Last week I described how John Oro discovered that five hydrogen cyanide molecules (HCN) could react to produce adenine (H5C5N5) one of the primary components of nucleic acids.
Oro was born in 1923 in a small village in the Catalonia region of Spain, where he grew up. He did the equivalent of an undergraduate degree in Barcelona, then moved to Texas and completed his PhD degree at Baylor College of Medicine about the time that I was graduating from high school. In 1961, John caught everyone’s attention when he published a paper suggesting that comets were a major source of the organic compounds required for the origin of life.
This was the first such proposal, and was followed up thirty years later when Chris Chyba and Carl Sagan published a series of papers that analyzed the quantitative amounts that might have been delivered in this way.
Oro and I got to know each other as colleagues in the mid 1970s when we independently realized that the origin of membranes was a significant gap in our understanding of how life began. One of his experiments had already influenced my thinking about the kinds of organic molecules available on the early Earth. Oro and his students passed hydrogen gas and carbon monoxide over hot powdered metal made from a fragment of the Canyon Diablo meteorite. (This is the meteorite that produced the famous crater in the Arizona desert.)
The aim was to demonstrate that a well-known reaction called Fischer-Tropsch type (FTT) synthesis could be catalyzed by the iron-nickel content of an actual meteorite. In the FTT reaction, the CO gas molecules transiently adhere to the hot metal surface, which keeps them close enough together so that they can form linear hydrocarbon chains when they react with the hydrogen gas. When John and his students analyzed the products of their reaction, they discovered that the mixture contained a large number of hydrocarbons and hydrocarbon derivatives such as alcohols and acids.
Meanwhile, Ed Anders, working at the University of Chicago, was analyzing extracts of the Murchison meteorite and found a variety of hydrocarbons that resembled those produced in the Oro experiment. In a paper published in 1983 Anders and his co-workers suggested that the hydrocarbons were products of FTT reactions that occurred on the asteroid parent body of the Murchison.
Well, if hydrocarbons could be synthesized so easily, maybe they were also synthesized in volcanic conditions of the early Earth. Bernd Simoneit and his research group at Oregon State University decided to test this idea, but instead of passing carbon monoxide and hydrogen gas over hot iron they simply sealed formic acid or oxalic acid into a small stainless steel pressure chamber and heated it up. These two acids are one step up the chemical scale from carbon dioxide. If we type carbon dioxide (CO2) as COO, then formic acid is HCOOH, where COOH is an acidic carboxyl group. Oxalic acid is simply two carboxyl groups linked together: HOOC-COOH. Formic acid gets its name from the Latin word for ant (formica) which use it as a defensive agent.
Disturb an ant hill, and you can smell formic acid. And if I chew on the stem of the sorrel (Latin name Oxalis) that grows in our weedy back yard, I can taste the pleasantly sour oxalic acid that gives its name to the plant. The word oxygen, by the way, has a related derivation. “Oxy-gen” means something that generates acid, and if oxygen reacts with elemental sulfur, phosphorus or carbon, the products are H2SO4 (sulfuric acid), H3PO4 (phosphoric acid) and H2CO3 (carbonic acid) that is produced when CO2 dissolves and reacts with water.
The trick used by Simoneit is that formic and oxalic acid, when heated to oven temperatures, break apart into hydrogen gas (H2) carbon dioxide (CO2) and some CO, or carbon monoxide, which are the starting materials for the Fischer-Tropsch reaction. When the gaseous products were heated in the steel container to simulate the temperature and pressure in a volcano, Simoneit discovered that a remarkable mixture of fatty acids and alcohols was produced. Bernd sent me a sample of the products, and in my lab we found that they readily assembled into membranous vesicles the size of bacterial cells.
Before I summarize these results, I want to point out that hydrocarbons are by far the most stable organic carbon compounds required for life processes. For instance, the oil we call fossil fuel is several hundred million years old, and persists long after the proteins, carbohydrates and nucleic acids of the original organisms are degraded by heat and pressure. Because they are readily synthesized and fairly stable, I think that hydrocarbons were relatively abundant in the prebiotic environment, perhaps even forming oil slicks on the ocean surface and washing up on beaches.
A reasonable conclusion is that these hydrocarbon compounds -- long chain acids and alcohols -- provided the self-assembling membrane components required for the origin of cellular life.