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The Murchison meteorite is an important piece of evidence for abiogenesis, because it contained nucleobases from extraterrestrial origin, as this Wikipedia article explains.

These results demonstrate that many organic compounds which are components of life on Earth, were already present in the early solar system and may have played a key role in life's origin.

My question is: How did these nucleobases arise? Is it possible for these nucleobases to have been synthesized naturally from simpler molecules in a specific environment, like in the Miller-Urey experiment for amino acids?

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I think it's fine here. Biochemistry is on topic to a degree on both sites, but I think this is closer to Bio's scope (I'm a mod on Chem). –  jonsca Jul 20 '12 at 3:30
    
The Wikipedia article about the meteorite mentions nucleobases, you're asking about nucleosides. Which one of the two are you asking about? –  Mad Scientist Jul 20 '12 at 8:24
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Just to clarify, what are you actually asking? I mean, what would be the alternative to them having been synthesised naturally? The only alternatives seem way out of scope for a biology site – namely, either spontaneous formation (e.g. supernatural creator) or aliens. –  Konrad Rudolph Jul 20 '12 at 12:43
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@KonradRudolph I think the alternative would be that we have no idea how they formed spontaneously. –  Mad Scientist Jul 20 '12 at 13:06
    
@MadScientist Sure. But whether we have an idea or not how they formed, they still formed naturally, or not at all. Or am I overseeing something here? –  Konrad Rudolph Jul 20 '12 at 13:10

2 Answers 2

up vote 5 down vote accepted

It is interesting to note that, in the Wikipedia article, it states that

The amino acids were racemic (that is, the chirality of their enantiomers are equally left- and right-handed), indicating that they are not present due to terrestrial contamination".

This implies that, if a 'life form' did create these proteins, they are fundamentally quite different to life on earth. (Particularly in light of the 70 unique amino acids they described!).

We cannot ever 'know' how they were made, unless a pretty revolutionary discovery is made that undisputably confirms evidence for extra-terrestrial life (although this is the best evidence yet!), so we are left to speculation.

The nucleobases either formed 'randomly' in one of the countless celestial events (although the chances of this seem pretty remote given the complexity of the molecules and the homogeneity in the sample), or they were made by extra-terrestrial 'life' (much more exciting, and indeed probably more likely, as we will see). (Or the 3rd option: intelligent design, however this is dramatically less probable than either of the other two).

The Miller-Urey experiment to which you refer has been subsequently enhanced, and evidence for the "catalytic potential of cosmic dust" is now reasonably robust (Hill, 2004). They demonstrate that simple nebular iron silicate catalysts can yield methane and water from CO2 and H2, and ammonia from N2 and H2. Allowing these to react yields nitrogen-containing organics such as methyl amine.

It is not a huge jump to suppose that with enough time, mass and energy, these reactants could combine and form a molecule (chain of amino-acid-like molecules - a protein) capable of faithfully replicating itself, and thus initiate life. There is no reason to suppose that Earth is unique in this respect; there are billions of suns in our galaxy alone, and billions of years of time for these reactions to take place.

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I am not sure that I would say that we are left to speculation. The fact that these chemicals are racemic would suggest that they were formed abiotically. This being the case, we can probably get to understand the chemistry which should be pretty basic. –  Poshpaws Jul 20 '12 at 17:18
    
But certainly the importance of dust should not be underestimated! –  Poshpaws Jul 20 '12 at 17:21
    
As I commented above, I thought one of the important findings from the Murchison meteorite is that it contains alpha-methyl-amino acids (which cannot racemize) where the L-form is in slight enantiomeric excess. See, for example, this paper by Breslow –  TomD Jul 21 '12 at 12:58

I think it's fair to say that we don't really know how these more complex organic molecules formed abiotically, but we can say that carbonaceous chondrites - a class of carbon and organic molecule bearing meteorites to which Murchison belongs (specifically CM2) - have experienced at least three distinct chemical histories:

  • In the interstellar medium (ISM) in cool (10 - 30 K), dense clouds where cold gas phase chemistry can lead to the formation of species such as HCN, C2O2 and CO.Polycyclic aromatic hydrocarbons (PAHs) have also be identified here.

  • In the early solar nebula. These simple organics can stick to dust grains forming "tarballs". The ultraviolet radiation field from the young star can then create some interesting chemistry through things like radical reactions and during the evaporation of ices. Things like diethyl ether have been detected in the pre/proto stellar nebulae of young stars.

  • In the parent body of the meteorite, such as in a meteoroid. Here most chemistry would occur through hydrothermal alteration over long timescales as the body orbited the parent star. As these bodies will invariably describe elliptical orbits, it will undergo many freeze-thaw cycles all in the presence of an intense radiation field.

Carbonaceous chondrites in particular undergo aqueous alteration due to the presence of water in liquid form.

Figure 2 of this article gives an basic overview of some of the chemistry involved. It actually refers to the chemistry in cold molecular clouds, but is almost equally applicable to that in things like meteroids/asteroids/comets.

Ehrenfreund, P, and Charnley, S.B., 2000, Annual Reviews of Astronomy and Astrophysics, 38, 427

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I've not read of complex molecules form in clouds before - are these molecules faithfully replicated in such a way as to call them living? –  Luke Jul 21 '12 at 0:35

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