Life may have started with both DNA and RNA

Published today by the Sutherland group, an exciting new possibility within the cyanosulfitic photochemical scenario, offering a potential way to solve two big problems within prebiotic chemical synthesis: the origins of DNA and the origins of purine nucleotides.

Find it here: https://www.nature.com/articles/s41586-020-2330-9 (you are welcome to ask me for a copy if there is difficulty accessing the paper).

A couple things worth noting: this possibility was first predicted a year ago by Bhowmik & Krishnamurthy (2019). Also worth saying that some of the key steps in this chemistry were found by Vaclav Chmela. This was the first synthetic chemistry he did as part of the Sutherland group, and this is his first paper.

Bhowmik, S. & Krishnamurthy, R. The role of sugar-backbone heterogeneity and chimeras in the simultaneous emergence of RNA and DNA. Nature Chemistry 11, 1009–1018 (2019). (https://www.nature.com/articles/s41557-019-0322-x)

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The putative geological plausibility of the sort of environment(dominated by hydrogen cyanide and/or various derivatives) envisioned by Sutherland and colleagues is hotly debated. Afaicg this is one of those areas that coming generations of planet hunting telescopes can help settle, if good spectral evidence is detected of planetary atmospheres for earth-like planets, containing non-neglible amounts of these compounds. If not, it seems to me this will remain entirely debatable for the foreseeable future.

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Mikkel, thanks for the reply. These are important insights, and exactly the sort of things I’m working on.

As well it should be. It’s not yet clear how ubiquitous such environments were, and its during a time where, on Earth at least, there is virtually no geological evidence. There is some preliminary data and work suggesting these conditions may have been global owing to a single giant impact (see Genda et al 2017; Benner et al. 2019, Zahnle et al. 2020), but its not clear whether this prebiotic chemistry is compatible with the hot and hazy environment that results (Zahnle et al. 2020).

It’s also not clear whether this chemistry could occur spontaneuously in these environments, even if they were available.

That’s right. Although Earth hasn’t kept a good record of its first 500 million years, we can look elsewhere in our solar system (especially Mars, see Sasselov et al. 2020) and outside our solar system. Specifically, we can look toward young rocky planets, going through the phase we might have, to see if the precursor compounds are present in those planets atmospheres.

On the much longer term, if life is reasonably common, we can use the distribution of life detections on exoplanets to figure out what scenarios are more plausible universally. Do they all cluster around stars with sufficient UV light (Rimmer et al. 2018)? Are they present on ocean worlds, up to a certain water content (those likely to have hydrothermal vents)? Or is their distribution based on some other criterion or criteria? It’s just as you say, looking at other worlds might give us the clearest insight about our own in this regard. If life is sufficiently common.

If life is rare, then we’ll learn that instead, but that’s about all we’ll learn from outside our solar system, at least in the foreseeable future.

References

Benner, S.A., Bell, E.A., Biondi, E., Brasser, R., Carell, T., Kim, H.J., Mojzsis, S.J., Omran, A., Pasek, M.A. and Trail, D., 2019. When Did Life Likely Emerge on Earth in an RNA‐First Process? ChemSystemsChem, 2, e1900035

Genda, H., Brasser, R. and Mojzsis, S.J., 2017. The terrestrial late veneer from core disruption of a lunar-sized impactor. Earth and Planetary Science Letters, 480, 25.

Rimmer, P.B., Xu, J., Thompson, S.J., Gillen, E., Sutherland, J.D. and Queloz, D., 2018. The origin of RNA precursors on exoplanets. Science advances, 4, p.eaar3302.

Sasselov, D.D., Grotzinger, J.P. and Sutherland, J.D., 2020. The origin of life as a planetary phenomenon. Science Advances, 6, p.eaax3419.

Zahnle, K.J., Lupu, R., Catling, D.C. and Wogan, N., 2020. Creation and Evolution of Impact-generated Reduced Atmospheres of Early Earth, The Planetary Science Journal, 1, 11

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How would you detect life? I’m curious.

Thanks,
Chris

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This is a topic all its own. One way to find life is to look for atmospheric biosignatures, spectroscopic evidence of gas-phase molecules that are best explained by the presence of life. There are two broad approaches here: look for what we know (oxygen and methane, nitrous oxide), and for what we don’t know (the ‘all small molecules’ project lead by Sara Seager). I’ve linked some nice talks about these. If people are sufficiently interested, I can start a thread about this, giving my own thoughts (and about the many problems that exist with all these approaches).

Clara Sousa Silva with a short introduction to biosignatures in general - YouTube
Sarah Rugheimer on conventional biosignatures - YouTube
Sara Seager on all small molecules (and other things) - YouTube
Clara Sousa Silva (again) about PH3 as a very promising candidate biosignature gas - YouTube

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