Related Lessons

Spontaneous Generation

• 1B The Nature of Science: Scientific Inquiry (6-8) #1


• 1C The Nature of Science: The Scientific Enterprise (6-8) #1, 3, 7


• 10I Historical Perspectives: Discovering Germs (6-8) # 1

• A Brief History of Microbiology
  
• The Slow Death of Spontaneous Generation (1668-1859)
  
• Benchmarks Online-The Nature of Science: The Scientific World View
  

• Each student will need a copy of the Science NetLinks student sheet, Spontaneous Generation: Building on Scientific Knowledge.
• Background-Spontaneous Generation, created by the biology department at Clermont College-(one copy)
  
• The following three experiments listed at Spontaneous Generation, a staff page on the Kent School District website (make copies of these experiments for the students).
  
• Francesco Redi's Experiment
• Lazzaro Spallanzani's Experiment
• Louis Pasteur 's Experiment
  

Independent Study:
If students complete this lesson independently, they will use Science NetLinks E-Sheet, Spontaneous Generation: Independent Study, to guide them through their learning. The independent study ends with a writing assignment that requires students to demonstrate an understanding of the Benchmark ideas. In order to understand what students should include in their writing assignment, be sure to read the Benchmarks, the entire lesson designed to be done in class (especially the Assessment section), and the background information listed above.


Motivation

Read aloud the first observation and conclusion from the Background - Spontaneous Generation page you printed ahead of time. Next, read the second observation, but NOT the conclusion.

Then ask students: Based on the first observation and conclusion, can you guess what people used to conclude? (Once students have made a few guesses, read the conclusion.)

Repeat this for the remaining observations. After students have named possible conclusions, read the conclusion and continue by asking the following questions:

• What do you think about ideas such as flies just coming from meat, or frogs from mud? (Students may laugh or find these conclusions unbelievable, but you should remind them that scientific knowledge is an ongoing process. You want to see whether students realize that there was no scientific evidence involved in the reasoning. Also, you may want to discuss that people look for reasons to explain outcomes whether there is scientific proof or not.)
  
  
• How do you think we've come to change these ideas? (This should give you insight to students' understanding of how scientific knowledge changes over time. Students should have some notion that someone along the way questioned the validity of the explanation and then tested it.)
  
  
• How can you prove that something is not true?
(Along with new ideas, come methods and experiments to test them. Here, get a feel for how students think scientific knowledge changes.)


• Can people just state new ideas without any science to back them up? (Of course they can, but they need valid experiments with good methodology to back them up to convince others. Sometimes the methodology is to disprove things, such as with spontaneous generation.)
  
  
• We know now that flies lay eggs on uncovered meat. We can't see the eggs, and back then people had little understanding of what they couldn't see (e.g., microscopic life). Do you think that if they'd known about microscopic things, they may have thought differently? (Here you can see if students understand the connection between new information, or a new theory and how it could cause reinterpretation of a situation. Screen for the misconception that students may NOT recognize the outcomes of changed theories -that they sometimes suggest new observations or reinterpretation of previous observations.)

• What was the goal of the experiments? (Many middle-school students think that experiments are conducted solely to produce a desired outcome. Talk about the motivation behind these three experiments. Was it to disprove spontaneous generation or to prove something else? If Redi had discovered that maggots do not come from meat, but not from flies either, would he have been disappointed?)
  
  
• How did the ideas about spontaneous generation change? (Students should think about how modification of scientific knowledge originates. Experiments and observations often have a hypothesis, or an idea that comes first, even if the idea is that the current thought may be wrong. If they don't see this right away, tell them to imagine being Redi, and to try to imagine what his thought process may have been. He was probably skeptical about spontaneous generation, but what would make him skeptical? Direct students to the questions and hypothesis at the top of each experiment.)
  
  
• Discuss Redi's "controlled" experiment. What is a controlled experiment? (Students should know that a controlled experiment is one in which as many things as possible are kept the same, while manipulating just one variable to discover the results. In Redi's experiment, what was that one variable? This was the covering of the samples-some were covered and others were not. Why is it important to only change one thing instead of two or more? Students should reason that it helps validate the experiment if only one thing has changed. If several have changed, the conclusion cannot be related to just one thing. In general, review with students the importance of controlling variables, as well as other techniques important for a good experiment.)

• Ask students to review how spontaneous generation was disproved. You may want to make a simple timeline on the board with the names of the three scientists, including information such as the following: In 1668, Redi conducted the meat covered/meat uncovered experiment to show that living things do not come from nonliving things; In the late 1700s, Spallanzani used broth-filled flasks to prove that even microorganisms could not come from nonliving things. (His experiments were not believed because they did not let air go through the flasks and people thought air would grow microorganisms from nonliving things); and finally
  In the mid 1800's, Pasteur was able to allow air to enter a flask of sterile broth.
  
  
• Ask students to discuss some things that can bring about change in scientific knowledge. One point that is interesting to note is that discovery spurs discovery. For instance, the microscope played a crucial role in the concepts discussed in this lesson.
  
  
• Ask students how Pasteur's experiment would have been different (if at all) if Redi had not come before him. Or if the microscope hadn't been discovered.
  
  
• Ask students if they think that all scientific knowledge is subject to change (modification). The answer should be yes, however point out that a lot of knowledge is fairly stable.

The essays should demonstrate student understanding of:

• how new information challenges or changes prevailing beliefs and/or theories;


• how new ideas lead to new investigations; and
• that scientific knowledge is subject to modification.

Created :10/20/2001