RAZD writes:
Do all the necessary elements for forming life need to come from meteors? Logically the answer is no.
I agree with you that not all of the necessary elements for forming life need to come from meteors, but assuming that life began with naturalistic spontaneous generation, where else would these essential compounds have originated? According to David Berlinski in his commentary on the origins of life (
I read it here)
According to the impression generally conveyed in both the popular and the scientific literature, the success of the original Miller-Urey experiment was both absolute and unqualified. This, however, is something of an exaggeration. Shortly after Miller and Urey published their results, a number of experienced geochemists expressed reservations. Miller and Urey had assumed that the pre-biotic atmosphere was one in which hydrogen atoms gave up (reduced) their electrons in order to promote chemical activity. Not so, the geochemists contended. The pre-biotic atmosphere was far more nearly neutral than reductive, with little or no methane and a good deal of carbon dioxide.
Nothing in the intervening years has suggested that these sour geochemists were far wrong. Writing in the 1999 issue of Peptides, B.M. Rode observed blandly that “modern geochemistry assumes that the secondary atmosphere of the primitive earth (i.e., after diffusion of hydrogen and helium into space) . . . consisted mainly of carbon dioxide, nitrogen, water, sulfur dioxide, and even small amounts of oxygen.” This is not an environment calculated to induce excitement.
A recent paper in Science has suggested that previous conjectures about the pre-biotic atmosphere were seriously in error. A few researchers have argued that a reducing atmosphere is not, after all, quite so important to pre-biotic synthesis as previously imagined.
In all this, Miller himself has maintained a far more unyielding and honest perspective. “Either you have a reducing atmosphere,” he has written bluntly, “or you’re not going to have the organic compounds required for life.”
So then, if the organic compounds required for life could not have already been present on earth, they must have been introduced.
In the thread that spawned this one, molbiogirl asserts that this was the case, citing Murchison as evidence, but all the missing pieces are not accounted for.
From the same article:
Among the questions is one concerning the nitrogenous base cytosine (C). Not a trace of the stuff has been found in any meteor. Nothing in comets, either, so far as anyone can tell. It is not buried in the Antarctic. Nor can it be produced by any of the common experiments in pre-biotic chemistry. Beyond the living cell, it has not been found at all.
When, therefore, M.P. Robertson and Stanley Miller announced in Nature in 1995 that they had specified a plausible route for the pre-biotic synthesis of cytosine from cyanoacetaldehyde and urea, the feeling of gratification was very considerable. But it has also been short-lived. In a lengthy and influential review published in the 1999 Proceedings of the National Academy of Science, the New York University chemist Robert Shapiro observed that the reaction on which Robertson and Miller had pinned their hopes, although active enough, ultimately went nowhere. All too quickly, the cytosine that they had synthesized transformed itself into the RNA base uracil (U) by a chemical reaction known as deamination, which is nothing more mysterious than the process of getting rid of one molecule by sending it somewhere else.
Robert Shapiro is not critical of just the synthesis of cytosine, but adenine as well.
In Origins of Life and Evolution of Biospheres he writes:
Many accounts of the origin of life assume that the spontaneous synthesis of a self-replicating nucleic acid could take place readily. Serious chemical obstacles exist, however, which make such an event extremely improbable.
Prebiotic syntheses of adenine from HCN, of D, L-ribose from adenosine, and of adenosine from adenine and D-ribose have in fact been demonstrated. However these procedures use pure starting materials, afford poor yields, and are run under conditions which are not compatible with one another.
Any nucleic acid components which are formed on the primitive earth would tend to hydrolyze by a number of pathways. Their polymerization would be inhibited by the presence of vast numbers of related substances which would react preferentially with them.
It appears likely that nucleic acids were not formed by prebiotic routes, but are later products of evolution.
Now that the rabbit trail has returned me to the topic, it seems that the assertion "
Adenine has been found" by molbiogirl that adenine has been found in meteorites is, at the very least, questionable.
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