Free Convection and the Origin of Life

Free convection is an important process in fluid dynamics. Yet Russ Hobbie and I rarely discuss it in Intermediate Physics for Medicine and Biology. It appears only once, in a homework problem analyzing Rayleigh-Benard convection cells.

How does free convection work? If water is heated from below, it expands as it becomes hotter, reducing its density. Less dense water is buoyant and rises. As the water moves away from the source of heat, it cools, becomes denser, and sinks. The process then repeats. The fluid flow caused by all this rising, sinking, heating, and cooling is what’s known as free convection. One reason Russ and I don’t dwell on this topic is that our body is isothermal. You need a temperature gradient to drive convection.

“Thermal Habitat for RNA Amplification and Accumulation,”
by Salditt et al. (Phys. Rev. Lett., 125:048104, 2020).

Is free convection ever important in biology? According to a recent article in Physical Review Letters (Volume 125, Article Number 048104) by Annalena Salditt and her coworkers (“Thermal Habitat for RNA Amplification and Accumulation”), free convection may be responsible for the origin of life!

Many scientists believe early life was based on ribonucleic acid, or RNA, rather than DNA and proteins. RNA replication is aided by temperature oscillations, which allow the double-stranded RNA to separate and make complementary copies (hot), and then accumulate without being immediately degraded (cold). Molecules moving with water during free convection undergo such a periodic heating and cooling. One more process is needed, called thermophoresis, which causes long strands of RNA to move from hot to cold regions preferentially compared to short strands. Salditt et al. write

The interplay of convective and thermophoretic transport resulted in a length-dependent net transport of molecules away from the warm temperature spot. The efficiency of this transport increased for longer RNAs, stabilizing them against cleavage that would occur at higher temperatures.

Where does free convection happen? Around hydrothermal vents at the bottom of the ocean.

A natural setting for such a heat flow could be the dissipation of heat across volcanic or hydrothermal rocks. This leads to temperature differences over porous structures of various shapes and lengths.

The authors conclude

The search for the origin of life implies finding a location for informational molecules to replicate and undergo Darwinian evolution against entropic obstacles such as dilution and spontaneous degradation. The experiments described here demonstrate how a heat flow across a millimeter-sized, water-filled porous rock can lead to spatial separation of molecular species resulting in different reaction conditions for different species. The conditions inside such a compartment can be tuned according to the requirements of the partaking molecules due to the scalable nature of this setting. A similar setting could have driven both the accumulation and RNA-based replication in the emergence of life, relying only on thermal energy, a plausible geological energy source on the early Earth. Current forms of RNA polymerase ribozymes can only replicate very short RNA strands. However, the observed thermal selection bias toward long RNA strands in this system could guide molecular evolution toward longer strands and higher complexity.

You can learn more about this research from a focus article in Physics, an online magazine published by the American Physical Society.

Salditt et al.’s article provides yet another example of why I find the interface of physics and biology is so fascinating.