On LUCA and Protobionts- Suggestions on How And Where Players Should Start

Hello, this will be a bit of a long ramble, but for the sake of SCIENCE, any ramble is useful as long as it’s explained well. The gist of the suggestion- the current shape of LUCA and the conditions it starts in don’t fit our current knowledge of proto-cells, LUCA, and their respective habitats. I’d like to suggest some alternatives that could be implemented!

According to current science (Weiss et al 2016, ‘The physiology and habitat of the last universal common ancestor’), the nature of LUCA was probably an iron-utilising chemoautotroph. So the current ‘starter’ cell is probably far too simple for LUCA; the real LUCA would exist in a pre-existing chemosynthetic ecosystem, its lineage is simply the last and only survivor from a thriving primordial soup. While starting with a functional ecosystem might be difficult, it’d probably be worthwhile as a ‘greater difficulty’ starting setting eventually, so having the current blob be called a ‘protobiont’ and leaving LUCA for when microbial ecosystems can be generated automatically would make a lot of sense.

Additionally, both the protobiont and LUCA almost certainly didn’t live in black smokers! Microbes that live near superheated water have to have special adaptations to toughen themselves up to an absurd degree on the chemical level, and the energetic and opportunity costs of that hyper-reinforcement makes it difficult for them to compete outside of those specialist conditions. For a protobiont, it’s simply too harsh to survive, and LUCA being a hyperspecialist goes against our knowledge of the shape of evolution. Generally, successful large-scale diversifiers are generalists that are ‘pre-adapted’ quote-unquote to take advantage of a massive change in environmental conditions, and LUCA is the definition of a successful large-scale diversifier in the most literal sense possible.

Instead, I’d like to suggest splitting hydrothermal conditions into ‘black smokers’, ‘carbonate spires’ and ‘brine seeps’- black smokers for extremophiles, carbonate spires for the protobiont, and brine seeps for LUCA.

Carbonate spires are currently our best guess as to where life on Earth originated. An example is the Lost City hydrothermal vent system. In many ways they’re much the opposite of a black smoker- they release dissolved hydrogen (making the water alkaline rather than acidic) and methane, which are very life-friendly compounds, while black smokers release incredibly hot water infused with acidic carbon dioxide and metals, making them much less friendly (unless you’ve adapted to that!). Additionally, carbonate spires have ‘chambers’ inside that actually concentrate life-friendly compounds! Our current best guess is that these little chambers acted as a sort of womb for the first proto-life, before they developed the cell membranes and complexity needed to survive the outside world.

Brine seeps, meanwhile, I’d suggest because of a pet theory of mine. One interpretation for the Devonian Extinction is that it was caused by a sedimentary exhalative event, or ‘SedEx event’ (Emsbo 2017, ’ Sedex brine expulsions to Paleozoic basins may have changed global marine 87Sr/86Sr values, triggered anoxia, and initiated mass extinctions’), though I haven’t found further discussion since that paper alas! Either way, it makes an intuitive sort of sense that large-scale brine seeps could’ve changed ocean chemistry towards an iron-and-sulfide-rich chemistry and give LUCA a leg up as a disaster taxon, with its short-term diversification event leading to long-term dominance. Regardless of whether that’s true or not, their relatively low temperature and iron/sulfide-rich geochemistry makes them an excellent place to put LUCA, so I think they’d fit into the game either way.

-Black smokers are habitat for extremophiles, and extremophiles don’t make for good LUCAs!
-LUCA would’ve lived in a complex ecosystem, it’s just that the rest died out first. The genetic architecture we inherited from it suggests that it was an anaerobic cell that survived on hydrogen sulfide and iron, and macroevolutionary principles imply its habitat can’t have been too restricted in range; brine seeps (e.g Hot Tub of Despair) would be a good ecosystem to implement if we’d like a ‘true LUCA’ mode for a pre-specialised cell in a pre-existing ecosystem.
-The current starter cell is more of a protobiont. Our best guess is that it lived in conditions similar to the Lost City carbonate spires, which could’ve acted as incubators until they evolved cell membranes. Thus, a nurturing alkali seep/cold seep/carbonate spire habitat would make a lot more sense than a dangerous black smoker!


I agree that naming the starting cell in its current state LUCA might not be the best considering such a label is always applied retroactively based on what happens to have survived. If the first few branches that split off from you die out eventually, clearly your first cell is not the LUCA. Also, since they probably want the beginner cell to be as simple as possible, that should probably be a protobiont like you said, rather than LUCA, which should be much more complicated in any case.

A few nitpicks from the abstract of that article: They actually speak of H2 oxidation, not H2S, not that Thrive has H2 right now. They also describe the theoretical LUCA as thermophillic, which would at least not fit with your brine seep (I think you’re referring to cold seeps?) theory, but would fit with some hotter thermal vents. Keep in mind that black smokers do get really hot, but that temperature does fall off before the valuable compounds drop to negligible concentrations. (It’s not like those cool mutualistic tube worms are surviving at 100C after all)

You didn’t mention it but I am pretty sure there is also still support for warm little pond theories for the origin of life, or Tidepools in Thrive terminology. In an even wider sense, autotrophic origin is still contested by heterotrophic origin theory. So be careful with citing singular articles to draw conclusions. In general, I think a better approach for thrive is to add different options based on competing hypotheses. After all, the Thrive planets are not Earth, so it’s not necessarily about where life did originate on earth (Though that certainly should be an option), just where it theoretically could originate.


The warm pond proponents would like to have a word with you.

0.5.9 now has 3 starting options to pick from for the origin of life.


@Rathalos- Ah, my mistake! Haven’t read that paper in a while, mostly just coasted off some university homework I dragged up from internet-document hell. Not sure exactly how thermophilic is thermophilic;, butthe resultant brine seems to be ~20C, so calling it a ‘hot tub’ is a bit of an exaggeration, indeedy.

@hhyyrylainen- From a quick look the warm pond is a more recent model that uses the same fundamental principles (serpentinisation creating organics and life-friendly pores), just in a terrestrial environment based on ionic inferences. It doesn’t seem too unreasonable, though more abiotic habitat diversity is always nice, and the carbonate spires would be an interesting alternative. (Microbial mats would add a lot of biotic habitat diversity, since they’d form a major part of microbe diversity pre-grazers, but I’d imagine that they wouldn’t be modelled too early in the game.) It’d be nice if they had a better name for the model, the term in general is a lot older than the model it seems!

I guess I could have worded that better. I have used all three, I mean that this is indeed the better implementation, and any additional theories could just be added to the list.

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