Chemical evolution in a small Gulf Stream

Chemical reactions driven by the geological conditions on the early Earth may have led to the prebiotic evolution of self-replicating molecules. Scientists at Ludwig-Maximilians Universitaet (LMU) in Munich are now reporting on a hydrothermal mechanism that could have promoted the process.

Life is a product of evolution through natural selection. That’s the homework lesson from Charles Darwin’s book, The Origin of Species, published more than 150 years ago. But how did the history of life on our planet begin? What kind of process could have led to the formation of the first forms of the biomolecules we know now, which later gave rise to the first cell? Scientists believe that on the (relatively) young Earth, environments must have existed that were conducive to prebiotic, molecular evolution. A dedicated group of researchers is trying to define the conditions under which the first tentative steps in the evolution of complex polymeric molecules from simple chemical precursors might have been possible. “To begin the whole process, prebiotic chemistry must be embedded in an institution that causes an appropriate combination of physical parameters that dominates an unbalanced state,” explains LMU biophysicist Dieter Braun. Together with colleagues based at the Salk Institute in San Diego, he and his team have now taken a major step toward defining such a state. Their latest experiments have shown the circulation of hot water (provided by a microscopic version of the Gulf Stream) through pores in volcanic rock can stimulate the replication of RNA strands. The new findings appear in the journal Physical review letters.

As carriers of hereditary information in all known life forms, RNA and DNA are at the heart of research into the origin of life. Both are linear molecules that consist of four types of subunits called bases, and both can be replicated – and therefore transferred. The sequence of bases encodes the genetic information. However, the chemical properties of RNA strands differ subtly from those of DNA. While DNA strands pair to form the famous double helix, RNA molecules can converge into three-dimensional structures that are much more varied and functionally versatile. Indeed, specifically folded RNA molecules have been shown to catalyze chemical reactions both in the red tube and in cells, just as proteins do. These RNAs therefore act as enzymes, and are referred to as ‘ribozymes.’ The ability to replicate and accelerate chemical transformations motivated the formulation of the ‘RNA world’ hypothesis. This idea postulates that, during molecular evolution, RNA molecules serve both as stores of information such as DNA, and as chemical catalysts. The latter role is performed by proteins in modern organisms, in which RNAs are synthesized by enzymes called RNA polymerases.

Ribozymes that can bind short RNA strands – and some that can replicate short RNA templates – were created by mutation and Darwinist selection in the laboratory. One of these “RNA polymerase” ribozymes was used in the new study.

Acquisition of the capacity for self-replication of RNA is seen as the crucial process in prebiotic molecular evolution. To simulate conditions under which the process could be established, Braun and his colleagues set up an experiment in which a 5-mm cylindrical chamber serves as the equivalent of a pore in a volcanic rock. On Earth early, porous rocks would be exposed to natural temperature gradients. Hot liquids percolated through rocks under the sea flower, for example, would encounter cooler water at the sea floor. This explains why submarine hydrothermal vents are the environmental setting for the origin of life that are most favored by many researchers. In small pores, temperature fluctuations can be quite significant, and can lead to heat transfer and convection currents. These conditions can be easily reproduced in the laboratory. In the new study, the LMU team verified that such gradients can strongly stimulate the replication of RNA sequences.

One major problem with ribozyme-driven scenario for RNA replication is that the initial result of the process is a double-stranded RNA. To achieve cyclic replication, the strands must be separated (‘melted’), and this requires higher temperatures, which are likely to deplete – and inactivate – the ribozyme. Braun and colleagues have now shown how this can be prevented. “In our experiment, local heating of the reaction chamber creates a steep temperature gradient, which introduces a combination of convection, thermophoresis and Brownian motion,” says Braun. Convection stares the system while thermophoresis transports molecules over the distance in a size-dependent manner. The result is a microscopic version of a contemporary ocean like the Gulf Stream. This is essential because it transports short RNA molecules into warmer regions, while accumulating larger, heat-sensitive ribozyme in the cooler regions and is protected from melting. Indeed, the researchers were surprised to discover that the ribozyme molecules aggregate to form larger complexes, which further improves their concentration in the colder region. In this way, the delivery times of the labile ribozymes could be significantly extended, despite the relatively high temperatures. “That was a complete surprise,” Braun says.

The lengths of the replicated strands obtained are still relatively limited. The shortest RNA sequences are more efficiently duplicated than the longer ones, so the dominant replication products are reduced to a minimum length. Hence, true Darwinist evolution, which favors the synthesis of progressively longer RNA strands, does not occur under these conditions. “However, based on our theoretical calculations, we are confident that further optimization of our temperature drops is possible,” says Braun. A system in which the ribozyme is composed of shorter RNA strands that can replicate it separately is also a possible way forward.