Monday, July 12, 2004

Evolution Of Biological Homochirality

I don't post much about evolution any more, because PZ Myers at Pharygula  and the whole gang at Panda's Thumb  cover that topic pretty well.  Every once in a while, though, I can't help myself.  Evolution is such an awe-inspiring process, it almost makes me believe in a higher power.  A few years ago, a college student, working on a paper of some sort, walked up to me at the mall with a tape recorder.  He asked me if I believed in a higher power.  I told him that I did, and that I though the higher power was a great, big, incredibly complex mathematical equation.  I figured the guy needed something interesting for his paper.  I always like to help a student when I can.  Maybe he's a blogger now.

Anyway, the inspiration for today's post comes from an article on Science Daily:

How Left-handed Amino Acids Got Ahead: A Demonstration Of The Evolution Of Biological Homochirality In The Lab
Date:  2004-07-09

A chemical reaction that demonstrates how key molecules in the biological world might have come to be predominately left or right handed has been reported by scientists at Imperial College London.

Ever since discovering that the building blocks of the biological world, such as amino acids and sugars, are distinctively left or right handed - possessing a quality known as chirality - scientists have been puzzling to answer how and why.

They believe that at the dawn of biological life there were even numbers of molecules in each form, but through hitherto unknown processes, one particular form came to completely dominate over the others (for example left-handed amino acids and right-handed sugars), a feature known as homochirality.

Now, using simple organic molecules, the Imperial researchers have demonstrated that an amino acid itself can amplify the concentration of one particular chiral form of reaction product. Importantly, the experiment works in similar conditions to those expected around pre-biotic life and displays all the signs to suggest it may be a model for how biological homochirality evolved.

I am not sure what the author means by "pre-biotic life."  Presumably, he or she means "pre-biotic conditions."  Minor quibble: the summary probably was written by a journalist, not a scientist.  The full text of the original article can be found here

The article demonstrates that it is feasible for homochirality to develop in a primordial soup.  The key point in the article is that the chemical reaction that causes proline to catalyze its own formation runs a lot faster when producing the left-handed version of proline.  This effectively debunks the claim commonly seen in anti-evolution or "scientific creationism" arguments, such as this one from Darwinism Refuted:

Let us for an instant suppose that life came about by chance as evolutionists claim it did. In this case, the right- and left-handed amino acids that were generated by chance should be present in roughly equal proportions in nature. Therefore, all living things should have both right- and left-handed amino acids in their constitution, because chemically it is possible for amino acids of both types to combine with each other. However, as we know, in the real world the proteins existing in all living organisms are made up only of left-handed amino acids.

Why would we assume that "the right- and left-handed amino acids that were generated by chance should be present in roughly equal proportions in nature"?  This would be true only if the two products were equally stable and were produced at the same rate, or if one were less stable but were produced at a correspondingly higher rate.  There is no particular reason to assume that either would be the case. 

See this site  for a discussion of this point. 

Remember: observations are gold; hypotheses, silver; and conclusions, bronze.