Chapter 1. Life: Chemical, Cellular, and Evolutionary Foundations

The scientific method shows that living organisms come from other living organisms.

Life is made up of chemical components that also occur in the nonliving environment and obey the same laws of chemistry and physics. Can life spontaneously arise from these nonliving materials? We all know that living organisms come from other living organisms, but it is worth asking how we know this. Direct observation can be misleading here. For example, raw meat, if left out on a plate, will rot and become infested with maggots (fly larvae). It might seem as though the maggots appear spontaneously. In fact, the question of where maggots come from was a matter of vigorous debate for centuries, until application of the scientific method settled the issue. In the 1600s, the Italian physician and naturalist Francesco Redi hypothesized that maggots (and hence flies) in rotting meat come only from other flies that laid their eggs in the meat.

To test his hypothesis, Redi set up an experiment in which he placed meat in three glass jars (Fig. 1.8). One jar was left open, a second was covered with gauze, and the third was sealed with a cap. The jars were left in a room with flies. Note that in this experiment, the three jars were subject to the same conditions—the only difference was the opening of the jar. The open jar allowed for the passage of flies and air; the jar with the gauze allowed for the passage of air but not flies; and the sealed jar did not allow air or flies to enter. Over time, Redi observed that maggots appeared only on the meat in the open jar. No maggots appeared in the other two jars, which did not allow access to the meat by flies. These observations supported Redi’s hypothesis that flies come from other flies, and did not provide support for the alternative hypothesis that maggots arise spontaneously from meat.

HOW DO WE KNOW?

FIG. 1.8

Can living organisms arise from nonliving matter?

BACKGROUND Until the 1600s, many people believed that rotting meat spontaneously generates maggots (fly larvae).

HYPOTHESIS Francesco Redi hypothesized that maggots come only from flies and are not spontaneously generated.

EXPERIMENT Redi used three jars containing meat. One jar was left open; one was covered with gauze; one was sealed with a cap.

RESULTS

FIG. 1.8

CONCLUSION The presence of maggots in the open jar and the absence of maggots in the gauze-covered and sealed jars supported the hypothesis that maggots come from flies and allowed Redi to reject the hypothesis that maggots are spontaneously generated.

FOLLOW-UP WORK Redi’s experiment argued against the idea of spontaneous generation for insects. However, it was unclear whether his results could be extended to microbes. Applying the scientific method, Louis Pasteur used a similar approach about 200 years later to investigate this question (see Fig. 1.9).

Redi demonstrated that living organisms come from other organisms, but some argued that his conclusion might apply only to larger organisms—microscopic life might be another matter entirely. It was not until the nineteenth century that the French chemist and biologist Louis Pasteur tested the hypothesis that microorganisms can arise by spontaneous generation (Fig. 1.9).

HOW DO WE KNOW?

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

FIG. 1.9

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.

Pasteur filled two glass flasks with broth that had first been sterilized over heat—one with a straight vertical neck and the other with a curved swan neck. As in Redi’s experiments, there was only one variable, in this case the shape of the neck of the flask. The straight-neck flask allowed airborne dust particles carrying microbes to fall into the sterile broth, while the swan-neck flask prevented dust from getting inside. Over time, Pasteur observed that microbes grew in the broth inside the straight-neck flask but not in the swan-neck flask. From these observations, Pasteur rejected the hypothesis that microbes arise spontaneously from sterile broth. Instead, exposure to microbes carried on airborne dust particles is necessary for microbial growth.

Redi’s and Pasteur’s experiments demonstrated that living organisms come from other living organisms and are not generated spontaneously from chemical components. But this raises the question of how life arose in the first place. If life comes from life, where did the first living organisms come from? Although today all organisms are produced by parental organisms, early in Earth’s history this was not the case. Scientists hypothesize that life initially emerged from chemical compounds about 4 billion years ago. That is, chemical systems capable of evolution arose from chemical reactions that took place on the early Earth. We’ll return to the great question of life’s origin in Case 1: The First Cell and in Chapters 2 through 8.