In his two columns (June 25 & July 1) Todd Whitcombe attempts to answer the questions, where did life on this planet originate? And perhaps more importantly, how did it originate? He asserts that a plausible theory resulted from the Miller-Urey experiment conducted in 1952 when Miller set up an apparatus that contained all of the ingredients that he thought were present in the early atmosphere of the Earth - hydrogen, nitrogen, methane and water. While it might have seemed realistic at the time, in 1966 geophysicist Philip Abelson published a paper declaring it wrong and in less than 10 years that was confirmed. (Klaus Dose & Sidney Fox, 1975).
Currently it's believed that the early Earth atmosphere consisted of nitrogen, carbon dioxide, carbon monoxide and water vapour. Such an atmosphere could not support the formation of prebiotic compounds.
Dr. Whitcombe stated that life requires certain features - the ability to replicate, the ability to adapt, the ability to utilize chemicals in the surroundings to achieve the first two objectives - but not a lot more. That's may be all that life requires, but for life to originate requires more.
Minimal independent life requires a minimum genome size of about 1700 gene products (proteins and functional RNAs) - [Colin Patterson, 1999.] Best probability estimate for a single gene product or protein to come into existence exclusively by natural means - one chance in 10 to the 75th power. Chance of even the simplest parasitic organism's 250 gene proteins could come into existence all at the same time - one chance in 10 to the 18,750th power (that's the number 10 with 18,750 zeros tacked on). [Hubert Yockey, 1992]. Compared to that, billions upon trillions upon quadrillions of chemical replicating systems is vanishingly small.
Not only must the starting materials be present on early Earth at the right concentrations along with energy sources or catalysts, the chemicals produced must remain stable and sufficiently concentrated long enough for subsequent chemical reactions to take place, while at the same time, chemical interference from other prebiotic compounds must not occur. There's also the problem that some prebiotic ingredients mutually exclude one another. For example, two nucleotide bases (adenine and guanine) require freezing conditions for their synthesis, while two other nucleotide bases (cytosine and uracil) demand boiling temperatures. The prebiotic soup would have to simultaneously freeze and boil. [J.L. Bada, C. Bigham, S.L. Miller, 1994]
It's suggested that the presence of RNA in the primordial soup would pretty much guarantee that life would emerge. All models for life's origin at some point involve the building and operation of RNA molecules. The stability of nucleotide building blocks that comprise RNA molecules and of the RNA molecules themselves provide a measure of the maximum time span required for any life origin. Such stability depends on temperature. At the time of life's beginning the Earth is believed to have been hot (70 - 90C).
All four of RNA's nucleotide building blocks degrade in warm temperatures in as little as 19 days (Matthew Levy, 1988). Unless life appears on a planet quickly, it will not arise at all. (Donald Goldsmith, 1997)
Where did the first RNA come from? Proteins are needed to make RNA, but RNA is needed to make proteins, so where did the needed proteins come from? RNA also requires a membrane made of proteins to protect it. It's a chicken and egg scenario.
The first life is believed to have appeared more than 3.8 billion years ago, just about as soon as conditions for life to exist were favorable, within a geologically very short 50 million years after the Late Heavy Bombardment event which saw the Earth's surface melted, leaving no liquid water, solid rocks or even basic prebiotic molecules.
It originated around the element carbon because that is the only element to possess a sufficiently complex chemical behavior to sustain living systems.
Carbon readily assembles into stable molecules comprised of individual and fused rings and linear and branched chains. It forms single, double and triple bonds, strongly bonds with itself as well as with oxygen, nitrogen, sulfur and hydrogen.
Back when Stanley Miller conducted his famous experiment, scientists were optimistic that the mystery of life's origin would soon be solved. However, while origin-of-life researchers have discovered many plausible chemical routes from simple compounds to biologically important compounds, for other critical biomolecules, no pathways can be discerned.
When it comes to the origin of life, as yet all we can say for certain is that it happened.
Art Betke
Prince George
