Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
Robert Frost, “Fire and Ice”
Fire and ice represent not only possibilities for the end of the world, but also for the beginning of life on Earth. Origin-of-life researchers have considered recently the temperatures under which life first emerged. While many scientists postulate that life arose amidst the high temperatures of deep-sea hydrothermal vents (go here to read one critique of this idea), a growing number are suggesting that life arose within ice.
In fact, these cold-temperature proponents maintain that an origin of life in ice solves a number of problems facing chemical evolution, as new research seemingly attests.1 Ironically, the same observations and lab experiments cited in support of an icy origin actually raise questions about the likelihood of this scenario ever having taken place on early Earth. When carefully considered, these studies demonstrate inadvertently that without the involvement of an intelligent Agent, life is not possible.
The Origin of Life in Ice
The possibility that frozen water covered the surface of early Earth helps fuel cold-temperature origin-of-life models. Such a “snowball-Earth” may well have occurred, because the Sun’s energy output at the time of the solar system’s formation was about seventy-five percent of what it is today. And unless early Earth’s atmosphere contained levels of greenhouse gases higher than those in today’s atmosphere, the planet would have been covered in ice. Whether or not this was actually the case remains controversial, but the chance that early Earth was a snowball planet opens up the possibility that life arose in the ice.
Microscopic regions of liquid water, called interstitial phases, reside within the solid structure of ice. Some origin-of-life researchers believe that the first steps in the emergence of life took place in these tiny locations. The interstitial phases serve as great locales for the origin of life. Even though cold temperatures slow down the rate of chemical reactions, they also increase the stability of the prebiotic materials, giving the reactants more time to react and preserving the reaction products for longer periods of time once formed. This stability and preservation may well offset the reduced rates of reactions, making it more likely for subsequent steps in the origin-of-life process to take place.
When water freezes, materials dissolved in the water concentrate in the interstitial phases of ice. This offers another advantage to the origin-of-life process. Many prebiotic reactions require high concentrations of the reactants, and the interstitial phases provide a milieu for high concentrations of reactants to be achieved.
The RNA World
Many origin-of-life investigators believe RNA was the first replicator and information-harboring molecule, predating both DNA and proteins. From this point-of-view, known as the RNA world hypothesis, RNA took on the contemporary biochemical function of both DNA and proteins by operating as a self-replicator that catalyzed its own synthesis. According to the hypothesis, numerous RNA molecules capable of a wide range of catalytic activity emerged over time. Eventually, this environment transitioned to an RNA-protein world that—with the addition of DNA—finally gave way to contemporary biochemistry. (For a more detailed discussion on the central importance of the RNA world hypothesis to the origin-of-life paradigm, please see here.)
In order for the RNA world scenario to account for the origin of life, researchers need to be able to demonstrate that RNA molecules can assemble from building block materials under plausible early Earth conditions. And if origin-of-life investigators claim that the evolution of the RNA world took place in ice, they need to demonstrate that this assembly can happen within the interstitial phases of ice.
Ribozyme-Assisted RNA Assembly in Ice
Recently a team of researchers from Cambridge, England, demonstrated that the ribozyme R18 RNA polymerase can assemble short segments of RNA within the interstitial phases of ice. Starting with a dilute solution of the ribozyme, magnesium chloride, and ribonucleotides (RNA building blocks), the scientists supercooled the solution to -7 °C and detected ribozyme activity. They determined that this activity stemmed from longer stability of the RNA polymerase and a two-hundred fold increase in effective reactant concentration in the interstitial phase of the ice.
On the basis of these exciting results, the researchers concluded that ice may have played a critical role in the origin of life, “promoting all the steps from prebiotic synthesis to the emergence of RNA self-replication.”2 They also note that within the interstitial ice phases, the diffusion of the RNA polymerase ribozyme is retarded, allowing ice to create “quasicellular” compartments that could have predated the emergence of the first true protocellular boundaries.
Trial by Fire
On the surface these impressive results seem to support a cold-temperature origin-of-life. However, deeper examination of the work reveals problems that raise questions about this model’s applicability to early Earth’s conditions.
For example, at cold temperatures most proteins and ribozymes usually undergo denaturation, losing their native structure and function (a phenomenon called cold denaturation). The ribozyme chosen in the study R18 RNA polymerase is unusual in that it doesn’t seem to experience loss of structure at cold temperatures. Other ribozymes the researchers considered for this work did lose activity at cold temperatures. This raises questions about the universality of the results. If R18 RNA polymerase is atypical, which it appears to be, but the loss of activity at cold temperatures is run-of-the-mill, then it is hard to believe that cold-temperature origin-of-life scenarios possess much validity.
Another problem was R18 RNA polymerase’s susceptibility to the presence of ions. Most ribozymes require magnesium ions to function; this is true for the R18 ribozyme. The researchers discovered, however, that in ice this ribozyme’s activity proved susceptible to the presence of all other positively charged ions. They also discovered that this ribozyme lost activity when the reaction mixture contained negatively charged ions other than chloride and sulfate.
Given R18 RNA polymerase’s sensitivity to ions, it is hard to envision how ribozyme-mediated RNA assembly could have taken place on early Earth. Surely, that environment consisted of ions other than magnesium, chloride, and sulfate.
In other words, these experiments do not have geochemical relevance. They only show that, in principle, ribozyme-mediated assembly of RNA could have happened on a snowball-Earth. However, these experiments also show the fastidiousness of the process, rendering the work all but meaningless for the origin of life.
Ironically, these experiments demonstrate that an intelligent agent is needed to assemble life from non-living systems. If it wasn’t for highly skilled chemists, who carefully selected the right ribozyme, judiciously controlled the solution conditions, thoughtfully excluded chemical interferrents, etc., ribozyme-mediated RNA assembly would never have succeeded in the lab.
So, was fire or ice responsible for life’s emergence? Neither, it seems. From what we learned about laboratory simulations of prebiotic chemistry, it looks like an intelligent Agent was responsible for life’s origin..
- James Attwater et al., “Ice as a Protocellular Medium for RNA Replication,” Nature Communications 1 (2010): doi:10.1038/ncomms1076.