ZingPath: History of the Earth and Life

Spontaneous generation is the hypothesis that life comes from nonliving matter. For many years, this hypothesis was not challenged because it was consistent with everyday observations. Many people also thought it could be an explanation for the origin of life. In 1668, a scientist named Francesco Redi conducted an experiment to disprove spontaneous generation. Redi believed that maggots that appear in meat come from flies that land on it. Redi observed two sets of flasks with meat. One flask was open to the air so flies could enter. The other was covered with gauze and blocked out the flies. As Redi predicted, maggots only appeared in the meat that came in contact with the flies. Redi's experiment was one of the first of a series of experiments that tried to disprove spontaneous generation.

After the invention of the microscope, scientists were able to see tiny organisms that they could not see before like bacteria. Experiments were done to test the growth of these microorganisms. Scientists found that bacteria could grow in broth. They also found that boiling the broth kills them. From these observations, new experiments were conducted to test spontaneous generation.

When a flask of broth was left open to the air, bacteria appeared. People who supported spontaneous generation thought that the bacteria came from the broth. Lazzaro Spallanzani designed an experiment to test this hypothesis. Spallanzani used two flasks of broth. He completely sealed one flask of broth from the air. The other was left open. Bacteria only grew in the flask that was open to the air. The supporters of spontaneous generation thought that Spallanzani's experiment was flawed because air was necessary for life.

Louis Pasteur did a similar experiment in 1864, except he allowed air to enter the flask of broth. Pasteur also believed that the bacteria entered the flask through the air so he decided to filter the air before it entered the flask. He used an s-shaped flask and a ball of cotton attached to the top of the flask. If bacteria did pass through the cotton ball, it would settle at the bottom of the bend in the s-shaped neck or along the inner wall. He also used a control sample to compare his results to. The control sample was a flask with broth that was open to the air. Bacteria did not grow in the flask with the s-shaped neck and cotton ball but did grow in the open flask. His experiment disproved spontaneous generation because he showed that the bacteria in the air caused the growth of bacteria in the broth. The idea that life comes from living matter is called biogenesis.