FIG. 1.9

Can microscopic life arise from nonliving matter?

BACKGROUND Educated people in Pasteur’s time knew that microbes grow well in nutrient-rich liquids like broth. It was also known that boiling would sterilize the broth, killing the microbes.

HYPOTHESIS Pasteur hypothesized that if microbes were generated spontaneously from nonliving matter, they should reappear in sterilized broth without the addition of microbes.

EXPERIMENT Pasteur used two flasks, one with a straight neck and one with a swan neck. The straight-neck flask allowed dust particles with microbes to enter. The swan-neck flask did not.

RESULTS

CONCLUSION The presence of microbes in the straight-neck flask and the absence of microbes in the swan-neck flask supported the hypothesis that microbes come from other microbes and are not spontaneously generated.

DISCUSSION Redi’s and Pasteur’s research illustrate classic attributes of well-designed experiments. Multiple treatments are set up, and nearly all conditions are the same in them all—they are constant, and therefore cannot be the cause of different outcomes of the experiment. One key feature—the variable—is changed by the experimenter from one treatment to the next. This is a place to look for explanations of different experimental outcomes.