One More Crack in the Mirror: Misplaced Hope in the Latest Model for the Origin of Life
Many of us are told that compelling scientific evidence exists for the emergence of life from inanimate matter via chemical evolution (also known as abiogenesis). Proponents some times defend this claim by pointing out in biology textbooks descriptions of laboratory experiments that simulate the chemical reactions on the early Earth.
In this article, I aim to make sense of these challenges to biblical creation. Here I discuss a team of chemists’ recent laboratory work simulating chemical reactions that could lead to homochirality, a necessary condition for life’s origin. I will point out problems with these experiments: namely, the conditions used to achieve homochirality in the lab are irrelevant to early Earth’s conditions.
Apart from having to clean up glass shards, why is it considered bad luck to break a mirror?
Some people have traced this superstition’s origin back to the Romans, who believed a mirror could confiscate part of one’s soul. So, if you broke a mirror, you fractured part of your soul.
While broken mirrors don’t really portend dire consequences, mirror image molecules bedevil origin-of-life researchers. These scientists are desperately trying to explain the genesis of homochirality—a strict requirement for life that involves molecular “mirrors.”
Amino acids and nucleotides (the building blocks of proteins and DNA, respectively) are made up of molecules that can exist in one of two forms. These different forms, called enantiomers, are mirror images of each other. Researchers often refer to enantiomers as either left-handed or right-handed because, as with the left and right hands, they are mirror images of each other. Yet, in biological systems, the amino acids and nucleotides used to build proteins and DNA are of a single handedness, a condition known as homochirality.
Image credit: NASA (https://nai.nasa.gov/library/images/news_articles/159_1.jpg)
Homochirality and the Origin of Life: A 50/50 Proposition
Until now, scientists working on the origin-of-life question have not found a real explanation for how homochirality emerged. When chemical reactions generate compounds that can exist as one of two mirror images, a 50/50 mixture generally results. Yet, to explain homochirality requires some means to generate 100 percent enrichment of only one of the enantiomers on early Earth.
For many people, the homochirality problem provides a basis for skepticism about the evolutionary explanation for life’s start. (See Origins of Life and Creating Life in the Lab for a detailed discussion of the most salient problems related to the origin of homochirality.) The video below (featuring a younger version of me) explains what homochirality is and why it’s important for the origin-of-life question.
Still origin-of-life researchers have not given up hope.Chemists from The Scripps Research Institute think they may have found a clue to help them resolve this problem.1
Feeling Lucky, Punk?
The Scripps chemists’ research built upon work done in the summer of 2009. This work, by researchers from the University of Manchester (Great Britain), reported on one of prebiotic chemistry’s most important breakthroughs in the last 50 years. In short, they discovered a chemical route (called the Sutherland reaction) that could generate ribonucleotides (a key step in the leading origin-of-life hypothesis) from simple chemical materials likely to be present on early Earth. This route involves less than a handful of steps. (Go here for a summary of this work and here for a critical assessment.)2 However, the ribonucleotides generated by this chemical pathway do not display homochirality; instead, a 50/50 mixture of enantiomers results.
In their research, the chemists from Scripps recognized that amino acids share a structural feature with a key chemical reactant in this pathway. They speculated that, if present, amino acids could influence the course of the reaction. The researchers also noted that a slight excess of some amino acids’ left-handed versions has been detected in meteorites like the Murchison.3 They reasoned that adding low levels of homochiral amino acids to the Sutherland reaction would generate chiral enrichment of one enantiomer relative to the other.
They evaluated a wide range of amino acids’ effect on the Sutherland reactionthat alanine, histidine, and tryptophan individually generated a 10 percent chiral enrichment of a ribonucleotide precursor; the addition of proline yielded a 55 percent chiral enhancement.
The researchers speculate that, on early Earth, a slight chiral imbalance in amino acids was amplified to nearly 100 percent via differential solubility of the left- and right-handed forms. These enantiomerically pure amino acids then caused a chiral enrichment of compounds in the Sutherland reaction. Once these compounds formed, they were chirally enriched to near 100 percent by differential solubility as well. Homochirality explained.
Cracks in the Mirror
Though interesting, this proposal is fraught with problems that become evident upon closer examination. For example, the Sutherland reaction lacks geochemical relevance and would not likely proceed in a productive manner on early Earth. This means that it cannot successfully generate homochirality either.
Another problem relates to the availability of amino acids that catalyze the chiral enrichment. Except for alanine, the other amino acids (proline, histidine, and tryptophan) would almost certainly never have occurred at high enough levels on early Earth to have had a meaningful impact on life’s origin. To my knowledge, no one has ever reported the production of histidine or tryptophan in spark-discharge experiments nor have these amino acids ever been isolated from meteorites. Proline has been generated in the lab and has been found in meteorites, but at trace levels.
These two problems are sufficient to cast aspersions on the proposed explanation for the origin of homochirality. In fact, the scientists at Scripps Research Institute have inadvertently provided direct, empirical evidence that apart from the work of an intelligent Agent, this prebiotic chemistry could not take place in a productive way on early Earth. If it wasn’t for chemists carefully controlling the amounts and purity of the chemical components added to the reaction mixtures and adjusting the reaction conditions with great precision, the generation of chirallyenriched ribonucleotide precursors would never result.
- Jason E. Hein, E. Tse, and D. G. Blackmond, “A Route to Enantiopure RNA Precursors from Nearly Racemic Starting Materials,” Nature Chemistry 3 (August 7, 2011): 704–6, doi: 10.1038/nchem.1108.
- I also discussed this work on the May 15, 2009 edition of Science News Flash.
- M. H. Engel and S. A. Macko, “Isotopic Evidence for Extraterrestrial Non-Racemic Amino Acids in the Murchison Meteorite,” Nature 389 (1997): 265–68.