Enduring Universe

June 6, 2011

We encountered this concept of a shadow biosphere that suggests some organisms on Earth may be relics of a previous genesis event. Right now, this is just speculation on the part of some astrobiologists who haven’t been able to find prove of this idea yet. And if life truly did arise on Earth more than once, where is it now? What does it look like and how does it compare to life as we know it now? How would we look for evidence that this actually happened? The most straightforward way to find answers would be to test the genome of bacteria from as many different locations as possible, and the more exotic, the better. (After all, it’s unlikely that something as big as a giraffe for example could have spawned from a different origin event, but something as small and elusive as a bacteria could go unnoticed unless we found it in the backdrop and sequenced its genome.) We would want to look for places that would be cut off from the rest of the biosphere; like miles underground. Scientists found bacteria colonies that consumed hydrogen bubbling from the crust miles below the surface in an abandoned South African mine. DNA analysis showed that this germ was related to all other life on Earth, but what does that mean? How would we know if life were alien to the planet?

If life as we know it didn’t use DNA or RNA, but relied on a different molecule for storing genetic information, then we might see something wildly different from life as we know it. Peptide nucleic acid (PNA) is composed of repeating N-(2-aminoethyl)-glycine units linked by peptide bonds; the structure is more stable under high temperatures and low pH so it’s possible that it could have been incorporated into early cells and that DNA only came later because DNA is more efficient at replication. Another analogue, threose nucleic acid (TNA) is a polymer used by biochemists in studies on DNA; this analogue can function like DNA or RNA, but TNA is simpler chemically, so it could have been a precursor to RNA.

While the evidence to support a TNA or PNA-dominated biosphere isn’t readily accepted by most molecular biologists, the RNA world hypothesis has enjoyed some support by researchers over the last 30 years. Because RNA can act as an enzyme protein and an information-storing nucleic acid, this versatile chemical is thought to have preceded the advent of DNA, leading some to suspect that early life ran off of RNA and not DNA. It wasn’t until later when a genetic accident could have spawned DNA, which would have dominated the life at the time because it could more efficiently copy itself.

Areiosan life runs off of PNA, but it has a triple helix structure. This idea isn’t new to biology; Linus Pauling suggested DNA might have a triple helix structure back in the 1950’s, but the discovery of DNA’s double helix structure by Watson and Crick completely shattered that idea. Still, when molecular biologists use PNA, it latches itself onto the double helix structure and forms a totally new triple helix that stabilizes DNA under higher temperatures and lower pH. The latter is imperative for Areiosan life because of the prevalence of sulfur in the environment. Sulfur dioxide and trioxide readily form sulfuric and sulfurous acid in contact with water, so increased acid rain has lowered the pH of Areiosan bodies of water. So while the acidified oceans would warp and denature Terroan DNA, Areiosan PNA doesn’t get bent out of shape by higher acidity.

DNA undergoes mutations regularly because the process of transcribing new DNA during cell replication is imperfect. DNA can be miscopied during transcription, but DNA polymerases that add base pairs to DNA during replication can correct some of these errors during a stage called elongation in eukaryotic cells. Because this PNA’s base pairs are harder to pry apart under moderate pH, the proofreading process in Areiosan cells is shorter than in Terroan cells. Because of this, a certain type of error called a mismatch error abounds in Areiosan cells because once replication is complete; there is no proofreading mechanism to follow like with our biology. Despite radically different mechanisms to mitigate mutations to the nucleic acid, both Earth life and Areiosan life experience identical rates of mutation. Mechanisms for repairing DNA are not perfect and these imperfections that allow mutations to occur spur the creation of novel genetic material. If there were no mutations, there would be no evolution and each individual would be a carbon copy of the parent; this would be disastrous because if every individual were exactly vulnerable to the same stresses, then a single event would wipe out an entire population. Mutation causes a varying ability for survival in individuals of a population, and this diversity can provide resilience to a population. Some of those mutations will prove to be deleterious to the individual, but the heritable mutations that don’t manage to wipe the individual tend to persist or even propagate within the population. These errors in DNA replication are the basis for new genetic traits, but if this process is too imprecise, it would kill off too many individuals and prevent that material from ever being propagated. The rate of mutation is therefore fine-tuned to allow “a balance between the evolution of species and the survival and reproductive success of individual organisms.”

This diagram shows the configuration a stable triple helix used on Earth in drugs for cancer treatment.


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