So, looking into design for the multicellular stage made me want to dive into the literature a bit. So I figured I might as well post my observations publicly. At the least, I will probably be using this to refer back to myself.
Don’t take any of this as solid game design or even suggestions for the final design. Just writing down my observations sticking close to the theory, with at most some speculation on how a game design sticking as close to the theory as possible could represent it. (Without regard for example for how much effort it would take to make some change for minimal benefit)
Diversity of ‘simple’ multicellular eukaryotes: 45 independent cases and six types of multicellularity
https://onlinelibrary.wiley.com/doi/10.1111/brv.13001
Very interesting article on simple forms of multicellular life (like before and just after the transition to MC stage). Gives a good overview of different structures, traits and methods in nature.
To summarise, the article splits up all its case study into several (sub-)categories. Largely based on how the multicellular colonies are actually formed. They also use a kind of “sliding scale” for degree of separation or complexity of the multicellularity:
- Formation of large cells with many nuclei, that may eventually be separated into separate cells. All of these are given a sliding scale generally based on how strict the division of cells/compartments is. Divided based on the type of the original cell:
- Just fairly large complex cells with many nuclei as in flagellates. “compartments” (each with nuclei) may be partially separated from each other by internal membranes, but the actual external cell membrane just wraps around the whole thing without dividing them up.
- Multi-nucleus amoeba-like cells that can potentially move their nuclei out very far so that you may eventually have small “cell bodies” each with their own nuclei, only connected by thin strips of cytoplasm.
- Cells with many nuclei that stretch out into very long strands with nuclei spread throughout. These can potentially be divided by the cell membrane into compartments or fully separated cells. (fungi are here).
- Normal cell division that simply does not have cells physically moving away from each other afterwards. Here there’s a kind of sliding scale from “only occasionally makes colonies” to “always makes colonies”, with the potential addition of “cells differentiate into different types“.
- Cells that are not actually physical touching but exist in a shared medium they produce. Note that the medium is metabolically active and under the collective control of the colony’s cells. So it really is like an extracellular matrix or body.
- Full physical non-seperation after cell division. (animals and plants are here)
- Internal budding: Dividing to make more cells inside the existing cells.
- Free floating cells coming together to form a colony. They put in a sliding scale here from “simply existing in the same area” to “cell membranes actually fuse together to make single cells” with addition of “making a body with a defined shape“.
- Cells physically moving together to associate. (slime molds are here)
- Amoeboid cells that extend cytoplasm arms to form networks.
(all of these have very nice diagram images of each subtype)
The authors don’t like setting any boundary between “colonies” and “multicellular organisms”, insisting that it’s a very gradual continuum. But one potential identifiers they do say:
…features that suggest a higher level of organisation. Three features seem to be especially informative and often co-occur: (i) different cell types; (ii) morphogenesis (presence of a conserved growth pattern, i.e. determinate growth); and (iii) presence of anatomical structures not present in individual cells.
Note that least 1 and 2 (probably also 3) are things that require and are enabled by the multicellular editor. So we can’t have these evolve fully separately that well. But this does mean that the stage transition with the editor change matches quite well with the biological transition to more advanced multicellularity. I do then also think having these three points fully evolved should be a requirement for the next stage transition.
We’ve previously had some requests/questions from the community about multi-nucleate cells, potentially as some sort of alternative to real multicellular organisms.
This article has convinced that such a thing is mostly indistinguishable from a “real” multicellular organism. Having multiple nuclei without separation of space in the cell just allows the cell to functionally get much larger, but without allowing specialisation. (We don’t really have space or reason for this in current Thrive planned design).
On the other hand, there are organisms that are technically single cells, but have nuclei in (mostly) separated compartments, either by barriers in the cell, or as “bodies” connected by thin strands . These could have specialised “compartments”. But they’re also functionally just the same as having separate cells (which in plants and animals can also have a lot of open connections). To the point that they can be difficult to tell apart for scientists.
So we can effectively just ignore this. At most, if we wanted to, we could have some “cell connections” option category for the whole organism in the MC stage.
This article reminded me of something else (that’s not the focus of the paper): the way our real life lineages of animals, plants and fungi fit into this model.
For animals and plants, as pointed out near the start, they are a very advanced form of “clonal colonies formed by cells dividing without separating”. Even their less complex relatives still mostly form colonies (if they do) via that same method, meaning the multicellularity evolved via this path. That is actually quite different from the current Thrive pattern of going from “colonies from cells coming together” to “multicellular organisms from cells dividing”. The end point matches, but the start does not.
Fungi meanwhile take an entirely different path where their close relatives just stay as very spread out filament-like cells with many nuclei, with some making partial compartments. Complex fungi just took the last step of (almost) fully separating those into cells. This is really quite unlike anything in Thrive.
For both of these cases though, I don’t think it’s worth it at this point to go through a great overhaul to make this “better match” earth life.