About 4 billion years ago, a collection of organic compounds somehow spontaneously organized themselves into a life form. But scientists are split on how this very important event occurred.
In a letter to John Hooker in 1871, Charles Darwin offered some speculation:
It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.
A test, of sorts, of Darwin's idea was the Stanley Miller/Harold Urey experiment in 1952, which sought to reproduce atmospheric conditions of the primitive Earth--methane, ammonia, and hydrogen gas floating over warm water representing the ocean. Sparks of electricity were introduced simulating lightning, and then the organic compounds that formed were analyzed. Lipids, sugars, and 11 of the 20 amino acids used in proteins were identified. In 2007, after Miller's death, the original vials from that experiment were re-analyzed using modern chromatography methods and mass spectrometry, which revealed over 25 different amino acids.
There is some evidence that rather than a warm little pond, the actual incubator of life was a very hot one. Experiments that add to Miller's mix hydrogen sulfide and sulfur dioxide, to represent volcanic eruptions, result in a much more diverse set of organic compounds. The discovery of the archaen thermophiles, bacteria that can live in hot springs as high as 120 degrees C, support the idea of a hot origin for life. Some thermophiles are anaerobes that use sulfur instead of oxygen as the electron acceptor during cellular respiration.
Hot biogenesis has some appeal, in that chemical reactions proceed much faster at high temperatures. On the other hand, organic compounds are more stable at cold temperatures. Psychrophiles are extreme organisms that live and carry on metabolic functions at as low as -15 degrees C, living proof that life is indeed possible at temperatures below freezing. Is it possible that life was actually born in a cold environment? Organic compounds have been found in meteorites, and various carbon and nitrogen compounds floating in interstellar space have been recognized by their spectroscopic signatures.
In a new study reported in the March 29 online version of Science, Fred Ciesla and Scott Sandford simulated the solar nebula of 4.5 billion years ago, at about the time planets were forming, and then followed the path of dust containing ice crystals of water, ammonia, methanol, and carbon doxide for a (simulated) million years. They determined that these would experience bombardment with high energy photons and temperatures high enough (>30 K) to form organic compounds. "These results imply that organic compounds are natural byproducts of protoplanetary disk evolution and should be important ingredients in the formation of planetary systems, including our own," according to the authors. In other words, the proto-Earth was preloaded with the building blocks of life.