[_ Old Earth _] Abiogenesis

[Packrat]

I came across this article, and I bet that it has been brought up before, but I'd like to read some input on it from you all: http://www.talkorigins.org/faqs/abioprob/abioprob.html

Any comments?

 

[Packrat]

I've been in contact with some others about this very question: the hypothetical primordial cell as opposed to the modern cell. But I have received no substantial input yet from my other contacts because they are working on addressing that specific article I believe. I think that there is a page from http://www.GodandScience.org that addresses the hypothetical primordial ur cell. See http://www.godandscience.org/evolution/chemlife.html

If this does not address the ur cell, please clue me in. <bump>

 

[Barbarian]

Right off the top, there's an error, the assertion that there is no way to explain the homochirality of amino acids. There are a number of natural ways homochirality can be achieved.

We are interested in the origin of biomolecular chirality. It is well known that biomolecules are all of one hand, but what determines which hand? Why are animals made of L-amino acids and not D-amino acids? This asymmetry in biology may be a feature of fundamental physics, because it turns out that the "natural" L-amino acids are slightly more stable than their "unnatural" D mirror images, due to the weak force. The weak force, carried by the Z boson recently discovered at CERN, is one of the four forces of nature - electromagnetic, weak, strong and gravity - and it is the only one of the four which can tell the difference between left and right. Due to the weak force, L and D molecules have slightly unequal energies because they are not in fact true mirror images: the true enantiomer of an L-amino acid is the D-amino acid made of anti-matter.

We calculate these small energy differences between enantiomers using ab initiomolecular orbital methods. In most cases our calculations do indeed predict the correct sign: not only are the L-amino acids more stable than the D, but the natural D-sugars are more stable than the L, and the right-hand DNA double helix is also more stable than its left-hand mirror image. We believe the slight enantiomeric excess from these "parity-violating energy differences" could be amplified kinetically in the pre-biotic soup to preferentially select today's L-amino acid/D-sugar biochemistry over D-amino acid/L-sugar "mirror life".

The parity-violating energy difference between enantiomers is not the only way in which the weak force could select biomolecular chirality. Radioactive beta decay is mediated by the weak force, and this causes a polarization of the electrons emitted in beta decay, which could produce selective destruction of one enantiomer. We are currently starting to develop the theory of this enantioselective beta-radiolysis.
http://www.ch.cam.ac.uk/staff/ajm.html

Then there is the false assertion that RNA cannot form in the absence of enzymes:

This last possibility was given credence by the discovery of some varieties of RNA, known as ribozymes, that can act as their own enzymes, snipping themselves in two and splicing themselves back together. Only with the evolution of DNA, the theory goes, was there a reallocation of labor with the more stable DNA molecule assuming the role of genome carrier and the more versatile protein-based enzymes taking on the job of biochemical catalysts.
http://www.daviddarling.info/encycloped ... world.html

And the argument that some components of RNA could not appear abiotically?

A significant sticking point is how RNA, self-catalyzing or not, arose in the first place. Until recently, two of RNA's building blocks, cytosine and uracil, had proven difficult to synthesize under the sort of conditions that might have prevailed on Earth about 4 billion years ago. However, in June1995, Stanley Miller and Michael P. Robertson, at the University of California, San Diego, reported1 that they had been able to synthesize cytosine and uracil under plausible prebiotic conditions. The technique involved placing urea and cyanoacetaldehyde in an environment equivalent to that of a warm tidal pool. As evaporation concentrated the mixture, the chemicals reacted to yield cytosine and uracil in large amounts.
[/img]http://www.daviddarling.info/encyclopedia/R/RNAworld.html And so on. You probably shouldn't be relying on people who don't understand the subject.