Tiny pores within volcanic rocks on ancient Earth may have provided the ideal conditions for replicating molecules, and could also have driven the evolution of longer and longer genetic sequences, researchers in Germany have shown.

One puzzling dilemma of origin of life simulations is that shorter fragments of genetic material replicate faster than longer ones and tend to out-compete them. This trend favours the loss of information over time rather than the development of longer strands.

But Dieter Braun and colleagues at the Ludwig Maximilian University of Munich think conditions that favour the opposite – as well as concentrating molecules in the primordial soup – could have existed inside tiny cracks within heated volcanic rocks on the sea bed millions of years ago.

‘We thought that a thermal gradient across a porous rock is a most simple and very common setting on the early Earth,’ says Braun. ‘That it can solve so many problems for the origin of life was actually not expected!’

The group set up an experiment to simulate conditions within individual rock pores. They immersed glass capillary tubes in continuously flowing water containing DNA fragments of varying lengths, and then heated them from one side to create a temperature gradient. They found that some fragments became trapped in the tubes and accumulated on one side, forming a concentrated pocket of DNA. Furthermore, when they fed fluorescently labelled primers, nucleotides and polymerase enzymes into the system and left it running for several hours, the fragments were able to replicate as they moved between hot and cold regions of the tube.

‘A millimetre-sized heated pore can accumulate molecules, keep them replicating by thermal cycling and feed them from an inflow,’ explains Braun. The process also favours longer DNA fragments: strands longer than 75 nucleotides replicate, while shorter ones are washed out of the tube. This survival of longer molecules, says Braun, is a prerequisite for the evolution of genetic information, and the rock pores could have acted as microscale ‘incubators’ for the building blocks of life.

Matthew Powner, who studies origin of life chemistry at University College London in the UK, says the team’s theory is convincing. ‘Thermophoretically driven growth of vesicles had been demonstrated before, but these new results demonstrate polynucleotide selection,’ he says. ‘If this selection can be coupled with mutation and, consequently evolution, really interesting things could be observed.’