzaro Spallanzani’s [spah-luh n-zah-nee] (1765) Boiled soups for almost an hour and sealed containers by melting the slender necks closed. The soups remained sterile indefinitely. 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 “life force” Therefore, spontaneous generation remained the theory of the time

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

Pasteur's Experiment - Step 1 Pasteur put broth into several special S-shaped flasks The special shaped was intended to trap any dust particles containing bacteria

Pasteur's Experiment - Step 2 Each flask was boiled and placed at various locations 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 Pasteur’s S-shaped flask kept microbes out but let air in.

The Theory of Biogenesis Proved microbes only come from other microbes (life from life) - biogenesis Figure 1.3

The end