At the dawn of Earth, about four billion years ago, the planet was a harsh place: a dim sun, frequent asteroid impacts, and air without oxygen. Under such conditions, the first steps of life seem almost impossible. But there is a hypothesis that life took refuge in warm shallow ponds, where water would dry up and then refill. Such cycles, like salt concentrating from seawater in the sun, concentrated the needed molecules, helping them link into more complex chains — future RNA.
One key ingredient in this process is hydrogen cyanide (HCN), a poisonous gas with the smell of bitter almonds. It can enter ponds by two routes: falling from the sky with meteorites or forming in the atmosphere itself and being washed out by rain. The researchers built a computer model of the atmosphere and compared these two sources. It turned out that atmospheric delivery of HCN is usually greater than meteoritic delivery, and this holds for a wide range of conditions: around Sun-like stars and red dwarfs, and with different air compositions.
This means that hydrogen cyanide was probably not a bottleneck on the road to life. If it is abundant, the chances of the first RNA molecules appearing on other rocky planets may be higher than previously thought. However, the authors caution: this is just one brick, not the whole construction.
as written by the authors arXiv:2603.18769
Hydrogen cyanide (HCN) is crucial for the RNA World hypothesis, forming biomolecules essential for early life. Life likely emerged around 4 billion years ago during the early Archean Eon, a period on Earth with a fainter sun, frequent impacts, and a weakly reducing atmosphere. Warm little ponds (WLPs) are hypothetical protective aqueous environments that help explain the emergence and evolution of fragile prebiotic chemistry in such a hostile environment. WLPs need to undergo cycles of evaporation and rehydration, concentrating prebiotic molecules that increase the likelihood of (de-)polymerisation and forming early RNA molecules. We use a 1-D model of atmospheric chemistry to compare atmospheric HCN delivery to WLPs with exogenous sources. Using early Archean Earth as our baseline, we examine the sensitivity of atmospheric HCN delivery to the atmospheric C/O ratio, semi-major axis, assumed stellar host type, and methane budget, exploring conditions across rocky exoplanets. We find that atmospheric HCN delivery is sensitive to these parameters but its values generally exceed that of meteoritic delivery and our baseline Archean Earth. Planetary atmospheres with higher C/O ratios within the habitable zones of G stars and those closely orbiting M-dwarfs deliver the most atmospheric HCN. We find that atmospheric HCN delivery is remarkably robust, so this molecule is likely not the rate limiting step for the emergence of prebiotic chemistry on rocky exoplanets. This finding, with important caveats, potentially increases the probability of life emerging on other worlds.
Hydrogen cyanide exists in space: it is found in comets and in the atmospheres of distant planets, and on Earth it could have accumulated in ponds like salt in a drying sea.
In Stanisław Lem's novel Solaris, the ocean of the planet spawns strange structures — perhaps the first steps of life there also began with simple molecules like HCN.