Saturday, February 12, 2011

Dr. Ralph Pudritz's CATP Seminar, January 14, 2011

Planetary Systems and the Origins of Life (Cambridge Astrobiology)
Some of the previous work by McMaster's Ralph Pudritz, a member of the Origins Institute and CATP Speaker on January 14, 2011 (Image is from Amazon.com)

Dr. Pudritz's CATP Seminar was built around the work he has done understanding the thermodynamics of biomolecules on the early Earth. The reason that this is interesting is because life is known to rely heavily on certain very specific compounds as building blocks, but not others. While there are almost an unlimited number of possibilities for carboxylic acids (which amino acids resemble) only 20 different amino acids are used by all life with an additional two found in some organisms, but not others.

Adding to the mystery is chirality, a sort of molecular handedness. Just like you could tell a right hand from a left one, if you found it on its own, all amino acids (with the exception of glycine) have at least two possible configurations. Since both forms are very similar, chemically speaking, nature produces essentially equal amounts of each through abiotic (that is, non-biological) processes. But where life is concerned, only one form is actually used. This incredible selectivity of biological processes is surprising, and sufficiently odd that it has often been talked about as a biomarker; a smoking gun that we have run across a biology or its byproducts.



Before discussing Ralph's contributions here, let's talk about some background information. What are amino acids and how are they produced in the first place? Amino acids are the basic building blocks of proteins which make up much of the solid substance of living things and which carry out the functions of cells, such as catalyzing necessary reactions like cellular metabolism. Proteins themselves are just polymerized amino acids, called polypeptides (named after the kind of bonding that takes place between amino acids). This is a very basic thing, and it's important to remember that when we talk about the code of life carried by DNA, what we're talking about is a recipe for fabricating proteins. Further, this means that since DNA evolved as a way to process amino acids into proteins, amino acids must have predated the emergence of DNA and other protein-producing, self-replicating molecules.
How then did the molecules of life arise?
                          -Carl Sagan, "Cosmos"
 Early on in the history of the Earth in a time called the Hadean, the surface of the earth was inhospitable; too hot for molecules like amino acids to form. However, once the Earth cooled sufficiently there were several ways of producing these molecules. Perhaps the most well-known pathway was elucidated by the Miller-Urey experiment in the 1950s. By combining water, a reducing atmosphere and energy (by sparking) they were able to produce amino acids. More recently, amino acids have been detected in giant molecular clouds and in abundance in primitive carbonaceous chondrites such as Murchison and Tagish Lake.

 

Recovery of the Carbonaceous Chondrite fall at Tagish Lake, British Columbia in 2000. Luckily the fall took place in the winter time which allowed recovery of the fragments. Top, a fragment in situ, below, Allan Hildebrand of the University of Calgary holds up a chunk collected. All samples have remained frozen since the fall. More pictures are available from the UWO Meteor group here.

Not only can we detect the amino acids present in the meteorites, but we can assay them and determine the relative quantity of each form. This is exactly what Ralph has done, compiling what exists in the literature and adding his own analysis. I've re-copied his figure from the astrobiology science conference below (the main abstract can be found here):


For the full article on which this figure is based, visit Astrobiology Magazine. What this figure shows is the relative abundance (concentration) in meteorites versus the gibbs free energy of formation of each amino acid under conditions of relatively low pressure and temperature. The ten you see plotted are, respectively Glycine (G), Alanine (A), Aspartic Acid (D), Glutamic Acid (E), Valine (V), Serine (S), Proline (P), Leucine (L), Isoleucine (I) and Threonine (T). These are the easiest amino acids to form abiotically, and they are the ten which Miller and Urey found in their soupy flask. Since they are so easily formed, they have gained the title of "early amino acids" which means that we think these were some of the earliest biomolecules present. (note that the other 12 "late" amino acids all have much higher gibbs energies of formation)

What you notice in the graph is an exponential decay in the amount of each amino acid present corresponding to the energy required to form it. This implies that this whole suite of molecules formed readily and formed together under conditions of thermal equilibrium at conditions compatible to those within meteorites or at the Earth's surface (i.e. relatively low pressure, relatively low temperature).

What this suggests to Pudritz (and primary author Higgs) is that genetic replicating systems similar to our own may be rather common in the universe. Since many of the places where life can arise (i.e. within differentiated asteroids, the surfaces of rocky planets) share the characteristics of these environments, it seems likely that the distribution of amino acids available in the starting molecular soup might be common. Since our self-replicating biochemistry has evolved to take advantage of these amino acids, other biochemistries elsewhere in the universe might be similar to our own.

Now, don't forget that similar biochemistry doesn't necessarily mean similar forms or even compatible forms. This work does not mean that "Star Trek" was correct in positing a universe filled with bipedal creatures. Furthermore, if other worlds hold biochemistries that are at all similar to our own, that makes these places more dangerous for us, as the potential is there for alien microorganisms to hijack our own biochemistry, as a virus does. You could say that this suggests that some of Fred Hoyle's late suggestions about an andromeda-strain like creature are slightly more credible. But still, it is neat to consider that the aliens out there might be a little less alien then our imaginations can envision.

Pudritz's work also addresses another interesting question. There are two main competing theories for the evolution of life on earth: "Bottom Up" or "Top Down." Basically, Bottom Up suggests that life forms easier in extreme environments with large gradients in temperature and concentration, such as in impact hydrothermal systems or near black smokers on the sea floor. Top Down is the classic molecular soup in shallow puddles on the Earth's surface.

While the conditions corresponding to "Top Down" match well with the abundances of amino acids in meteorites, when you recompute the energies for the "Bottom-up" case, the correlation between the abundance and the energy disappear. That's potentially bad news for those of us who would like to see evidence of life on worlds where the only possibility for the emergence of life is bottom-up. These include places within our own solar system like Europa and Enceladus and any extrasolar planet with enough water to bury the surface. It's not fatal, however, it just implies that the distribution of amino acids produced in such environments would be very different. So long as the particular amino acids that we classify as "early"are not critical to the development of self-replicating systems, life could certainly arise in these more extreme, high pressure and temperature environments as well.

As a final note, speaking of the production of amino acids in asteroids, recently evidence has been building up that there is an excess in the "left-handed" varieties amongst carbonaceous chondrites. That is the same handedness used by life on Earth. As I mentioned before, chirality has been thought to be one of our most promising ways to distinguish abiotic from biological activities. As such, this new data has some serious implications. If the synthesis of the left handed forms in asteroids are abiotic, it suggests that chirality may not be as good a tool as we had hoped. However, the other possibility is that chirality is indeed proof of biological activity and thus the insides of differentiated asteroids might have been a good place for life to evolve in the first place. 

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