Humans have created a huge number of myths to account for the presence of life on Earth. It's only recently, however, that researchers have been able to investigate how the conditions on a primordial Earth could have generated molecules critical to life, such as proteins and DNA.

We've made lots of progress when it comes to understanding the chemistry that can create things like the individual bases of RNA, and shown that long RNA molecules can catalyze a variety of chemical reactions. But there's still a gap between the two; specifically, researchers haven't identified conditions that would allow individual RNA bases to condense into long chains that could potentially be catalytic.

Now, researchers have found conditions that would favor the replication and presence of longer nucleic acid sequences. They turn out to be remarkably simple: heat dissipation across an open rock pore. This sort of environment would have likely been widespread in the hot early Earth due to the presence of porous volcanic rock.

So far, most tests that involve copying nucleic acid molecules have been done in containers where the raw materials are all well mixed and then heated evenly. But that's unlikely to reflect the conditions on a pre-biotic Earth. Reaction materials probably diffuse away from a source location; heat might also come from a single source; and the whole reaction might take place on a rocky surface—or in cracks within the rock itself.

To model these sorts of conditions, the authors used a DNA-copying reaction called PCR. They set up the PCR reaction in a small tube called a capillary; the raw materials of the reaction flowed in from one end of the capillary, constantly diluting the DNA molecules undergoing reactions.

PCR normally requires a rhythmic cycling of temperatures to separate and anneal DNA. For their test, however, the authors instead set up a heat gradient, with one end of the tube hotter than the other. To complete a copying reaction, a molecule would have to migrate within the tube, going from the hotter to the cooler end, and returning to the hot end to start the cycle again. The reactions were seeded with a mixture of DNA molecules of various sizes, and then let go to see if any of the DNA would end up being copied.

Because of the flow the researchers set up, the volume trapped within the capillary tube was exchanged 150 times over a 7 hour period. During this time, the authors took samples of the reactions going on within the tube. The products were then separated based on size to find out which molecules ended up being copied.

Researchers observed that only the longer strands were able to replicate at a high enough rate to overcome dilution effects of the flow through the pore. The shortest strands quickly became extinct in the system. These results were supported by an analysis of how the DNA growth kinetics compared to the dilution rate. This analysis confirmed that bias towards longer replicating strands works best if the copying reaction is inefficient. Under these conditions, the dilution of the short strand occurs before it can be replicated efficiently.

Researchers also investigated how a well-mixed system, one that lacks temperature gradients, behaves. They found that in well-mixed solutions, copying short DNA strands was favored over longer ones, That's consistent with earlier work in the field, which had suggested it would be very hard to build up the sorts of long molecules that have interesting behaviors.

While this doesn't definitively show that hot rock pores were involved in key steps during the origin of life. But it does suggest that the barriers to the creation of longer nucleic acid molecules might not be as substantial as we thought.

Nature Chemistry, 2014. DOI: 10.1038/NCHEM.2155 (About DOIs).