Cell Specialization (Early Multicellular)

Theory
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Overview

I think one of the key elements that will be needed to make 2D Multicellular more fun is a natural incentive to evolve different specialized cell types. For example, there ought to be more benefit to a single cell with lots of digestive power versus many cells with only a little digestive power.

Additionally, another method is to make placement of cells matter more. Make damage affect the outermost cells first, so the player is incentivized to evolve their outer cells to be their skin cells / protective cells. Make neural cells grant a bonus to adjacent sensory cells, but make sensory cells ineffective unless placed on the exterior. Make digestive cells grant a bonus to adjacent absorptive cells / feeding cells.

With these two elements, the player will have to think more carefully about what cell types to evolve, and also where to place them in his colony. This will only apply to 2D Multicellular, because after the transition to 3D we will no longer simulate individual cells and the player’s creature will be organized into a series of tissues instead.

Finally, I think it would be good design to ensure that any mutations we consider adding to flesh out the cell types be available in both Microbe and Multicellular. It doesn’t make sense for an organelle to just appear only for colonies but not evolve in unicellular life.

Finalizing the Design

Since this is a topic that can be implemented soonish, I think it would be good to have a discussion and nail down the concepts, and then document it on the wiki. However, to be clear, I’m not pushing that we do this right away. This is meant just as a design discussion to document some concepts which we can refer to later when we actually get to implementing this. I’ll start by listing all of the common biological “cell types” that we will want to incentivize the player to evolve:

  • Skin Cell / Protective Cell. Cell types that focus on protecting the colony as much as possible.
  • Absorptive Cell. Cells that maximize their Phagocytosis Ability to be able to engulf the largest possible particles from the surrounding water, that the colony can then eat.
  • Digestive Cell. Cells that maximize their digestion efficiency and digestion speed to extract the max amount of nutrients from the food the colony eats.
  • Locomotive Cell. Cells that focus on producing locomotion for the colony through cilia, flagella, mucilage jets, or other means.
  • Endocrinal Cell / Hormonal Cell. Cells that maximize the production of agents, either for internal or external use.
  • Storage Cell / Fat Cell / Adipocyte. Cells that maximize their storage space to store the colony’s food.
  • Biosynthetic Cells. Cells that generate sugars or other nutrients (like ammonia) for the colony, using Photosynthesis, Chemosynthesis, or Thermosynthesis.
  • Sensory Cells. Cells that focus on maximizing sensory reception.
  • Neural Cells / Neurons. Cells that focus on maximizing nervous capacity / signal proteins.
  • Muscle Cells / Myocytes. Cells that are very fluid and flexible that maximize movement speed through contractile fibers.
  • Bioluminescent Cells. Cells that focus on producing light biologically.
  • Gametic Cells / Reproductive Cells. Cells in a sexually reproductive colony that focus on produce gametes.

Then we can discuss these cell types one at a time. What are the incentives to evolve such a specialized cell? What mutations would such a cell utilize? What traits would it try to maximize? Should it have any placement/adjacency effects? I’ll start with the Protective Cell:

Design: Protective Cells

As far as I know, damage to a colony only affects the outermost cells right? So the base incentive for this is already in place. The player will naturally want to evolve his exterior cells to be the most damage resistant.

What are some mutations that skin cells would evolve?

Scientifically speaking, there are not many mutations a skin cell will evolve at this point. It’s only when organisms get much larger that they will evolve many many layers of skin, with perhaps additional layers of scales, shells, and more.

When an organism is microscopic and soft-bodied, the best he can do is just evolve a layer of useless outer cells to sacrifice if he is ever attacked. These cells would have as few organelles as possible so that they cost as little as possible to maintain, and just serve to be the first line of defence. In Thrive, since we aren’t going to model the individual death and regrowth of cells in a colony, the benefit of skin cells will just be that they increase your overall health.

The alternative is to not be soft-bodied and evolve cell walls. I’ve seen many Let’s Players evolve cell walls for their outer cells, even when playing as very mobile organisms. From what I know this is pretty unrealistic, because any multicellular organisms with cell walls or mineralized shells (calcium carbonate or silicate) sacrifice most or all of their mobility. Or at the minimum, they keep a portion of their organism uncovered so that they have one region by which they can flap some flagella to generate thrust. So I think to address this the game should look at how many exterior cells have cell walls, and apply a penalty to the colony’s mobility based on what percentage of exterior cells this is.

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This is already in the game when the Godot bullet physics engine integration is not bugging (this has been a problem basically since we switched to Godot). If we get bad physics callback data damage etc. is assumed to have hit the colony lead cell as we just can’t know anything more specific.

There’s two bugs regarding this that need to be ironed out before cell placement can really matter without making the bugs much more annoying:

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Just a note: I think we should be very wary of implementing so many specialized organelles/parts with only a single role in the editor and instead implement whatever additional unique multicellular adaptations as multicellular-exclusive upgrades. We don’t want to clutter the editor with, for example, myosin/actin for muscular-system cells, but should instead include cool little organelle variants which can be utilized only once the player crosses the threshold to multicellularity.

Extending the above into more general principles, we should also be very wary of bloating the early multicellular stage with a huge amount of parts with completely different functions. Many cell types you mention - neural cells, (advanced) muscle cells, and (advanced) fat cells for example really only show up in diploblastic organisms. Organisms with a single germ layer, such as choanoflagellates, simple plants, and sponges have a very limited capacity for advanced cellular specialization. Specialization advanced enough to create nervous networks, vascular systems, fat cells, etc. only start showing up in diploblastic organisms or plants with multiple structural layers, and even then require a third germ layer to create further specialization (muscles in particular). Besides, for very simple organisms, many of these specialized cell types aren’t necessary yet; neurons aren’t very useful for organisms who don’t need to maintain various processes simultaneously yet or who aren’t really composed of multiple cells. If you are able to get your hands on a copy, I really recommend “On the Origins of Phyla” by James W. Valentine; it paints a very good picture of exactly when certain features and traits arrived on the path from simple multicellular organisms to incredibly complex animals.

I do think that looking completely through a gameplay lens, the 2D multicellular stage should almost serve as a cumulative and whacky closing act to the microscopic portion of Thrive. I envision it almost like an all-out brawl after the player proves they have successfully created a viable unicellular build; certain upgrades and modifications are granted now that the player has the capacity of multiple cells, such as poison/slime that spreads really fast, or cells with membranes that essentially act as a hardened shell if maximum rigidity is applied (which you mentioned of course), or cilia that are able to heavily manipulate currents. Then they must essentially cope with supercharged cellular traits, presenting one final hurdle on the way to the macroscopic aspect of Thrive.

It would be a lot for a player to deal with macroscopic-level concepts (nervous systems, muscular structures, etc.) through the microscopic editor, so I think we should instead be presenting macroscopic-level concepts when the game goes fully 3D. As such, the 2D multicellular stage should serve as the conclusion to the systems unique to the microscopic stage.

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@hhyyrylainen : Okay that is good to know, both of those do seem high priority to solve.

I would tend to agree and disagree with this. I agree that we shouldn’t bloat with too many organelles with little purpose, but I don’t agree that we should add multicellular exclusive organelles as a result. I think all organelles available in microbe should be the same toolset available to designing your colony, but have the fact that you now are able to place multiple cells act to simply unlock new potential or synergies with the existing organelles. Like for example, if we added a contractile fibers organelle (which would be the organelle that myocytes would use), it would also be a useful organelle in unicellular gameplay by increasing the movement speed of ameboid movement, so it would serve a gameplay purpose in both stages. It doesn’t make sense scientifically why new organelles would suddenly appear after multicellularity.

Yeah that is true actually, we could knock off Nervous Cells as a type we want, and we could look at Storage Cells as more simple cells with many vacuoles instead of advanced Adipocytes.

I would be hesitant to reduce this list too much though, since many of these other types do exist in early multicellular organisms, like digestive cells, sensory cells, myocytes (sponges do have them from what I see), etc. The player doesn’t necessarily need to create one cell type for each of these roles, they could have some of their cell types fulfill several of these roles. For example a typical “animal” colony could have a skin and sensory cell type, an absorptive and digestive cell, a storage cell, a locomotive cell, a muscle cell, a gametic cell, and their stem cell (so 7 types total).

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I was thinking it made sense because multicellularity allows cells to emphasize certain aspects of themselves without compromising their overall fitness since they get compensated by sharing resources with other cells. For example, a muscle cell would have practically no use in generating the amount of tension it can within its fibers if it was an individual cell, but it would certainly benefit an organism with a larger body plan. That amoeboid situation would be such a niche situation that I struggle to justify adding a completely distinct part for that situation unless we made a contractile fiber universally useful, which wouldn’t be really accurate (could also deal with the fact that we haven’t really established a concrete pseudopodia concept). That’s why I think we should deal with such examples through upgrades more and less through adding individual parts; upgrades as currently conceptualized are more useful for specific situations and require a bit less work in terms of modelling and all that.

We do see these features in non-diploblastic or non-triploblastic organisms, but only in larger organisms. For example, myocytes are useful for sponges because they can control the flow of water into and out of a macroscopic body to facilitate external digestion; but for tiny just recently multicellular organisms, I really don’t see the use considering the scale of forces at play. And other simple multicellular organisms like Placozoa display some interesting and unique features, but a quick Google Search tells me they consist of roughly a thousand cells (and their genomes seem to suggest that they have been simplified from much more complex organisms). For context, I think I remember @hhyyrylainen said that we should probably limit the maximum amount of cells in a 2D organism to around 25 due to the amount of processing power it would take. And besides, I think that managing that many cells on a cell-by-cell basis could really start getting unruly around that range. Of course, we can either abstractly represent these cells as a 3D texture (I saw a concept for that somewhere here) or hope for increased performance in the near future, but we do have to be mindful of not tasking the player with finetuning 30 or 40 cells on a cell-by-cell basis.

That’s why I had the thoughts I had above regarding the 2D-multicellular phase; I think it should represent an amped up unicellular experience, a guns-blazing farewell of sorts to unicellular gameplay since the player has a limited capacity to implement complicated multicellular features. I think we should start by considering existing cellular upgrades/features/parts and by finding ways to turn them up to 11, and then see where that gets us in terms of the 2D-multicellular phase.

For example, I can imagine one cool thing we can implement is having it so that a max-rigidity silicia/chitin cell takes absolutely no damage like a shield in a 2D multicellular organism, but also completely shuts off a cell to their environment so that they cannot absorb compounds and the such. We would need to make it so that cells with no membrane exposed (cells surrounded completely by other cells) have reduced compound uptake efficiency to both encourage the development of a simple digestive system and actually make it so that the above rigidity setting actually means something, but I think that’s pretty doable.

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Yeah. The limit to reach macroscopic is now 20, and even that seems a bit of a struggle given the current performance levels. So designing for 10-15 cells rather than over 20 would be much appreciated by the programming team (unless an absolute saviour arrives sometime soon and can solve our performance problems).

In macroscopic with metaballs we basically no longer have a performance limit. So we can have species made up of like 200 separately placed metaballs if we want. I think though that about 50-100 metaballs would likely be the “normal” / expected amount of spheres a species is made up of before reaching aware stage.
So there the limit is purely what will give the best gameplay experience (tedium vs control).

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@Deus

Yeah I see what you’re saying, perhaps it’s not worth making the 2D Multicellular Stage too detailed. Due to performance we’ll currently want to cap colonies at around 20ish cells, and the 2D-ness can be limiting to certain mechanics or features we’d want to implement. Better to save some systems for once you have a 3D organism and environment.

I feel like the following would be a good updated list of cell roles to incentivize for 2D multicellular: Protective, Offensive, Absorptive, Respiratory, Digestive, Locomotive, Exocrinal, Biosynthetic, Sensory, Biolumiescent

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