Biogenesis Heath Biology Chapter 2 Abiogenesis or Spontaneous
Biogenesis Heath Biology, Chapter 2
Abiogenesis or Spontaneous Generation • Def: Living things can arise from non-living things • First stated by Aristotle (325 BCE), unquestioned for almost 2000 years • Supported by common observations • Examples: Bread --- Mold Meat --- Maggots Mud --- Frogs
Jan Baptist van Helmont (circa 1600) • Recipe for Mice: • 1) Place dirty rags in an open pot or barrel containing some grains of wheat or some wheat bran • 2) After 21 days, mice will appear • Note: There will be both male and female mice present. They will be capable of mating and reproducing more mice
Francesco Redi (1668) • Challenged the idea of spontaneous generation • Question: Why do maggots appear on the surface of decaying meat? • Experiment: • Phase 1: Allowed meat to decay in an open jar • After several days, maggots appeared • Continued observing for 20 days and the maggots formed hard shells around themselves • One week after this adult flies emerged from the shells
• Experiment (cont. ) • Analysis/Conclusion from Phase 1: • Redi recalled seeing the same type of flies on the meat earlier in the experiment • Concluded that the maggot was not a worm but in fact an early stage of a fly’s life • Phase 2: Experiment with a control group to test if maggots came from eggs laid by flies • First group of containers: Meat and left open • Second group of containers: Meat and sealed with a lid • Third group of containers: Meat and piece of cloth covering the jar
• Experiment (cont. ) • Results from Phase 2 : • Maggots only appeared on the meat of the jars left open and on the cloth net of the third group (none on group 2) • Analysis/conclusion from Phase 2 • Spontaneous generation dictates that ALL the meat should have maggots • It was also believed that air had an “active principal”, which would explain why the sealed jars (group 2) had no maggots • Group 3 should have therefore had maggots on the meat if spontaneous generation was correct as the air was able to reach the meat through the cloth
Why is Redi’s experiment impressive? • Given when it was conducted, it was a wonderful example of scientific rigor: • # of samples • # of trials • Variables • Controls • Use of non-biased observations • Repeated in different seasons
Anton von Leeuwenhoek (1675) • A step backwards/return to spontaneous generation • First use of the microscope in this line of experimentation • Leeuwenhoek studied pond water and using the microscope observed what we now call microorganisms • Leeuwenhoek named them “animalcules” and “cavorting beasties”
• Common experiments supporting “cavorting beasties” and spontaneous generation involved studying the growth of microorganisms in mixtures of water and food • Small food particles were mixed with water and then this water was strained, leaving a clear liquid (broth) • The broth became cloudy over time and when observed with the microscope contained countless microorganisms • Conclusion: The living organisms came from the non-living broth OR spontaneous generation
John Needham (1748) • Similar to Redi’s meat experiment • Meat Broth Experiment: • • • Known: Heat kills microorganisms Experiment: Meat broth was boiled for 5 -10 minutes then sealed with a cork Control: Second boiled broth left unsealed Results: Both containers had microorganisms reappear Conclusion: Spontaneous Generation
Lazzaro Spallanzani (1767) • Recognized a weakness in Needham’s experiment • Made two changes to the design • 1) Boiled the broth for 1 hour • 2) Used a melted glass seal instead of cork (porous) • Results: The broth in the sealed containers stayed free of microorganisms as long as the containers remained sealed • Objection: Sealing the containers did not allow access for the “active principal” • Conclusion: Spontaneous generation not disproven
Louis Pasteur (1861) • Successfully disproved Spontaneous Generation • Repeated Needham’s/Spallanzani’s experiments but used Sshaped flasks • Experiment: • Broth placed in S-shaped flasks and heated for 1 hour+ • As the flasks cooled, condensation formed in the “S” and trapped the air containing the microorganisms • Flasks were left unsealed, therefore “active principal” had access • Control: Broth placed in straight neck flasks and heated for 1 hour+ and left to cool
• Results: • Flasks with the S-curve did not develop microorganisms • Flasks without the S-curve did • When flasks with the S-curve were tipped to allow the liquid trapped in the neck to enter the flasks, microorganisms developed • Conclusion: Spontaneous Generation is disproven • Fun Fact: Some of Pasteur’s original flasks are still on display in Paris and still free of microorganisms (over 120 years later) • Bill Nye: Spontaneous Generation
Thomas Henry Huxley (1870) • Introduced the term Biogenesis • Definition: All living things arise from other living things of the same type • Biogenesis is still accepted in Biology today and is in fact one of the characteristics by which life is defined
- Slides: 16