© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

7.3.1  Prebiotic Synthesis

For many years it was known that mixtures of carbon dioxide, ammonia and water vapor would produce small amounts of simple organic chemicals if energy was supplied. But the results of these experiments were generally very discouraging and the yields miniscule under these oxidizing conditions. To originate life in such a poor, thin broth would be well-nigh impossible.

In 1953 a graduate student named Stanley Miller, working under Nobelist Harold C. Urey at the University of Chicago, constructed an apparatus to imitate the conditions of the primitive Earth (Figure 7.1). Previous investigators had always assumed the atmosphere to be oxidizing or neutral. Miller and Urey, following the suggestions of A. I. Oparin in the Soviet Union and J. B. S. Haldane in Britain during the 1920’s, took the unprecedented step of devising a reducing environment instead.²²⁵⁸

Figure 7.1 Miller Apparatus for Prebiotic Synthesis²³¹⁵

In this schematic of the apparatus used in Stanley Miller's s historical experiment, a variety of organic compounds are synthesized as the atmosphere of methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapor (H₂O) is subjected to an electric spark discharge. Circulation is maintained in the system by the boiling water on one end and the condensing jacket on the ether.

After one week of continuous operation, the water was removed and tested by paper chromatography. A great abundance of amino acids and other organics was detected.

Miller mixed together methane, hydrogen, ammonia and water, and carefully eliminated all oxygen from the system. This gaseous concoction was then circulated past an electric spark discharge, followed by a water bath to simulate the primitive sea. After about one week of continuous operation, the "ocean" had turned a deep reddish-brown.

The experiment was halted and the contaminated water removed for analysis. Miller discovered to his amazement and delight that many amino acids had been produced in surprisingly high yields. Two percent of the total amount of carbon in the system was converted into glycine alone. Sugars, urea, and long tarlike polymers too complex to identify were also present in unusually high concentrations.

Of course, electrical energy was only one of the many sources of energy available on the primitive Earth (Figure 7.2). In fact, ultraviolet radiation was probably the principle source: UV would have been able to penetrate to the surface be cause the protective ozone layer in the upper atmosphere did not yet exist. A Miller-type experiment using ultraviolet rays and a reducing atmosphere was performed in 1957 by the German biochemists W. Groth and H. von Weyssenhoff at the University of Bonn.²³⁰⁷ Their results closely paralleled those obtained at the University of Chicago half a decade earlier.

Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth

Countless prebiotic simulations have since been achieved which confirm Miller’s original conclusions. One bibliography, current through 1974, lists more than three thousand papers on the subject.¹⁶⁷⁹ An exhaustive treatment of all of them is clearly beyond the scope of this book, but the interested reader in encouraged to dive into the literature (Table 7.1).

Table 7.2 lists the sources of energy believed to be present during the first eon or so of Earth’s history. Ultraviolet radiation leads the pack. Carl Sagan and others have completed experiments with UV which seem to indicate rather high yields for prebiotic amino acids, the building blocks of proteins. Over the first billion years of chemical evolution on this world something like a hundred kilograms of amino acids per square centimeter may have been produced, resulting in a "soup" of about 1% concentration. This is the approximate consistency of chicken bouillon.

But ultraviolet radiation is a two-edged sword. While it may be the most abundant form of energy for molecule building, it is also the most destructive. Early researchers were concerned that organics would be destroyed as fast as they were created. Fortunately, the primitive oceans probably turned opaque like the brownish glop in Miller’s apparatus rather quickly. Vital chemicals newly synthesized and carried a short distance beneath the surface of the soup by convection undoubtedly escaped decomposition.

Of the remaining energy sources, electrical discharge was the most potent. As much as 5-15% of the carbon in a mixture of methane, ammonia and water may be converted to amino acids and other organics by the energy of the discharge. Various forms of ionizing radiation give high yields as well. a particles, b particles, and g rays were common on the surface of the primitive Earth because of the presence of intense natural radioactive sources in the crust -- such as potassium-40, thorium-232, and isotopes of uranium.

Volcanic heat was another prebiotic power supply.^2368,2380 It has been shown that lava-heated seawater and underwater volcanoes may be effective in producing biologically important compounds. Heat and sonic energy would have been released by infalling meteorites -- certainly a significant factor in the environment of the primitive solar system.^1417,2375 In fact, experiments performed recently by Bar-Nun and others have conclusively demonstrated that as much as 30% of the nitrogen in an ammonia atmosphere can be converted into amino acids in this manner.^{315,1664,2375} Torrential rains have even been suggested as a possible source of energy for prebiotic synthesis, and experiments have shown that a flask of formaldehyde, allowed to stand for a few days at room temperature, will produce some simple sugars.

The great lesson appears to be that the exact nature of the power supply is relatively unimportant. Amino acids, sugars, and other chemical precursors to life probably arise on any planet possessing an initially reducing atmosphere and quantities of hydrogen, carbon, nitrogen and oxygen in gaseous reduced form -- regardless of the particular source, or sources, of energy available.*

* Other factors may also be important. For instance, early-type stars (F) are more likely to emit ultraviolet radiation in copious quantities than are late-type stars (K, M). The speed of chemical evolution in primitive planetary environments may actually slow as we move from class F through classes G to K stars among habitable solar systems.

Last updated on 6 December 2008