First Organic Compounds

All of the elements found in organic compounds are thought to have existed on Earth and in the rest of the solar system when the Earth formed. But how and where were these elements assembled into organic compounds? An important hypothesis to solve this puzzle was proposed in the 1920s by two scientists: Soviet Alexander I. Oparin (1894-1980) and American John B. S. Haldane (1892-1964). They thought that the early atmosphere contained ammonia, NH3; hydrogen gas, H2; water vapor, H2O; and compounds made of hydrogen and carbon, such as methane, CH4.

According to Oparin, at high temperatures, these gases might have formed simple organic compounds, such as amino acids. When Earth cooled and water vapor condensed to form lakes and seas, these simple organic compounds would have collected in the water. Over time, these compounds could have entered complex chemical reactions, fueled by energy from lightning and ultraviolet radiation. These reactions, Oparin reasoned, ultimately would have resulted in the macromolecules essential to life, such as proteins.

Synthesis of Organic Compounds

Oparin carefully developed his hypotheses, but he did not perform experiments to test them. So, in 1953, an American graduate student, Stanley L. Miller (1930—), and his professor, Harold C. Urey (1893-1981), set up an experiment using Oparin's hypotheses as a starting point. Their apparatus, illustrated in Figure 14-6, included a chamber containing the gases Oparin assumed were present in the young Earth's atmosphere. As the gases circulated in the chamber, electric sparks, substituting for lightning, supplied energy to drive chemical reactions. The Miller-Urey experiment, and other variations that have followed, produced a variety of organic compounds, including amino acids.

Since the 1950s, scientists have used similar experiments to test and revise hypotheses about the origin of simple organic compounds. In such experiments, scientists have combined a variety of chemicals and energy sources to produce an assortment of organic compounds, including amino acids, ATP, and nucleotides. Scientists are convinced that basic organic compounds could have formed on early Earth in many ways.

Furthermore, scientists who study planet formation have proposed new hypotheses about early Earth's atmosphere. For example, one hypothesis holds that the atmosphere of early Earth was composed largely of carbon dioxide, CO2; nitrogen, N2; and water vapor, H2O. Laboratory simulations of these atmospheric conditions have shown that both carbon dioxide and oxygen gas interfere with the production of organic compounds. Therefore, the production of organic compounds might only have been possible in areas protected from the atmosphere, such as those that exist in undersea hot springs.

Organic Compounds from Beyond Earth

Some scientists hypothesize that organic compounds could have been carried to Earth by debris from space. In 1970, a broad mixture of organic compounds was found in a newly fallen meteorite. Because the meteorite was recovered before it was contaminated with organic compounds from Earth, these compounds must have formed in space. So, organic compounds from space could have accumulated on the surface of early Earth.

The apparatus of the Miller-Urey experiment was intended to test Oparin's hypothesis about the conditions for formation of organic molecules on early Earth. Although similar experiments and hypotheses continue to be revised and tested, the experiment was an important milestone in this area of scientific investigation.

figure 14-7

Membrane-bound structures, such as these, have been formed in the laboratory under conditions that may have existed on early Earth. Structures such as these may have enclosed replicating molecules of RNA and may have been the forerunners of the first cells.

figure 14-7

Membrane-bound structures, such as these, have been formed in the laboratory under conditions that may have existed on early Earth. Structures such as these may have enclosed replicating molecules of RNA and may have been the forerunners of the first cells.

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