The source of organic molecules

Almost everything about the origin of life is mysterious, but when we turn to the sources of organic chemicals such as amino acids and sugars we have almost an embarrassment of riches. There are broadly three candidates. (1) Organic molecules were made from inorganic ones on the Earth's surface. (2) They were present in the protoplanetary disc from which the Earth formed so they were (and, according to Gold, still are) trapped in the planet's fabric. (3) They were imported by way of meteorite impacts or cometary fragments.

(1) The first suggestion, manufacture from simple inorganic components of the primitive atmosphere, is historically important. The concept of a "primordial soup" in which organic compounds were formed and life originated was first proposed by Haldane in the 1920s, but it awakened scientific interest only in the 1950s, when an attempt was made to simulate prebiotic conditions in the laboratory. In 1952-3, Miller and Urey showed that if electric sparks (simulating lightning) were fired for several days through a gas mixture containing ammonia, hydrogen, carbon dioxide and water (allegedly representing the primitive Earth atmosphere), a tarry mixture formed that contained simple organic compounds such as amino acids and sugars. Miller and Urey were almost certainly wrong about the composition of the atmosphere; in reality it probably contained little or no hydrogen or ammonia, without which no amino acids would have formed in the experiment. As it was, Miller and Urey obtained only a few of the necessary amino acids. Moreover, a dilute solution of organic compounds in the prebiotic sea would hardly give rise to a rich "primaeval soup" from which life could have arisen, as they suggested. The importance of the Miller-Urey experiment is not that it elucidated the origin of life, but that it made it a subject of reputable scientific inquiry. It has retained this status ever since.

The argument behind the experiment was flawed but the conclusion that Miller and Urey drew might be valid. Volcanic vents rich in iron and nickel sulphides could have acted as primitive hydrogen sources, reducing nitrogen to ammonia, so there could have been enough ammonia locally in these environments. Amino acids and other organic compounds could therefore have been manufactured from inorganic materials in volcanic vents, particularly hydrothermal vents.

One variant of this idea holds that the organic products of vent reactions became trapped in iron sulphide bubbles, the precursors of cell membranes, the surfaces of which catalysed the formation of protein-like polymers from amino acids. Such bubbles could have formed at the interface between hot alkaline water from the vent and cold acidic sea water. The electrical potential across the iron sulphide membranes could have served as an energy source. This is an attractive possibility because amino acids do not polymerise efficiently if they are simply dissolved in water (though prolonged heating might help them to do so). Efficient polymerisation usually needs a solid surface. On the other hand, the hypothesis implies that proteins appeared before and independently of nucleic acids, which is not a currently popular view (see below).

  • 2) The second suggestion, that organic molecules were present when the Earth was formed, is supported by astronomical data and is the basis of Gold's idea about where life began (previous section). Infrared telescopy shows that simple organic molecules are widely distributed around the galaxy in interstellar dust: carbon monoxide, formaldehyde, methanol, polyaromatic hydrocarbons, and some amino acids. Some meteorites - a type known as carbonaceous chondrites - contain organic compounds including amino acids. Since these meteorites are believed to be remnants of the protoplanetary disc of the Solar System, the primitive Earth probably contained the same compounds.
  • 3) The third suggestion, that organic matter reached the primitive Earth surface via meteorite and comet impacts, is almost certainly true. It is supported by the same astronomical data as (2). Many comets are rich in simple organic compounds. The comet storms that scarred the planet in its youth were probably the source of most of the Earth's water (water is the main ingredient of most comets) so they could have been a major source of organic compounds as well. Even today, when impacts are very much rarer, some 50,000 tons of meteorite dust fall on the Earth every year, and this too contains traces of organic constituents33.

Whatever their source, it seems clear that simple organic molecules, the raw ingredients of life, were abundant on the prebiotic Earth. Most of them arrived ready formed, either native to the planet or delivered by meteorites. Some might have been made by Miller-Urey processes in such environments as volcanic vents. The provision of simple organic constituents is one facet of the origin of life that no longer seems problematic.

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