What is the Theory Of Spontaneous Generation?
After van Leeuwenhoek discovered microorganisms, the scientific community became interested in the origins of these tiny living things.
spontaneous generation, the hypothetical process by which living organisms evolve from nonliving matter; also the archaic theory that used this process to explain the origin of life.
Many believed in spontaneous procreation because it explained such occurrences as the appearance of maggots on rotting meat.
According to that theory, pieces of cheese and bread wrapped in rags and left in a dark corner, for example, were thus thought to produce mice, because after several weeks there were mice in the rags.
In the 18th century, it had become clear that higher organisms could not be created from inanimate material. However, the origin of microorganisms such as bacteria was not fully understood until Louis Pasteur proved in the 19th century that microorganisms multiply.
Early Discussion on Spontaneous Generation
The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation. He suggested that life emerges from inanimate material when the material contains pneuma (“vital heat”).
As evidence, he cited several cases of animal appearances from environments where such animals had not previously existed, such as the seemingly sudden appearance of fish in a new puddle of water.
This theory persisted until the 17th century when scientists undertook additional experiments to support or refute it. At the time, proponents of the theory cited that frogs simply appeared on the muddy banks of the Nile in Egypt during the annual floods.
Others observed that mice simply emerged among grain stored in thatched barns. When the roof leaked and the grain formed, mice appeared.
Jan Baptista van Helmont, a 17th-century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal sources of food and shelter for mouse populations to thrive.
Slow Death of Spontaneous Generation
From the earliest times, people believed in spontaneous procreation, that living organisms could develop from inanimate matter. This view was eventually challenged by the Italian physician Francesco Redi (1626-1697), who carried out a series of experiments on rotting meat.
Francesco Redi’s Experiment: Challenged the spontaneous generation theory over the First time
Italian physician Francesco Redi set out in 1668 to show that maggots did not arise spontaneously. Redi filled two glasses with rotting meat.
The first was left unsealed so that flies could lay eggs on the meat, which then developed into larvae. The second jar was locked and since the flies could not get in there were no maggots.
Yet Redi’s antagonists were not convinced; They claimed that fresh air is needed for spontaneous conception. So Redi set up a second experiment, Redi left the meat in each of the six containers, two were open to the air, two were covered with gauze, and two were tightly closed.
His hypothesis was supported when maggots developed in the uncovered jars, but maggots did not appear in either the gauze-covered jars or the tightly closed jars.
He concluded that maggots can only form if flies are allowed to lay eggs in the meat and that the maggots are the offspring of flies and not the product of spontaneous procreation. Redi’s results were a severe blow to the long-held belief that large forms of life could arise from non-life.
However, Leeuwenhoek’s communications on microorganisms renewed the controversy. Some suggested that microbes were created spontaneously, but larger organisms did not. They indicated that cooked hay or meat extracts formed microorganisms after prolonged sitting.
John Needham’s Experiment: Strengthened the spontaneous generation theory
The case for the spontaneous generation of microorganisms seemed to be confirmed in 1745 when John Needham found that the chilled solutions, even after heating chicken broth and corn broth before pouring them into sealed bottles, were soon teeming with microorganisms. Needham claimed that microbes developed spontaneously from the fluids.
Lazzaro Spallanzani’s Experiment: Challenged John Needham’s finding
Lazzaro Spallanzani did not agree with Needham’s conclusions. Twenty years later, he suggested that microorganisms from the air probably entered Needham’s solutions after they were boiled.
Spallanzani took sealed glass flasks that contained water and seeds and then placed the flasks in boiling water for about 45 minutes. He found that Heated but sealed flasks remained clear, without any signs of spontaneous growth, unless the flasks were subsequently opened to the air.
He proposed that air carried germs to the culture medium but also commented that external air might be required for the growth of animals already in the medium.
In response to Spallanzani’s findings, Needham responded by claiming the “vital force” necessary for a spontaneous generation had been destroyed by the heat and was kept out of the flasks by the seals.
Spallanzani’s observations were also criticized because there was not enough oxygen in the sealed flasks to support microbial life.
Franz Schulze and Theodor Schwann Experiment
Needham insisted that air was essential to the spontaneous production of microscopic beings. This argument was answered some 60 or 70 years later independently by two other investigators, Franz Schulze (1815-1873) and Theodor Schwann (1810-1882).
Schulze passed air through strong acid solutions into boiling infusions, whereas Schwann passed air into his flasks through red-hot tubes. In neither case did microbes appear.
But the die-hard advocates of spontaneous generation were still not convinced. Acid and heat altered the air so that it would not support growth, they said.
H. Schroder and T. von Dusch Experiment
In about 1850, H. Schroder and T. von Dusch performed a more convincing experiment by passing air through cotton into a flask containing heated broth. Thus the microbes were filtered out of the air by the cotton fibers so that growth did not occur, and a basic technique of plugging bacterial culture tubes with cotton stoppers was initiated
Louis Pasteur’s Experiment: Disapproved Spontaneous generation theory
Louis Pasteur is credited with conclusively disproving the theory of spontaneous generation with his famous swan-neck flask experiment. He subsequently proposed that “life only comes from life.”
Finally, in 1861, a French scientist named Louis Pasteur designed a series of flasks with the necks bent into an S shape.
The necks were fashioned so that fresh air could reach the flasks but were bent in such a way that any air-borne microbes would be trapped at the bottom of the curves. He boiled the broth inside the flask and did not see any microbes in the broth for many months.
When he eventually removed the top from the flask and left it off, he found the liquid to be teaming with microorganisms within a few days. Therefore, he proved that the microbes from which life arises are present within the air and are not spontaneously generated!
A detailed explanation of this experiment is given in the Theory of Biogenesis read more.