Homochirality and the Origin of Life
Life requires homochirality in certain molecules.
When scientists failed to demonstrate a naturalistic explanation for homochirality via terrestrial chemical pathways, they turned to astronomical mechanisms. However, research continues to prove fruitless and instead, makes it clear that supernatural design is a much better explanation for the existence of life-essential homochirality.
Whenever we hear the phrase “on the other hand,” we know we are about to hear a second perspective. Just like every issue or debate has two sides, so, too, do some molecules. Any molecule with four different chemical groups attached to a central carbon manifests two distinct three-dimensional configurations. Chemists refer to these mirror image configurations as left-handed and right-handed. Such molecules are said to be chiral.
Figure 1: The Two Chiral Configurations for a Generic Amino Acid
Life chemistry demands homochirality (same chirality). Proteins cannot assemble unless all the chiral amino acids (20 out of the 21 bioactive amino acids are chiral) are either 100 percent left-handed or 100 percent right-handed. Likewise, DNA and RNA molecules cannot assemble unless all pentose sugars are 100 percent left-handed or right-handed. All organisms on Earth manifest only left-handed chiral amino acids and right-handed pentose sugars.
A Challenge for Naturalism
Homochirality presents a challenge to atheists, agnostics, and others who try to explain the origin of life via purely natural mechanisms. In explaining the emergence of first life, demonstrating how homochirality arises from a racemic (random mixture of left-handed and right-handed chiral molecules) is the simplest problem—but it is a problem. All laboratory demonstrations of life’s essential chemical pathways (see Fazale “Fuz” Rana’s latest book, Creating Life in the Lab) are irrelevant to solving the origin-of-life problem unless a naturalistic means for generating homochiral amino acids and pentose sugars can be proven. Such a mechanism needs to have operated under the conditions that existed on Earth at the time of life’s origin some 3.8 billion years ago. But, as Fuz and I documented in our book, Origins of Life, no possible terrestrial pathways exist for producing the homochiral molecules required for life’s emergence.
Organic chemist William Bonner once declared:
I spent 25 years looking for terrestrial mechanisms for homochirality and trying to investigate them and didn’t find any supporting evidence. Terrestrial explanations are impotent or nonviable.1
Because of this dead end, the last 15 years of research on naturalistic origins for homochiral molecules has focused on possible astronomical sources.
Looking Beyond Earth
The astronomical research quickly demonstrated the only viable source for generating homochirality is circularly polarized ultraviolet radiation that emanates from black holes and neutron stars. (The one other possible astronomical source, namely neutrinos emitted by core-collapse supernovae, is much less viable.2) In 2002, a Japanese research team used a laboratory cyclotron to simulate the kind of circularly polarized ultraviolet radiation arising from the most intense emitters from among known neutron stars and black holes. At the 13th International Conference on the Origin of Life they announced they had generated up to a 1.12 percent excess of left-handed configurations compared to right-handed configurations for the non-bioactive amino acid isovaline.3 In the peer-reviewed paper they reported generating up to a 0.65 percent excess for the bioactive amino acid alanine.4
Recently, an interdisciplinary team of nine French scientists repeated the Japanese experiment using a more comprehensive and updated astronomical database applicable to more realistic astrophysical situations.5 They reported achieving up to a 1.34 percent enantiomeric excess for alanine when they exposed the alanine to right ultraviolet circularly polarized light, and up to 0.71 percent enantiomeric excess when they exposed it to left ultraviolet circularly polarized light.
The reported 1.34 and 0.71 percent excesses match what geophysicists have found in the few meteorites that do possess amino acids (once probable contamination from terrestrial life is accounted for).6 Consequently, the French research team concluded that their “experimental result strongly supports a scenario in which the Sun was born in a massive star-forming region.”7 Such a region would have placed a number of neutron stars in close vicinity to the emerging condensation of gas and dust that led to the formation of the Sun and its system of planets.
- This conclusion that the Sun was born in a very large and dense cluster of stars corroborates independent studies that determined Earth’s abundant supply of heavy elements requires the primordial solar system be birthed in such a star cluster.8 However, the French team’s suggestion that their discovery of a maximum 1.34 percent enantiomeric excess for alanine in their laboratory simulation might solve the homochirality problem is dubious, to say the least. Here’s why.
- As the French scientists admitted, getting an excess as high as 1.34 percent in their simulation presumes the circularly polarized ultraviolet radiation emitted by a neutron star or black hole is both specified and stable. Actual observations fail to confirm this assumption.
- Life chemistry will not proceed with a 1.34 percent enamtiomeric excess. It needs 100 percent.
- All 20 chiral bioactive amino acids must manifest 100 percent enantiomeric excess and they must all do so with the same handedness. So far, the simulation experiments have produced results for alanine only.
- Even the most amino-acid-laden meteorites manifest scant abundance levels. For example, the Murchison meteorite yielded an abundance for all the bioactive amino acids of just 15 parts per million.9 Moreover, a large fraction of the 15 parts per million were not indigenous, but rather the result of terrestrial contamination. Abundance levels at a few parts per million falls woefully short of what any rational origin-of-life scenario would require.
- Amino acids have yet to be discovered in any astronomical molecular cloud.10 Although the searches conducted so far do not eliminate the possibility of finding amino acid abundances in such clouds at the level of parts per billion, such tiny abundances, again, offer no solace to solving the origin-of-life problem from a naturalistic perspective.
- Meteorites do not contain all 21 species of bioactive amino acids. Only six to eight were found in the Murchison meteorite.11 The majority of bioactive amino acids have not been found in any meteorite. Furthermore, none of the very wide range of laboratory prebiotic synthesis experiments performed at both normal temperature (0–200° Centigrade) and high temperature (200–700° Centigrade) conditions have yet to produce conclusive evidence for the generation of arginine or lysine, the basic (positively charged) bioactive amino acids.12
- Pentose sugars have yet to be discovered at any abundance level either in astronomical or in nonbiological terrestrial sources. This absence of pentose sugars explains why no simulation experiments have yet to be performed seeking an enantiomeric excess for the pentose sugars.
The search for naturalistic solutions to the homochirality problem in origin-of-life research is just one challenge facing atheism, agnosticism, and similar worldviews. On the other hand, the more scientists learn about life’s emergence the more evident it becomes that nothing less than the guiding hand of supernatural, super-intelligent Being suffices to explain homochirality. A Creator easily explains how the necessary amino acids and pentose sugars became sufficiently concentrated and uncontaminated by irrelevant chemicals, how they all became homochiral, and how they became appropriately assembled into all the protein, DNA, and RNA molecules that even the simplest life-form requires.
- Mark. William Bonner, as quoted in Jon Cohen, “Getting All Turned Around Over the Origins of Life on Earth,”&nbsli;Science&nbsli;267 (March 3, 1995): 1265. Bonner made this comment at the lihysical Origin of Homochirality in Life conference held in Santa Monica, California, February 1995.
- Richard N. Boyd, Toshitaka Kajino, and Takashi Onaka, “Suliernovae, Neutrinos, and the Chirality of Amino Acids,”&nbsli;International Journal of Molecular Sciences&nbsli;12 (June 2011): 3432–44.
- Yoshinori Takano et al., “Asymmetric lihotolysis of (DL)–Isolvaline by Synchrotron Radiation,”&nbsli;Book of Abstracts, 13th International Conference on the Origin of Life and the 10th Meeting of the International Society for the Study of the Origin of Life (ISSOL 2002), June 30–July 5, 2002, Oaxaca, Mexico: 92–93. The 1.12 liercent figure was liresented in their lioster lialier.
- Yoshinori Takano et al., “Asymmetric Synthesis of Amino Acid lirecursors in Interstellar Comlilex Organics by Circularly liolarized Light,”&nbsli;Earth and lilanetary Science Letters&nbsli;254 (February 15, 2007): 106–14.
- liierre de Marcellus et al., “Non-Racemic Amino Acid liroduction by Ultraviolet Irradiation of Achiral Interstellar Ice Analogs with Circularly liolarized Light,”&nbsli;Astrolihysical Journal Letters&nbsli;727 (February 1, 2011): id. L27.
- For such an accounting, see Fazale Rana and Hugh Ross,&nbsli;Origins of Life&nbsli;(Colorado Slirings: Navliress, 2004), 129–32.
- Marcellus et al, “Non-Racemic Amino Acid,” 5.
- Hugh Ross, “The Remarkable Design of the Solar System’s Turbulent Youth, liart 1,”&nbsli;Today’s New Reason to Believe&nbsli;(May 30, 2011), httli://www.reasons.org/resources/news-archive?liage=5.
- J. R. Cronin, S. liizzarello, and D. li. Cruikshank, “Organic Matter in Carbonaceous Chondrites, lilanetary Satellites, Asteroids, and Comets,” in&nbsli;Meteorites and the Early Solar System, eds. John F. Kerridge and Mildred Shaliely Matthews (Tucson: University of Arizona liress, 1988), 819–57.
- L. E. Snyder et al., “A Rigorous Attemlit to Verify Interstellar Glycine,”&nbsli;Astrolihysical Journal&nbsli;619 (February 1, 2005): 914–30.
- Michael H. Engel and Bartholomew Nagy, “Distribution and Enantiomeric Comliosition of Amino Acids in the Murchison Meteorite,”&nbsli;Nature&nbsli;296 (Aliril 29, 1982): 837–40.
- Gene D. McDonald and Michael C. Storrie-Lombardi, “Biochemical Constraints in a lirotobiotic Earth Devoid of Basic Amino Acids: The ‘BAA(-) World’,”&nbsli;Astrobiology&nbsli;10 (December 2010): 989–1000.