Did life begin in ice?
Posted May 4, 2005
Special to World Science
New findings are backing up a theory that life originated in ice, researchers said. If it’s true, they add, it could boost the chances that life might turn up in places considerably colder than our planet.
The theory departs from mainstream thinking on the origins of life, which assume that warm, or hot, and wet conditions were necessary.
“Conditions associated with freezing, rather than ‘warm and wet’ conditions, could have been of key importance” for the first chemical reactions that led to life, wrote four researchers in the July 21 issue of the Journal of Molecular Evolution, a research publication.
The scientists, including Laura F. Landweber of Princeton University in Princeton, N.J., argue that ice might have been a favorable environment to generate the first self-replicating molecules, a precondition for life.
These molecules would be of a type called ribonucleic acids, or RNA—a chemical cousin of DNA, which makes up genes. Many researchers believe the first self-replicating molecule was RNA, not DNA. This is because RNA can do various things in addition to carrying genetic information, which is all that DNA basically does.
Some of RNA’s activities seem to be similar to what would be required for self-replication, something that DNA can’t do, strictly speaking. DNA needs the help of other molecules to copy itself. Also, RNA still exists in living cells, where it has various functions—some so basic to life that many scientists think RNA must have been there from the beginning.
The theory that RNA started it all, a 20-year-old proposal called the “RNA world hypothesis,” holds that RNA was not only the first self-replicating molecule, but also that it initially carried out most of life’s functions, such as metabolism and cell formation.
Most biologists consider the RNA world hypothesis at least plausible, but it has some problems. It’s not easy to explain how the first self-replicating RNA molecules, a prerequisite to life under the hypothesis, might have arisen.
RNA molecules tend to fall apart under warm conditions outside of cells. This would prevent the buildup of the rather long, complex RNA molecules that would presumably be needed to conduct life processes, according to Landweber and her colleagues.
Various conditions can prevent RNA molecules’ breakdown, the researchers argue. These include various types of water solutions, and freezing. But freezing may have been the one that most likely occurred on the early Earth, they argued.
Freezing usually slows down chemical reactions, which is why cold places are generally considered hostile to life. But freezing actually speeds up some of RNA’s key activities, Landweber and colleagues argue.
This is because ice contains hard, tiny compartments that hold the molecules together in one place, where they can react together. Some of these reactions result in the creation of bigger RNA molecules.
In liquid water, by contrast, the molecules don’t come close enough together often enough to react as much. Thus they tend to fall apart faster than they can react to create bigger products.
In essence, the small compartments in ice play the role that cells today play in bringing the molecules together to react, Landweber and her colleagues argue. Dehydrated substances—a sort of primordial sludge, for instance—could also have provided a function similar to ice, they added, but ice works better.
Landweber’s group conducted an experiment to test the theory. Led by Alexander Vlassov of
SomaGenics, a Santa Cruz, Calif-based biotechnology company, the researchers broke to pieces some RNA molecules found in normal cells. This process yielded more, smaller, RNA molecules.
By doing this, the researchers produced RNA molecules of sizes that proponents of the RNA-world theory think might have been available on early Earth. They then experimented to find out what sort of capabilities these smaller RNAs had.
Reporting their results in the May 25, 2004 issue of the jourrnal Nucleic Acids Research, the researchers noted that these broken-up RNAs still could carry out some of the same functions as normal
RNAs, but only in ice or sometimes other extreme conditions, such as dehydration.
These activities included grabbing other pieces of RNA and attaching them together, an activity called “ligation” that is similar to self-replication.
To fully self-replicate, a molecule must attach others together in such a way as to match the sequence of chemical pieces that characterize the first molecule. This process is called “template-directed”
But the ligation alone—even without the self-replication—is capable of building up ever larger and more complex RNA molecules, which according to the RNA world hypothesis could eventually develop self-replicating abilities.
The theory that an icy environment might have helped jump-start life isn’t new. Researchers proposed as early as 1994 that repeated cycles of freezing and thawing could help accelerate some of the chemical reactions necessary for life.
Such an environment could have existed on early Earth, where according to some researchers, repeated meteor and comet impacts might have periodically melted an otherwise icy environment.
However, Landweber and her team seem to be the first to have provided an account of how the “RNA world” might have fit into this scenario, according to Leslie
Orgel, an origins-of-life researcher at the Salk Institute for Biological Studies in San Diego, Calif.
The work “has important implications,” said Jeffrey L. Bada, director of the NASA Specialized Center in Research and Training in Exobiology in La Jolla, Calif., one of the original proponents of the freeze-thaw cycle theory.
Although Landweber and her colleagues also wrote that freeze-thaw cycles are helpful for the processes they describe, such cycles aren’t strictly necessary in their proposal.
Moreover, they wrote in their Journal of Molecular Evolution paper, “It is worth noting that Jupiter’s moon Europa and even Mars are also thought to contain large amounts of liquid water and ice now or at some time in the past.”
The possibility of RNA activities in ice, they added, “lends some credibility to claims that the rather extreme environments of these extraterrestrial locations could have provided suitable conditions for the emergence of life.”
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