1. Divine or Special Creation Theory - Life originated from a supernatural power whom we call God
2. Abiogenesis or Spontaneous Generation - life originated spontaneously from non-living things
3. Cosmozoic or Interplanetary Theory - Life originated from outer planets in the form of a resistant spore propelled by radiation pressure, reached earth and started the first form of life
4. Philosophical Theory of Eternity - Life has no beginning and no end. It states that whatever forms of life we have now have actually been here right from the beginning of time
5. Marine Theory - Life originated from the sea
6. Naturalistic or Evolutionary or Physico-Chemical Theory - This is the most scientific and the most accepted theory. Life came about as a result of a chain of chemical reactions that gave rise to a mass of living protoplasm, which then gradually modified, giving rise to the present forms of life. This chemical evolution has never been duplicated.
7. Biogenesis - Life comes from other living things.
SCIENTIST Proving and Disproving some of these theories:
1. Francesco Redi’s Experiments (late 1600s)
Redi was and Italian physician and one of the first to formally challenge the doctrine of spontaneous generation. Redi's question was simple, “Where do maggots come from?”
According to abiogenesis, one would conclude that maggots came from rotting food. Redi hypothesized that maggots came from flies and designed an experiment, elegant in its simplicity, to challenge spontaneous generation.
Redi put meat into three separate jars:
•Jar #1 he left open. He observed flies laying eggs on the meat and the eventual development of maggots.
•Jar #2 he covered with netting. Flies laid their eggs on the netting and maggots soon appeared.
•Jar #3 he sealed. Flies were not attracted to this jar and no maggots developed on the meat.
This seems to be a clear demonstration of life giving rise to life. Yet it took another two hundred years for people to accept abiogenesis as a fallacy.
2. Anthony van Leeuwenhoek’s “Animalcules” (1600-1700s)
Leeuwenhoek was a Dutch cloth merchant, and due to his trade, frequently used lenses to examine cloth. Rather than employing lenses made by others, he ground his own, and the expertise that he gained through lens crafting combined with a curious mind eventually led to an interest in microscopy.
During his life, Leeuwenhoek assembled more than 250 microscopes, some of which magnified objects 270 times. Through magnification, he discovered presence of “micro” organisms--organisms so tiny that they were invisible to the naked eye.
He called these tiny living things “animalcules,” and was the first to describe many microbes and microscopic structures, including bacteria, protozoans and human cells.
3. John Needham & Lazzaro Spallanzani (1700s)
The debate over spontaneous generation was reignited with Leeuwenhoek’s discovery of animalcules and the observation that these tiny organisms would appear in collected rainwater within a matter of days. John Needham and Lazzaro Spallazani both set out to examine this apparent microscopic abiogenesis.
John Needham was a proponent of spontaneous generation, and his beliefs were confirmed when, after boiling beef broth to kill all microbes, within the span of a few days, cloudiness of the broth indicated the respawning of microscopic life.
Lazzaro Spallazani noted a flaw in Needham’s experiment. The containers holding Needham’s beef broths had not been sealed upon boiling. So Spallazani modified Needham’s experiment, boiling infusions, but immediately upon boiling he melted the necks of his glass containers so that they were not open to the atmosphere. The microbes were killed and did not reappear unless he broke the seal and again exposed the infusion to air.
4. Louis Pasteur Settles It (1800s)
Pasteur, a French scientist who made great contributions to our understanding of microbiology and for whom the process of “pasteurization” is named, repeated experiments similar to those of Spallazani’s and brought to light strong evidence that microbes arise from other microbes, not spontaneously.
Pasteur’s Swan-Necked Flasks
Pasteur created unique glass flasks with unusual long, thin necks that pointed downward. These “swan-necked” flasks allowed air into the container but did not allow particles from the air to drift down into the body of the flask.
The End of Abiogenesis
After boiling his nutrient broths, Pasteur found that these swan-necked containers would remain free of microbes until he either broke the necks of the flasks, allowing particles from the air to drift in, or until he tilted the flask so that the liquid came in contact with dust that had accumulated at the opening of the flask. It was these carefully controlled experiments of Pasteur that finally put to rest the debate over spontaneous generation.