Spontaneous generation refers to the theory that certain forms of life, such as flies, worms, and mice, can develop directly from nonliving things, such as mud and decaying flesh. This theory dates to prehistoric times and was widely accepted for thousands of years. It was challenged by scientific experiments, such as those performed by the Italian biologist Francesco Redi in 1668. Redi demonstrated that maggots (the young of flies) did not appear in meat from which adult flies were excluded. Previously, many people had believed that flies developed from decaying meat.
The theory of spontaneous generation was largely abandoned in the mid-1800’s. By then, improvements in microscopes and other scientific instruments had enabled scientists to see the eggs and sperm of higher animals, the ovules (eggs) and pollen of plants, and bacteria and other microorganisms. For example, in the mid-1800’s, the French scientist Louis Pasteur observed reproduction and growth in microorganisms. He demonstrated that the microorganisms would grow in sterilized broth only if the broth was first exposed to air that contained their spores (reproductive cells). Pasteur’s discoveries led to the development of the cell theory of the origin of living matter. The cell theory states that all life originates from preexisting living material.
Today, most scientists believe that spontaneous generation took place at least once—when certain chemicals came together to form the first simple living organism more than 3 billion years ago. This process is not thought to be occurring in nature today because conditions on the earth no longer favor such chemical combinations. In addition, any simple organisms that did form in this way would almost certainly fail to compete successfully against more complex existing organisms. However, laboratory experiments since the mid-1900’s have showed that many molecules found in living organisms can be synthesized (produced artificially). Most biologists believe that it will eventually be possible to produce simple forms of life in the laboratory.
See also Biogenesis; Life (Spontaneous generation).
