A rudimentary form of natural selection likely existed in the primordial soup even before life arose on Earth. If so, the complex 鈥渆cosystem鈥� of prebiotic molecules may have made the eventual arrival of life much more probable.
Most experts presume that life arose from complex molecules such as nucleic acids and proteins, which were assembled from a mix of simpler units strung together with chemical bonds.
To examine how this might occur, and Hisashi Ohtsuki, mathematical biologists at Harvard University, used simple equations to model the growth of such chains of building-blocks.
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The model shows that because longer chains require more assembly reactions, they should be much less common than short chains. And if some assembly reactions run faster than others, then chains built from these fast-assembling sequences of building blocks grow to be most abundant.
Threshold of life
This bare-bones equivalent of natural selection makes the prebiotic soup an interesting place, they say.
鈥淚t generates a rich evolutionary dynamic 鈥� or what I would want to call a 鈥榩revolutionary鈥� dynamic 鈥� where you have diversity, you have information, you have complicated chemistry,鈥� says Nowak.
Such a system, full of novel, interacting molecules, would be the ideal milieu to generate a molecule with attributes that would favour the assembly of copies of itself. Nowak鈥檚 prebiotic selection could then act to refine this ability by ensuring that better replicators become more common.
At some point, Nowak鈥檚 model predicts, the best replicator may get fast and accurate enough to dominate the population, sucking up all the resources and driving all the other prebiotic sequences extinct. This is the threshold of life.
鈥淯ltimately, life destroys pre-life,鈥� says Nowak. 鈥淚t eats away the scaffold that has built it.鈥�
鈥楳urky area鈥�
In showing that selection actually precedes the origin of life, and helps to shape it, Nowak helps bridge the gap between nonliving and living systems. In a sense, he says, the prebiotic soup is constantly testing possible replicators, making it much more probable that one might eventually reach the threshold of life.
Nowak鈥檚 model helps clarify a murky area of research on prebiotic mixtures, but it offers little direct guidance to experimentalists, says , an origin-of-life researcher also at Harvard.
鈥淭he tricky part is figuring out exactly what the relevant chemicals to use are,鈥� she says. 鈥淢artin鈥檚 model is basically agnostic about that question.鈥�
Journal reference: (DOI: 10.1073/pnas.0806714105)
