Posted by jlwile on November 4, 2011
A commenter left this link on an unrelated post. Since the commenter has, in the past, tried to support several unscientific positions, I assume he or she thought that the experiment demonstrated the plausibility of abiogenesis, the the idea that life might have emerged through a series of incredibly unlikely interactions between nonliving chemicals. Of course, such an idea contradicts everything we know about the study of life, since all life we have ever studied comes from other living things. I have written several articles (here, here, here, here, here, and here) that demonstrate how the data speak against abiogenesis, but those who want to ignore the scientific evidence desperately hope for some special time in the past when all our current evidence doesn’t apply and life could actually spring from nonliving chemicals.
One of the many, many problems associated with any naturalistic origin-of-life scenario is that of stereoisomerism. As I explain here, there are certain biological molecules that can be formed in two different ways. They have the same chemical formula and form mirror images of each other. However, these mirror images are not identical. Think about your hands. When you hold them palms together, they are mirror images of one another. However, no matter how you tilt or turn it, you cannot make your left hand look identical to your right hand. If you put the palm of one hand on the back of the other hand, for example, one hand’s thumb will be where the other hand’s pinky finger is. So while your hands are mirror images, they are not identical. There are many biological molecules that are like that, and we call them chiral molecules. The two mirror images that are formed by a chiral molecule are called enantiomers.
All origin-of-life scenarios start with simple molecules that do not form enantiomers. We call these achiral molecules, since they cannot form two mirror images that are different from one another. This is a problem, because in the lab, when achiral molecules react to form a chiral molecule, an equal amount of each enantiomer is formed. As a result, you end up with a mixture that is 50% one enantiomer and 50% the other enantiomer. We call this a racemic mixture. The problem is that life isn’t like that. In most chiral molecules of life, only one of the enantiomers is used. We call this an enantiopure compound, since it is purely one enantiomer, without any of the other. So any origin-of-life scenario has to figure out a way of producing just one enantiomer, or it has to figure out a way to get rid of the other enantiomer once it has formed.
This is a major problem, of course, and the link that the commenter left claims that a “plausible” solution to this problem has been found. Of course, when you look at the actual paper you find that the process is anything but plausible in an origin-of-life scenario.
The paper1 discusses how researchers were trying to create enantiopure precursors for the production of RNA. Currently, the most fashionable origin-of-life scenarios involve RNA forming first. Thus, to make these scenarios work, we have to first form the molecules that are necessary in order to make RNA, and some of them are chiral. To make RNA, you can use only one enantiomer, so you have to figure out some way of forming these RNA precursors in an enantiopure way.
In the researchers’ experiments, they started with an amino acid mixture that had slightly more (1% to be exact) of one enantiomer than the other. Thus, it wasn’t really a racemic mixture of the two mirror images, but it was close. They suspended it in a solvent (either chloroform or ethanol) and removed any excess solid. They then allowed the solvent to evaporate, so they were left with just the solid that had been suspended in the solvent. To this solid, they added some racemic mixtures of two chiral molecules, dissolved everything in water, and let them react. The reaction produced another chiral molecule that is a precursor to RNA, and it had the correct biological enantiomer in 20-80% excess, depending on the conditions. If they then cooled the mixture to 4 degrees Celsius, the precursor formed a crystal that was enantiopure.
So an amino acid sample that has slightly more of one enantiomer than the other can force a chemical reaction to produce an enantiopure chiral molecule, as long as the right conditions are met. The authors say that this is:
…a concerted sequence of physical and chemical amplification processes that make use of a small initial imbalance in amino-acid enantiomers as the only molecular asymmetry in the system.
In other words, the slight imbalance of enantiomers in the amino acid ends up being amplified in the products of a chemical reaction that occurs in their presence.
Of course, from an origin-of-life perspective, you have to ask a few questions. First, where did the slight imbalance in amino acid enanatiomers come from? Remember, chiral molecules are formed in racemic mixtures, not with 1% excess of one enantiomer. As detailed in a previous post, researchers were able to produce such an excess in amino acid enantiomers, but it required a ridiculously unrealistic scenario. Second, how do we get the nice situation where this sample is then suspended in a solvent that is not water, after which the solvent evaporates? Third, how do we get the racemic molecules to wait until the solvent (that is not water) evaporates to start reacting in the presence of the amino acids? Fourth, how do we suddenly get water into the picture, since the solvent we started with wasn’t water? Fifth, how do we get the lower temperature at just the right time to crystallize out the enantiopure sample? Sixth (and most important), how do these precursors actually react in an uncontrolled environment in order to produce actual RNA, which is just the starting point for the currently fashionable view of abiogenesis? In the experiment outlined in the paper, most of these questions were answered by intelligent intervention – the researchers controlled each step so it happened just right.
Now don’t get me wrong. This is a nice bit of research, and it produces a completely unexpected result. If the experiments are replicated, I expect this technique will be utilized in many synthetic processes. Thus, it is really great research. I just don’t think it is a plausible means by which the chirality problem can be solved in origin-of-life scenarios. It requires a lot of intelligent intervention just to get the precursors of RNA synthesis, while abiogenesis requires no intelligent intervention at all to go significantly further: produce self-replicating RNA that can also catalyze other chemical reactions.
1. Jason E. Hein, Eric Tse, and Donna G. Blackmond, “A route to enantiopure RNA precursors from nearly racemic starting materials,” Nature Chemistry 3: 704–706, 2011.
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