Origins of Life – What are the odds?

I hope to make this the first in a series of posts about current (relatively) origins of life research.  The reasoning behind this is two-fold.  The first is because I’m curious and the blog provides an opportunity to share what I learn.  The second is to counter the creationist plays against origins of life.

I know of no scientist who will not admit that we will never know exactly what happened on this planet that resulted in life.  Of course, that’s not an admission that creationism is just as legitimate as science with this regard.  The thing is, if science can show that there are no physical or chemical reasons that life couldn’t form naturally, then there is no reason to invoke the super-natural.

I hope that everyone will agree that there are no restraints against the formation of organic compounds from non-organic reactants.  This has been demonstrated since 1952 when Stanley Miller and Harold Urey did their famous experiment.   Be sure look at the “recent related studies” in the Wikipedia article.  A recent examination of some of the original vials of material using modern methods resulted in a large increase in the number and kinds of organic compounds that were made in the original experiment.

In fact, organic compounds have been found in the oddest places; Titan,  nebula, and meteorites.  So I hope that there are no concerns with the fact of organic compounds forming.  In fact, the atmosphere and conditions on Titan have been shown to be able to form all five nucleotide bases as well as amino acids.

Now, the question, of course, is how did these disparate pieces come together and form the living system we see today?  Creationists will go on and on about how unlikely it is for complex RNA, DNA, and proteins to form.  They will say something like, “X process requires a DNA of Y length and the odds against that forming are 23895798148574925926594365094359043594632953295734587428574902875 to 1 or worse.

The problem is that creationists do not keep up on the research and the paper I will talk about today will show them just how wrong they are.

First, on the subject of odds, one must consider the length of time involved and the available area for such reactions to take place.  If we consider that one reaction takes place per second for a million years that reduces the odds of it occurring by 31.5 trillion.  (13 orders of magnitude) If we assume that one reaction takes place per every square kilometer on the Earth’s surface, then that reduces the odds by another 500 million.(8 orders of magnitude.  Of course, if you think in square meters rather than square kilometers, then you reduce it by a further 6 orders of magnitude.

So time and area can play a huge role in the reduction of the improbability that creationists push on an unsuspecting innocents .  [Of course, creationists only have 6000 years or so, not several billion to go from bare rock to modern life.]

But what about the other part, what about the nucleotide sequence itself?  Is that really necessary?

The answer is no.

Let’s take some process, like translation for example.  How large does an RNA need to be to catalyze this kind of process.  Well, the answer researchers at the University of Colorado and California have found is stunning.

Five.  That’s it.  An RNA strand that is a mere five nucleotides long can catalyze RNA 2’(3’) aminoacylation (that is, the process of adding an aminoacyl group to a compound. It produces tRNA molecules with their CCA 3′ ends covalently linked to an amino acid).  The active site is only three nucleotides.  It’s actually a complex with complimentary 4 nucleotide chain with it.  GUGGC/GCCU.

This image from the paper shows the kind of difference we’re talking about here.

These are all small trans-aminoacylating RNA complexes.  (A) C3 RNA.  (B) Intermediate trans complexes.  (C) Final GUGGC/GCCU complex.

So what does this mean for the odds of something like this occurring?

I’ll let the authors speak to this:

The ultimate importance of these observations may lie partly in the unknown number of other reactions that can be accelerated by comparably small RNAs. This is because for each such minuscule RNA reaction, there is a prima facie case that it would become accessible even after the most primitive ribonucleotide polymerization.

To see this, consider that, to pick every possible RNA pentamer sequence from arbitrary pentamers (with probability 0.9975), one needs only accumulate 4.1 × 10−18 gm of RNA. To possess every tetramer (with probability 0.9975) from a pool of  arbitrary tetramers, one would need 3.4 × 10−18 gm RNA. In a real polymerization, one would have a distribution of lengths; nonetheless, with only attograms of total RNA of distributed short lengths from some geochemical source, one would have not only our ribozyme, but every activity of comparable size.

So, the odds of getting this particular activity approach one with a bare 0.0000000000000000041 grams of material.  In fact, to get every possible RNA sequence of this length would require only 0.0000000000000000034 grams of material.

That means that the odds that creationists tout as nearly impossible are based on totally incorrect assumptions.  Reactions do not need 200 nucleotide RNAs or even 26 nucleotide RNAs to catalyze.  Five will do just fine and the odds of getting a five RNA sequence are very, very, very good.  I’d put money on it.

Addendum

I just had a brief e-mail conversation with the lead author of this paper (congratulations Dr. Turk) and she has mentioned that there is an additional paper (presently in review with JACS) regarding further reactions with the 5-nt ribozyme.  Keep an eye out.

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Turk, R., Chumachenko, N., & Yarus, M. (2010). Multiple translational products from a five-nucleotide ribozyme Proceedings of the National Academy of Sciences, 107 (10), 4585-4589 DOI: 10.1073/pnas.0912895107