Today, after twenty years of generous funding and intense research, astrobiology still ranks as science’s only data-free discipline. However, a team of biochemists and physical chemists at Lawrence Livermore National Laboratory now seeks to deliver researchers from their futile endeavors to find terrestrial and extraterrestrial solutions to the origin-of-life problem by combining elements of both.1
As the team notes, astronomers have discovered carbon- and nitrogen-containing molecules in comets (see figure 1 and 2) and especially in interstellar molecular clouds (see figure 3). The number of different carbon- and/or nitrogen-containing molecules detected in comets and interstellar clouds now exceeds 120. It is also true that comets have struck Earth at various times throughout its history and did so very frequently at the time life originated about 3.8 billion years ago (see figure 4).
What happens to comets and their supply of these molecules when they pass through Earth’s atmosphere and when they strike the planetary surface presents a big problem, however. Calculations and measurements show that both events generate so much heat (atmosphere passage generates 500°+ Centigrade while the collision generates 1,000°+ Centigrade) that they break down the molecules into components useless for forming the building blocks of life molecules.
Figure 1: Comet 81P Wild
In 1974, comet 81P Wild passed within 500,000 miles of Jupiter, which caused the comet to be perturbed into orbiting within the inner solar system. This new orbit enabled NASA to send the Stardust Spacecraft to the comet in 2004 to recover samples, which were returned to Earth and analyzed for organic molecules. The only amino acid indisputably detected in the sample was glycine at an abundance level of just 20 trillionths of a mol per cubic centimeter.
Image credit: released into the public domain by Chesnok.
Figure 2: Nucleus of the Comet 81P Wild
This image was taken during the close approach phase of the Stardust Spacecraft’s flyby on January 2, 2004.
Image credit: NASA.
The Lawrence Livermore research team challenged the idea that comets passing through Earth’s atmosphere to the surface will fail to produce, or leave unscathed, any prebiotic molecules with potential use in assembling life molecules. In their molecular dynamics simulations, the team showed that
- “impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine”2; and
- oligomers can break apart on post-impact quenching and, thus, lower pressures “to form a metastable glycine-containing complex.”3
Consequently, the team concluded that comet impacts could deliver amino acids, the building blocks of proteins, to Earth’s surface.
Figure 3: Sagittarius B2—A Giant Molecular Cloud near the Center of Our Galaxy
This map of Sagittarius B2 was created by the Very Large Array radio telescope in New Mexico. Red indicates stronger radio emission; blue, weaker. This gas cloud holds the record for highest number of detected carbonaceous molecules. Image credit: National Radio Astronomy Observatory/Associated Universities Inc./National Science Foundation.
Just how relevant are the Lawrence Livermore research team’s simulations to the origin-of-life problem? Let me raise five critical assessments and then conclude with what I believe to be the most positive research contribution made by the team.
First, glycine is the smallest and simplest of the 20 different kinds of amino acids found in proteins. It contains just 10 atoms—one nitrogen, two oxygen, three carbon, and four hydrogen—and has a molecular weight of 75. For comparison, biological amino acids arginine and tryptophan contain 26 and 27 atoms and possess molecular weights of 174 and 204, respectively. Also, glycine is the only non-chiral biological amino acid, that is, it comes in only one configuration. The rest come in two configurations, left-handed and right-handed. Proving the viability of glycine production is very different from establishing that the team’s proposed reaction can explain the production of the other 19 biological amino acids.
Second, the team’s proposed reaction did not produce stand-alone glycine. Rather, they demonstrated the possible production of complexes that “resemble glycine” and “a metastable glycine-containing complex.”
Third, the team did not demonstrate that their proposed reaction can deliver accumulating concentrations of glycine. They did not establish that the glycine or glycine-like complexes are not at some point destroyed as rapidly as they are produced. They admitted as much in their use of the adjective “metastable.”
Fourth, molecular dynamics simulations are not the same as laboratory experiments. Simulations offer a rough guide as to what can be expected from a laboratory experiment. But given the complexity of the chemistry under investigation, no result should be trusted until it is confirmed by laboratory tests.
Fifth, no matter how long the list of amino acids proven possibly producible by certain chemical reactions, that production is irrelevant to the origin-of-life problem. One must also show that the amino acids can be produced in adequate concentrations with adequate stability with perfect chirality (100 percent left-handed or 100 percent right-handed) under the physical and chemical conditions that existed on the 740-million-year-old Earth. So far, no proposed naturalistic reaction, including the one proposed by the Lawrence Livermore team, meets these conditions.
All that said, the team did make some important contributions. Their proposal offers a possible explanation as to why glycine or any other amino acid has yet to be detected in an interstellar molecular cloud. It also offers a possible explanation for the disparity in glycine abundance levels in the sampled comet 81P Wild (only 20 trillionths of a mol per cubic centimeter) and in the <a href="http://en.wikipedia.org/wiki/Murchison_meteorite">Murchison meteorite</a> (several parts per million).
The team showed that the terrestrial impact, rather than the comet’s indigenous inventory, might be responsible for higher levels of glycine. This conclusion would explain why a landed meteorite would possess a much higher concentration of glycine than a comet in interplanetary space. Follow-up laboratory experiments could determine, for example, exactly how much of the glycine in the Murchison meteorite was present before the meteorite entered Earth’s atmosphere.
More importantly for origin-of-life research, chemists can develop laboratory experiments to determine the quantities, if any, of glycine, other amino acids, nucleobases, and five-carbon sugars produced by various kinds of possible comet collisions with Earth. Molecular dynamics simulations, such as those performed by the Lawrence Livermore team, can help guide the design of such experiments. Chemists can use similar laboratory experiments to determine the rate at which various kinds of collisions destroy amino acids, nucleobases, and five-carbon sugars in the vicinity of the collision site. When all this is done, teams of chemists, physicists, and astronomers can develop realistic models simulating the comet and asteroid collision rates that occurred at the time of life’s origin on Earth (see figure 4). This would allow them to see to what degree, if at all, the collisions contributed to the inventory of abiogenetically-produced amino acids, nucleobases, and five-carbon sugars.
Figure 4: Late Heavy Bombardment Previous to the Origin of Life
Between 3.9 and 3.8 billion years ago, an estimated 17,000 large comets and asteroids bombarded Earth. This bombardment increased the planet’s surface temperature, making it hundreds of degrees hotter than the highest temperature that would allow amino acids and nucleotides to survive in an undamaged state. Once Earth’s surface cooled to where rocks and liquid water stably remained, the origin of life occurred.
Image credit for the background images: NASA/ESO.
Such experiments will not yield any insights into how, by natural means alone, all the amino acids would manifest a left-handed configuration and all the five-carbon sugars would manifest a right-handed configuration. Nor will such experiments show how amino acids, nucleobases, and five-carbon sugars could assemble naturally into proteins, DNA, and RNA.
However, by showing that the raw supplies of stable abiogenic amino acids, nucleobases, and five-carbon sugars on the early Earth were far too low to be of any value for any conceivable naturalistic origin-of-life scenario, scientists could help put to rest any notion that life’s emergence can be explained apart from the agency of a supernatural, super-intelligent Creator.