The concept of spontaneous generation, the notion that inanimate matter could suddenly become alive, goes back as far as Aristotle ... maybe further. "According to Descartes (viewing maggots in meat): "Since so little is required to make a being, it is certainly not surprising that so many animals, worms, and insects form spontaneously before our eyes in all putrefying substances" (Margulis and Sagan, 1995). However, the same text also states that "... all worms found in meat were derived from flies, not putrefaction" (Redi). Experiments, nevertheless, continued to support the concept until, in the mid 1800’s, it was finally disproved by Pasteur. Spontaneous generation was not a specifically evolutionary concept, simply the best explanation of a phenomenon that scientists had until Pasteur's experiments. Creationists claim, somewhat absurdly, that Pasteur was opposed by the biological establishment because of his opposition to spontaneous generation and to Darwinism. Yet, in a time of belief that at least some illnesses resulted from possession or sin, Pasteur, by recognising the microbial origin of disease, actually set the treatment of disease on a naturalistic path for the first time.
DiscussionAt some point in the distant past the first living cells formed ... exactly when, and by what mechanism, is as yet unknown. The oldest speculated life forms are, at least, 1 billion years old so we can be certain that it happened sometime before that. The origin of life was not a process of evolution but of abiogenesis and scientists have speculated on the possible ways in which it might have occurred. The earliest chemical steps are believed to have been as follows:
- The spontaneous formation of ribose. The probability for this to have occurred is good.
- The spontaneous formation of nucleotide bases. The probability for this to have occurred is good.
- The assembly of ribose and bases into nucleosides (probability of this occurring is low).
- The activation of these nucleosides into nucleotides. The probability for this to have occurred is good.
The Chemicals of LifeThere has been no particular mystery to the formation of the first organic chemicals ever since Friedrich Wöhler (1928) demonstrated that no special requirements were needed for the spontaneous synthesis of Urea. Since then the exploration of space has further demystified organic chemistry. Space is permeated by a tenuous “cloud” of microscopically fine particulate matter (typically referred to as “Interstellar Dust”) which contains carbon, hydrogen, oxygen, nitrogen, sulphur and silicon. These molecules tend to be highly reactive free radicals which under normal conditions (on Earth) would react with other chemicals to form stable compounds, many similar to those found in living organisms. Further, amino acids have been discovered on celestial bodies e.g. the Murchison meteorite (Australia 1969) and Haley’s comet which was analysed from spaceborne instruments during its recent passage through the solar system. The Murchison meteorite was carbonaceous and carried concentrations of amino acids as high as 100 ppm, the same kind that Stanley Miller’s pre-biotic experiments produced in the 1950’s. Saturn’s satellite Titan also is believed to have “seas” composed of hydrocarbons. With evidence such as this, scientists widely agree that complex organic chemicals are not necessarily the product of life but form spontaneously by banal reaction. It is highly likely that, given the proper conditions, the first building blocks of life could arise (on Earth or elsewhere) as do simple chemicals ... spontaneously and according to the rules of simple thermodynamics. Stanley Miller (1953), a graduate student at the University of Chicago in Harold Urey’s lab (the discoverer of Heavy Hydrogen and a widely acknowledged authority on planetary formation), designed a series of experiments to test the effect of lightening on Earth’s primitive atmosphere. On the assumption that Earth’s early atmosphere was a mixture of hydrogen, methane, ammonia and water vapour Miller found that more than 15% of the methane carbon had converted to a variety of amino acids in only a few days. Recent thought is that the atmosphere would not have been as rich in hydrogen but data from the Murchison meteorite and elsewhere strongly suggests that Miller’s results are still of significant value. Miller’s work forms the basis of the new discipline abiotic chemistry ... the chemistry of compounds formed without life and specifically concentrating on the abiogenetic events of the early Earth. Experiments within this discipline have yielded amino acids, sugars, organic acids as well as purine & pyrimidine bases (some of the components of DNA & RNA) and other biologically significant substances. Although contrived some of the experiments give clear indication that abiogenesis is the most likely method for the formation of the early chemicals and the emergence of early life on Earth.
Conclusion"How far in the direction of biochemical complexity the rough processes studied by abiotic chemistry may lead is not yet clear. But it seems very likely that the first building blocks of nascent life were provided by amino acids and other small organic molecules such as are known to form readily in the laboratory and on celestial bodies. To what extent these substances arose on earth or were brought in by the falling comets and asteroids that contributed to the final accretion of our planet is still being debated." Christian de Duve According to Miller the basic chemicals and a reducing atmosphere are all that’s required for the formation of life. How much space dust contributed to that process is uncertain, certainly meteorites tend to feature high levels of chemicals such as hydrogen cyanide (essential to prebiotic synthesis of amino acids), but Miller feels that if they could form naturally in space then they could also do so on Earth. Ultimately, however, he feels it doesn’t really make much difference where compounds like HCN came from as long as it was present at the time and the proper conditions existed.
- "The Probability of Abiogenesis", Andrew Ellington (1995)
- "From Primordial Soup to the Prebiotic Beach (an interview with exobiology pioneer, Dr Stanley L. Miller, University of California San Diego)", Sean Henahan (1996)