Spontaneous Generation and Experimental Design

Introduction

The goal of science is to investigate and understand the natural world, to explain the events in the natural world, and to use those explanations to make useful predictions. Scientists collect and organize information in a careful, orderly way, looking for patterns and connections between events. Scientists propose explanations which can be tested by examining evidence.

Scientific thinking usually begins with observation, the process of gathering information about events. The information gathered from observations is called data. There are generally two types of data. Quantitative data are expressed as numbers, obtained by counting or measuring. Qualitative data are descriptive and involve characteristics that can’t usually be counted, like color or appearance.

After initial observations, researchers will propose a hypothesis. A hypothesis is a proposed scientific explanation for a set of observations. A scientific hypothesis must be proposed in a way that enables it to be tested. Some hypotheses are tested by performing controlled experiments and/or gathering more data. Some hypotheses are ruled out while others are supported and confirmed. In most cases, the work of one researcher will be followed and improved upon, leading to new hypotheses and, sometimes, the rejection of previous hypotheses.

Whenever possible, a hypothesis should be tested by an experiment in which only one variable is changed at a time. All other variables should be kept unchanged or controlled. This type of experiment is called a controlled experiment. The variable that is deliberately changed is called the independent variable. The variable that is observed and that changes in response to the independent variable is called the dependent variable.

A good example of this is the belief in spontaneous generation. The history of this debate is outlined below:

Historical Background

            Around 2000 years ago, the idea that life could come from nonliving material (spontaneous generation) was accepted by most people. Maggots on meat, mice on grain, and beetles on cow dung were compelling pieces of evidence at the time.

            In the late 1660s, Francesco Redi noticed flies on meat prior to the appearance of maggots and hypothesized that they must have laid eggs. He predicted that keeping the flies away from the meat would prevent the appearance of maggots. He covered some meat-filled jars with gauze, while leaving others open. Maggots appeared in the uncovered jars only. He concluded that maggots form only when flies come in contact with meat, thus the spontaneous generation of maggots did not occur.

            In the mid 1700s, while trying to challenge Redi’s work, John Needham hypothesized that spontaneous generation could occur under the right conditions. He sealed gravy in a bottle and heated it to kill any living things. After a few days, the bottle was full of activity. This observation lead Needham to conclude that the microorganisms must have come from the juices of the gravy.

            Lazarro Spallanzani, after reading the work of Redi and Needham, decided that Needham hadn’t heated his samples enough. He boiled two jars of gravy and then sealed one immediately after boiling. After a few days, the unsealed jar was full of microorganisms while the sealed jar remained clean. Spallanzani concluded that the microorganisms were the offspring of microorganisms in the air, not the product of the gravy.

            Despite this evidence, many people still believed in spontaneous generation well into the 1800s; since air was considered essential to life, Spallanzani’s experiment was not seen as a fair one. In the mid 1860s, Louis Pasteur designed a flask with a long curved neck that would allow air to flow, yet it hindered the flow of dust and microorganisms. Broth was boiled and left in the flask for an entire year, where it remained free of microorganisms. The neck of the flask was then broken and, within a day, the broth became clouded with life. This evidence finally convinced the scientific community that spontaneous generation was incorrect. With the rejection of spontaneous generation, scientists’ views about living things changed dramatically. Pasteur demonstrated that all living things come from other living things.

Questions

1. Why do you think reasonable individuals once accepted the idea of spontaneous generation?

2. How does a hypothesis compare to a scientific theory?

3. For each of the four scientists

a) State their hypothesis.

b) State the independent variable.

c) State the dependent variable.

d) List any controlled variables.

4. What assumption did Needham make that made his results invalid?

5. A jar of salsa can remain in the cupboard for months. What can you infer from Pasteur’s work about why this jar, once opened, must be stored in the refrigerator?

6. Why is Redi’s experiment considered a controlled experiment?

7. Describe Pasteur’s flask. How did this unique design help him successfully refute the hypothesis of spontaneous generation?

8. What was the impact of Pasteur’s investigations on scientists’ ideas and people’s lives. (hint: look up pasteurization)

9. In looking at the plants in your home, you notice that the plants in one room are healthy while those in another room are struggling. Usually your mother waters and feeds the plants, but sometimes she forgets or asks your father to do it. You wonder what could be causing the difference. Propose a hypothesis to explain your observation and then design a controlled experiment to test your hypothesis.