The biogenesis theory claims that all living things arise from living things. This theory is completely opposite to the spontaneous generation theory. But before we go into the details of the theory of biogenesis, let us understand the spontaneous theory of generation.
Spontaneous Theory of Generation
Long before 1900's, people believed that organisms could evolve from non-living things. One of the early thinkers who believed that non-living things could spontaneously give rise to living things was Aristotle. For example, it was a common belief that logs gave rise to crocodiles, dirty sewers gave rise to rats, maggots came from dead bodies, wet soil lead to creation of toads, etc. This spontaneous theory of generation had a strong belief system among people over many centuries.
A Dutch tradesman and amateur microbiologist, Antoni van Leeuwenhoek, observed small organisms in dirty water and some material he scrapped from his teeth. These organisms were called animalcules, what we call as protozoans today. This discovery took Europe by storm and scientists were thrilled to find these animalcules. The most pricking question in the minds of many was about the origin of these tiny creatures. This doubt had only one answer, the one provided by the spontaneous theory of generation.
Over the years, many intelligent minds came up with theories that defied the theory of spontaneous generation. An Italian physician and naturalist, Francesco Redi, could prove in 1668 that life cannot evolve spontaneously. However, people who supported biogenesis did not think that Redi's theory was applicable to microbes. Many other scientists continued their attempts to dissolve the spontaneous theory of generation.
However, in 1745 an English biologist, John Needham, added chicken broth to a flask and allowed it to cool. After few days, microbes did grow in the broth, and Needham proposed it as a proof of spontaneous theory of generation. Needlam claimed that vital life is needed for spontaneous generation of animalcules. His claim was challenged by the Italian biologist Lazzaro Spallanzani in 1768, who repeated the experiment in a sealed container. He found that there was no growth of microbes in the container.
Then, in 1858, a German doctor and pathologist, Rudolf Virchow, challenged the spontaneous generation theory by proposing the theory of biogenesis. He stated thatm "living cells can arise only from pre-existing living cells". This theory partly explained the presence of animalcule under the microscope. However, without any concrete scientific evidence, Virchow's theory was not accepted by everyone.
Louis Pasteur and Theory of Biogenesis
Louis Pasteur was a French chemist and microbiologist who is well-known for the principle of vaccination, fermentation, and pasteurization. He was the first person to successfully prove the theory of biogenesis. He proposed that there are organisms present in the air that are not visible to the naked eye. However, he emphasized on the fact that air did not give rise to living things. He carried out an experiment that would dissolve the spontaneous theory of generation and prove the theory of biogenesis.
In his experiment, Pasteur heated a number of short-necked flasks containing beef broth. After heating the flasks, he immediately sealed the mouths of some flasks and left a few unsealed. After a few days, microorganisms were observed in the broth that was in unsealed flasks. The sealed flasks had no microorganisms in them. This proved that the creatures were present in the air and could contaminate the unsealed beef broth.
Now, to prove that air did not give rise to organisms, he demonstrated another experiment. He again filled long-necked flasks with beef broth. The necks of the flasks were bent into S-shaped curves. These flasks were then heated to kill any presence of life in the beef broth. He then left the flasks as they were for a few days without sealing them. This way, the air could reach the broth as it was not blocked. After a few days, Pasteur observed the broth and found no microorganisms in it. This showed that the air can access the broth, but the organisms are trapped in the S-shaped neck of the flask and thus are not able to reach it.
This simple and ingenious experiment was enough to break the age-old theory of spontaneous generation. The world could now understand that only a living thing can give rise to another living thing. Thus, the belief of non-living things giving rise to living things was shattered by a simple experiment conducted by Louis Pasteur.