Last week, I was talking about the components necessary for life to have evolved on this planet. While we don't know exactly how it started, scientists have come up with plausible theories to explain the origin of life.
For a while, in the 1950's, several prominent astronomers contended that life could or must be extra-terrestrial in origin. Their argument was based on the detection of small organic molecules in the inter-stellar medium, including spectroscopic lines that could be interpreted as belonging to porphyrins.
Porphyrins are the basis of hemoglobin, chlorophyll, and a host of other molecules essential for life as we know it and so the hypothesis that these molecules found their way to Earth from space didn't seem farfetched.
We now know that organic molecules have, indeed, been brought to the surface of the Earth as carbohydrate rich meteorites.
But the idea that little green men from Mars or elsewhere deliberately seeded our planet with life is the work of science fiction in my opinion. It is an unnecessary step in the story behind the origin of life.
Still, the theory has its proponents and they argue life was seeded from the interstellar medium and so it must pervade the galaxy.
A more realistic theory, though, resulted from the Urey-Miller experiment conducted in 1952 and reported in 1953.
This was a very simple and straightforward demonstration that the constituents of life can be synthesized in the laboratory from simple compounds with no outside intervention and with the chemicals that would have been present on the early Earth.
Harold Urey had been working on the chemical composition of the Earth's early atmosphere trying to determine what the primordial soup might contain. When Stanley Miller approached him about doing a graduate project, Urey gave him the task of re-creating the conditions from which early life might have evolved.
The experiment is not very complex.
In essence, 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.
He warmed this mixture of gases up, applied an electrical spark to simulate lightning and condensed the results. In the space of a single week, the mixture turned brown. Analysing this material, he found it was rich in amino acids - the building blocks of all proteins.
He didn't actually find proteins, just the building blocks and some have argued this means his approach couldn't be connected to life. But Miller's and Urey's published results suggest organic life may have had an inorganic origin. It is a bit like saying, "Well, we have all the pieces. Now we just need to put them together correctly."
Other scientists repeated and expanded the experiment to show that many of the amino acids can be obtained this way. They also tried different combinations of gases to simulate the primordial soup.
Perhaps the most significant experiment was Joan Or discovery in 1961 that the nucleic base adenine could be synthesized from hydrogen cyanide and ammonia in water.
Adenine is essentially five hydrogen cyanide molecules linked together and hydrogen cyanide was one of the products of the Urey-Miller experiment.
Other experiments showed that the other nucleic bases could be synthesized in a similar fashion from small molecules. It is also important to note adenine has a number of other roles in biochemistry beyond being a constituent of both DNA and RNA. It is the "A" in ATP which is the molecular energy system employed by all living organisms.
Did life emerge out of this primordial chemical soup? Given the time span involved, why not?
Consider that in a single week, Urey and Miller were able to synthesize amino acids in a very simple apparatus.
How many weeks have passed since the formation of the Earth? Enough time that a lot of chemistry has occurred.
Further, the formation of nucleotides has led both Francis Crick and Leslie Orgel to separately but simultaneously publish papers arguing the presence of RNA in the primordial soup would pretty much guarantee that life would emerge. The collection of amino acids would be transformed to proteins by RNA and the resulting proteins, in turn, would help to generate more RNA.
We know from chemical analysis that the Earth's atmosphere has changed dramatically over the past four-and-half-billion years. Changed as a result of living organisms. But the original atmosphere was filled with all sorts of small molecules that needed only a spark to generate more complex chemical species.
I don't think that we need to look to space or beyond as an explanation for the origin of life. I think that life is very probable.
Within our solar system - which is the only test case that we know of - it is batting 1.000.
After all, we are here.
