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A Catalyst for Life

Credit: agsandrew/iStockphoto

Credit: agsandrew/iStockphoto

By Rowena Ball

A chemical found in hair bleach may have catalysed life, and can even explain why new life is no longer being created from non-living building blocks on modern Earth.

On the prebiotic Earth more than 3.6 billion years ago there were no living cells and no proteins. Instead it is thought that life began with RNA molecules replicating in rock pores around hydrothermal vents prior to the evolution of DNA, cell membranes and proteins. This is the “RNA World” hypothesis.

Cell-free RNA replication requires thermal cycling – heating to separate the base-paired double strands, and a cooling phase to anneal complementary strands into newly replicated duplexes. While this fact is often overlooked in hypotheses about the origin of life, new research published in the Journal of the Royal Society Interface has proposed that this temperature cycling may have been provided by a natural thermochemical oscillation of hydrogen peroxide (H2O2). Exothermic reactions of H2O2 give rise to robust, self-sustained thermochemical oscillations.

H2O2 is ubiquitous on the modern Earth and in the biosphere, albeit in small quantities. Since the most primitive photosynthetic cells are thought to have used H2O2 as an electron donor, it is reasonable to assume that H2O2 was produced long before those organisms evolved.

Many scientists hold that the geochemical environment for the emergence of life was provided by submarine hydrothermal systems and hot springs. Experiments have shown that H2O2 is produced near hydrothermal vents when oxygenated seawater mixes with vent fluid.

Another source of H2O2 production in such an environment involves a surface reaction of pyrite (FeS2) with H2O. It can be shown that high concentrations of H2O2 may build up locally and be supplied at a constant rate for long periods of time.

Our study tested the oxidation of thiosulfate ions (S2O2–3) by H2O2. This hydrogen peroxide oscillator has just the right period and temperature amplitude to drive the replication of small RNA molecules:

  • if the period is too long, the RNAs decay faster than replication can increase them;
  • if the period is too short, the strands do not separate completely and replication fails;
  • if the temperature amplitude is too high the RNAs break up.

Thiosulfate ions occur in hydrothermal waters of Yellowstone National Park, so it is reasonable to conjecture that this ion was present in hydrothermal environments on the early Earth.

Porous rocks around hydrothermal vents could have provided microenvironments with naturally self-sustaining thermo­chemical oscillations. If an RNA supply is added to this recipe for the primordial soup, replication may be driven by thermo­chemical cycling.

Previous research has used artificially imposed thermal cycling to achieve cell-free RNA replication and amplification by complementary strand-pairing. We tested whether our H2O2 oscillator could have driven this process.

Computer Simulations

When we ran computer simulations that input experimentally measured thermokinetic and thermophysical data and output the temperature and concentration histories of the system, we found that the hydrogen peroxide oscillator can indeed drive rapid RNA replication and amplification, with the concentration of duplex RNA doubling every 0.3 seconds.

The phase relationships of our hydrogen peroxide oscillator (Fig. 1) show that the H2O2 reaction heats and cools RNA at the right rate to allow it to replicate. The exothermic reaction generates heat, so the temperature spikes to a maximum before declining as the reactant becomes depleted. The temperature reaches a minimum before the reaction cycle begins again.

The single RNA strands are consumed as the temperature rises while duplex RNA production, which requires heat, increases. Near the temperature maximum, single strands begin to accumulate again, but there is only a small spike before the temperature declines.

Interestingly, the oscillation is quasiperiodic, so the system is capable of complex periodic behaviour. This could give this molecular replicating system some powerful capabilities. For example, a biperiodic temperature response is capable of replicating two different RNA species. Nature may well have done exactly this in the primordial rock pores.

A Mechanism for Natural Selection

The response of RNA to H2O2 is a mixed story. Some small RNAs are very stable in the presence of H2O2, with a half-life of 12–30 minutes. There is also evidence that oxidation by H2O2 causes RNA dysfunction – but not degradation.

Oxidised RNA evidently may still replicate. For example, oxidised mRNA has been successfully converted to complementary DNA by reverse transcriptase, with the oxidised RNA bases inducing mutations in the complementary DNA.

Although H2O2 may damage or modify RNA (as could many other possible ingredients of the primordial soup), there is no reason to suppose that it cannot replicate and pass out of the reaction zone faster than it is damaged or degraded. RNA that is modified by the action of H2O2 in such a way that it can withstand H2O2 damage would then be favoured by natural selection – surviving and passing on this H2O2 tolerance.

Thus the hydrogen peroxide thermochemical oscillator provides a mechanism for natural selection and evolution, as well as a driver for replication and amplification! It may have created and driven the first truly living system!

Life on Other Worlds?

Hydrogen peroxide occurs abundantly on Jupiter’s moon Europa, and is believed to have occurred formerly on Mars, which suggests that these planetary bodies may have evolved their own RNA worlds.

The chances of a hydrogen peroxide thermochemical oscillator arising spontaneously on the early Earth in the presence of nucleotide precursors are perhaps very small. However, the Earth is large, rock pores are innumerable and there was plenty of time on the early Earth for improbable events to happen.

But why don’t we find spontaneously self-replicating and evolving RNA communities on modern Earth? Quite simply, there are no longer the amounts of hydrogen peroxide around that were there in prebiotic times!

Rowena Ball is an ARC Future Fellow at The Australian National University’s College of Physical and Mathematical Sciences.