2025MNRAS.538.1653M

Recent detections of carbon-bearing molecules in the atmosphere of a candidate Hycean world, K2-18 b, with JWST are opening the prospects for characterizing potential biospheres on temperate exoplanets. Hycean worlds are a recently theorized class of habitable exoplanets with ocean-covered surfaces and hydrogen-rich atmospheres. Hycean planets are thought to be conducive to hosting microbial life under conditions similar to those in the Earth's oceans. In this work, we investigate the potential for biological evolution on Hycean worlds and their dependence on the thermodynamic conditions. We find that a large range of evolutionary rates and origination times are possible for unicellular life in oceanic environments for a relatively marginal range of environmental conditions. For example, a relatively small (10 K) increase in the average ocean temperature can lead to over twice the evolutionary rates, with key unicellular groups originating as early as ${\sim} 1.3$ billion years from the origin of life. On the contrary, similar decreases in temperatures can also significantly delay the origination times by several billion years. This delay in turn could affect their observable biomarkers such as dimethyl sulphide, which is known to be produced predominantly by Eukaryotic marine phytoplankton in Earth's oceans. Therefore, Hycean worlds that are significantly cooler than Earth may be expected to host simpler microbial life than Earth's oceans and may show weaker biosignatures, unless they orbit significantly older stars than the Sun. Conversely, Hycean worlds with warmer surface temperatures than Earth are more likely to show stronger atmospheric biosignatures due to microbial life if present.