recap

4.2 recap

Life does not arise repeatedly through spontaneous generation, but comes from preexisting life. Water is an essential ingredient for the emergence of life. Chemical synthesis experiments provide support for the idea that life’s simple molecules formed in the prebiotic environment on Earth. Meteorites that have landed on Earth provide some evidence for an extraterrestrial origin of life, as do experiments with landers on Mars showing possible chemical interconversions that may reflect life.

learning outcomes

You should be able to:

  • Explain how Redi’s and Pasteur’s experiments disproved spontaneous generation.

  • Describe conditions on early Earth and theories about how these conditions affected the origin of life on Earth.

  • Justify the conditions used by Miller and Urey in their experiments.

Question 1

What conditions existing on Earth today might preclude the origin of life from the prebiotic molecules Miller and Urey used?

The presence of O2 in the atmosphere produces an oxidizing condition that prevents the reduction reactions observed in the Miller–Urey experiment

Question 2

The interpretation of Pasteur’s experiment (see Figure 4.6) depended on the inactivation of microorganisms by heat. We now know of microorganisms that can survive extremely high temperatures (see Chapter 26). Does this change the interpretation of Pasteur’s experiment? What experiments would you do to inactivate such microbes?

If microbes survived heat, the initial part of Pasteur’s experiment might begin with microbes already present. They would grow in both the open and closed flasks. To get the results that Pasteur did, his flasks must not have contained such microbes. An answer for the proposed experiment on heat-stable microbes might be to inactivate them using reagents, such as mercaptoethanol, that destroy proteins.

Question 3

The Miller–Urey experiment (see Figure 4.7) showed that it was possible for amino acids to be formed from gases that were hypothesized to have been in Earth’s early atmosphere. These amino acids were dissolved in water. Knowing what you do about the polymerization of amino acids into proteins (see Figure 3.6), how would you set up an experiment to show that proteins can form under the conditions of early Earth?

A suggested experiment might be to dry the samples after the Miller–Urey experiment (allowing condensation reactions—polymerization) and then apply energy in the form of heat. This condition might have existed in volcanic rock on early Earth.

Chemistry experiments modeling the conditions of ancient Earth provide clues about the origins of the monomers (such as amino acids) that make up the polymers (such as proteins) that characterize life. How did these polymers develop?