Spontaneous Generation vs. Biogenesis Theory | Overview & Origins

Definition of Biogenesis

Where do kittens come from? What about baby birds? No, these aren't philosophical questions. Kittens come from cats, and baby birds come from bird eggs. Along the same lines, microorganisms, or living things that are too small to see with the naked eye, come from other microorganisms. That seems pretty obvious, right? Well, it hasn't always been that way.

In fact, the origin of microorganisms was a major debate following their discovery in the 1670s by Antoni van Leeuwenhoek. It wasn't until the 1850s to 1860s, nearly 200 years later, that scientists came up with the theory that all living organisms have to come from other living organisms. This theory is called biogenesis because bio means 'life' and genesis means 'beginning.'

Background on Biogenesis

Let's back up just a little bit. What did people believe during those 200 years after Leeuwenhoek's discovery? The answer is something called spontaneous generation, which is when living things simply appear, usually because of food. In fact, people used to believe this about more than just microorganisms. For example, if food wrapped in cloth were left in a corner, mice would show up. Their appearance was explained as spontaneous generation, meaning that they literally appeared there.

By the time microbes, another word for microorganisms, were discovered, few people believed higher order animals like mice came from spontaneous generation. However, it was the reigning belief for nearly 200 years that microbes were produced this way.

Theory of Biogenesis

Some scientists were skeptical of the spontaneous generation hypothesis, especially in later decades. In 1858, a scientist named Rudolf Virchow came up with a counter-hypothesis, claiming that life can only come from life. He called his hypothesis, you guessed it, biogenesis. However, he did not have any experiments to back it up. In 1861, Louis Pasteur solved this problem by setting up his own experiments to test the biogenesis hypothesis. His and future experiments were successful, which is why biogenesis is now a solid theory, and not just a hypothesis.

Pasteur's experiments were designed to prove that microbes live in the air and can contaminate food and liquid, but that the air itself is not the original source of these microbes. They do not just appear. First, he boiled beef broth in several different containers. Even at that time, people knew that heat killed microscopic organisms, and Pasteur's experiments further supported this.