When life began on Earth it didn't resemble modern cells, with their modern fancy DNA and cell nuclei and ribosomes. The fundamental property shared by all living systems is replication with heredity, and the first things that naturally arose on Earth capable of doing that weren't complex modern microbes... most likely they were tiny constellations of tangled nucleic acids. Our biosphere seems to have begun as an "RNA World", where nucleic acids bubbled and stewed inside the plumbing of seafloor hot springs, replicating and complexifying over time. Millions of different types of RNA tangles occupied such places, constantly reforming and copying themselves, constantly making copy-errors. Errors leading to better replication were naturally preserved... while molecular swarms that didn't hold themselves together very well got cannibalized by competing assembly reactions. Eventually some forms of RNA tangles grew to dominate their little plumbing-worlds. Those are the ones that led to true cells.

That's the vision offered by modern origins-of-life research. A recent study carried out by botanists working on planet pathogens has made that vision a little clearer. Selma Gago and colleagues at the University of Valencia in Spain have measured the rate at which mutations occur in a plant-disease viroid called Chrysanthemum chlorotic mottle. Viroids are simple life forms composed of just a few RNA strands bound together in a tiny bundle. Inside plant cells viroids insert themselves into the cellular machinery and co-opt it into making more of themselves, but early in Earth's history the simplest forms of life to first arise were probably something akin to viroids. In Earth's primordial seas, RNA bundles capable of building themselves from the sea-soup of amino acids were the first replicators... the first RNA replicons, which preceded true cells probably by several million years at least. Gago and colleagues' new study measured the mutation rate of their viroid subject, and found that it's a veritable mutation machine, with a stunningly high rate of approximately one mutation per copy. At that rate, a human embryo would quickly melt apart into a sickening goo of stuttering, dying cellular machinery.

Mutations add diversity to our genetic code, but too many mutations at once can be deadly. In simpler life forms mutations occur more frequently, partly because simpler life forms reproduce faster, but also because simpler life is beholden to less complicated and finicky bodily infrastructures in which mistakes can be costly. Bacteria mutate much more than animals do, but bacteria can double exponentially in hours... offering plenty of cannon fodder to the battle for genetic survival. Animal reproduction is more costly and yields fewer offspring, making mutations more dangerous and instilling stronger selective pressures favoring error-correction. But bacteria just don't need to worry as much (metaphorically speaking) about such things.

Viroids worry even less. As genome size decreases, mutation rates increase... and according to Gago and company's new study the rate at which viroids can mutate hedges very close to the highest mutation rate possible prior to absolute molecular disintegration. Enough mistakes and you shatter, but mutation rates lower than that critical threshold allow very simple replicators to persist with maximum adaptability. RNA replicons in Earth's primordial oceans would have likely behaved similarly... pushing the limit of mutability, beyond which lay total fragmentation, but within which a tiny tinkertoy complex of folded RNA could wobble and transfigure itself into any composition that yielded working copies. Given those circumstances, it's not all that surprising that at least one successful lineage was born that could survive down the eons in open water.

As Dawkins says, there are vastly more ways of being dead than of being alive. The trick with mutable replication is throwing out enough modified copies of yourself to explore as much potentially-being-alive territory as possible. The simpler the replicon, the more it can afford to vary... and those lucky few that stumble upon good engineering solutions to life's challenges are the ones who can push further into possibility-space.

mutationrates_2

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Tags: Emergence of life, Mutation, Mutation rates, Origins of life, RNA world

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Comment by cj the cynic on March 17, 2009 at 11:53am
Very informative. Thanks.
Comment by Ravenone on March 17, 2009 at 11:52am
HOLY SHIT...that was interesting. I just learned something cool today! Thanks DUDE!

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