Imagine pond scum: there is an interface between two fluids, and living cells are confined to this 2 dimensional structure. complexity has evolved hierarchical up to the point of having internal organelles (essentially eucaryotes) giving the cell some thickness, but that's been maxed out. A cell must have one side in the top fluid and one side in the bottom fluid to survive without rupturing. Furthermore, each side has a distinct material that is meant for the respective fluid (that is, they can’t pitch or flip over).

Now they’ve evolved a single cell thickness biofilm, daughters sticking together and performing different roles within the community.

But, given that it must remain only a single cell thick, how might it continue to evolve more complexity? How might it even develop intelligent behavior?

Note: I came up with this while pondering the scope of the tag, and how it doesn’t imply a 2-d .

  • $\begingroup$ What exactly keeps it flat? In a realistic setting, unless there is a very, very strong pressure to stay flat, it will eventually expand as it evolves as a species. $\endgroup$ – Zxyrra Jan 27 '17 at 0:38
  • $\begingroup$ @Zxyrra it is destroyed if the proper part of its surface contacts the wrong fluid. $\endgroup$ – JDługosz Jan 27 '17 at 0:42
  • $\begingroup$ The cells can’t get any thicker. I’ve not specified a reason; trying to keep the problem generic and abstract. $\endgroup$ – JDługosz Jan 27 '17 at 1:02
  • $\begingroup$ there are already organisms that are large sheets of cells only two cells thick, Trichoplax. $\endgroup$ – John Jan 27 '17 at 2:50
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    $\begingroup$ There are ample of youtubes where the mold Physarum polycephalum solves geometrical puzzles; it is almost as if it has some rudimentary intelligence even though it's just a blob of ooze. Sure, it will take any thickness from one cell to several when moving around, but you shouldn't limit the thickness to 2D as that's not entirely true for foams. A stabilized foam can have quite thick layers of liquid between the air bubbles and can allow at least bi-layers of cells. $\endgroup$ – Mrkvička Jan 27 '17 at 12:18

There's many arguments for how complexity can form in 2 dimensions. My favorites are:

  • Rule 110 - Rule 110 is a Turing complete ruleset which is done in one dimension. Typically it is visualized in 2 dimensions, where the other dimension is "time" (or number of iterations). This means your creature can not only compute, but it can visualize its own computations and analyze what happened.
  • The Holographic Principle - A lot of fundamental forces which operate in 3d can actually encode all of their information in 2 dimensions. In fact, some string theorists theorize that the entirety of the universe can be encoded in 2 dimensions using such a principle.

From what I remember, the biggest issue with 2d life is the inability to form a gut. Our gut is open on both ends, one end is the mouth and one is our anus. We can pass matter through it and, generally speaking, have what goes in come out. If we don't like what we ate, we can generally operate around that stuff until it is excreted because the material is moving through a hollow tube through our bodies. If you are in 2 dimensions, such a gut would bisect you into two independent creatures, unable to communicate except through whatever unknown medium you were eating. You lose the ability to eat first and ask questions later.

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    $\begingroup$ While this is interesting, note that the creature in question is three-dimensional, just relatively flat - so a gut could still form, a thin brain of normal neurons could still work, etc. $\endgroup$ – Zxyrra Jan 27 '17 at 0:49
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    $\begingroup$ @Zxyrra The OP stated that the cells are maxed out, so there's no way to thread a gut through them such that the cell surrounds it from above or below. I suppose it would be possible if extracellular material was permitted out of plane. You could have each cell that is part of the gut tied together with "ropes." $\endgroup$ – Cort Ammon Jan 27 '17 at 1:00
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    $\begingroup$ Cells could disconnect and reattach, like Plainivers’s zipper organs. $\endgroup$ – JDługosz Jan 27 '17 at 1:04
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    $\begingroup$ @JDługosz Yes, but that is fraught with difficulty. It's much easier to do things if you don't separate. Imagine how much harder it would be to walk if some mistakes could cause your leg to disconnect from your body and need to be re-attached. It also opens up the question of "reattach to what?" A malicious foodstuff could attempt to do a man-in-the-middle attack on the body by forging the signals of the other cells. $\endgroup$ – Cort Ammon Jan 27 '17 at 1:06
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    $\begingroup$ @a4android Gotta love it. The 2d world... where making a brain is easier than making a stomach! $\endgroup$ – Cort Ammon Jan 27 '17 at 1:35

This is a really interesting question, and I wanted to take a swing at it. The fact that we have an essentially 2D organism living in a 3D universe leads to some interesting possibilities and challenges. Let's a few examine them:

  • Eating: This one was addressed extensively in the comments on Cort Ammon's answer. I think the best way to solve it is the suggestion of "arms" that reach out to engulf a foodsource and pull it in. This is essentially how amoebas eat, and the technical term for these arms is pseudopods. The process operates easily in this almost-2D setup we've got going, as long as the cells' foodsource doesn't have significant thickness, so that it can be enclosed from the top and bottom as well. I'm assuming this is possible, since your cells themselves need to be closed on the top and bottom. The whole process is called phagocytosis.

  • Thinking: You could actually use a pretty traditional nervous system setup, with some differentiated daughters acting as neurons, passing electrochemical signals back and forth and relaying them to the non-neuron cells with chemical stimuli. The main difficulty here is size; our brains are a dense 3D network with a fairly small volume. Something of similar complexity laid out into two dimensions is going to require a lot of surface area to have a similar level of complexity. Unfortunately, we can't just expand outward forever; eventually, the distance is going to be a problem. Whatever nervous system architecture this creature has, it would need to be very different from what we see in 3D creatures; the "brain" will probably end up being spread out through the whole creature.

  • Moving: This is probably one of the easiest problems. 2D locomotion turns out to be surprisingly easy. Check out this YouTube video to see an example where someone has used an evolutionary algorithm to design creatures that can locomote in a 2D aquatic environment. There are a lot of different ways to solve this problem, some of them fairly non-obvious.

  • Communication/Sensing: If you have more than one of these creatures, they probably need to be able to communicate somehow (especially if you're looking for intelligence), and of course sensing their environment is crucial. Given sight isn't the best option for a 2D film, we probably want to be looking at chemical or acoustic signals. Chemical signals are obvious: "taste" the surroundings for signal chemicals, and release chemicals into the environment to send signals. In terms of sensing, this could also take the form of "tasting" foreign objects to see what they're made of/how they are structured (at least in the vicinity of the phase boundary). Acoustics is promising for longer-range communication. The film could vibrate the boundary by undulating up and down out-of-plane, sending out ripples that can be interpreted as sound (like we humans vibrate air to send acoustic messages). Electromagnetic communication might also be possible for even longer-range communication (generating bioelectric fields like an electric eel, for example), but I think it would probably be more evolutionary trouble than it's worth.

  • Tool Use: This one is going to be difficult given the 2D constraint. Probably the best bet will involve collecting grains of fine sediment trapped in the phase boundary, and sticking them together using some kind of bio-glue (which probably evolved for a different purpose, like attaching to solid surfaces). This could be used to make small objects, or even fairly large objects if the bottom phase is sufficiently more dense than the particulate (eg mercury and sand should work very well). This condition would let you even do things like make 2D "houses" for your creatures (nearly-closed polygons of glued sediments with a single entry hole that can be stoppered from the inside, something like what a trap door spider does, but in 2D). If your creatures somehow got intelligent enough,

There's lots of interesting stuff that can happen here if you get into specifics. For example, what are the two fluids? Being at a phase boundary means there's a potential to exploit differences in the phases for various purposes. For example, if there's some mixing, the cells could allow demixing of small droplets through special channels in their bodies, harvesting energy from the potential gradient (essentially, using gravity as a food).

In general, I think it's going to be fairly easy to have a complex creature of moderate intelligence (eg, insect level), though it would likely be a very different type of intelligence from what we're used to because of the unusual nervous system architecture it would have. So, we humans probably wouldn't be talking with these things, but we may see evidence of their intelligence if they build tools and structures.


I think that it is unlikely.

Intelligence, in our case, came from competitive pressure. Our ancestors weren't the strongest or the fastest. In order to survive, we had to be the smartest. We used hands and tools to compensate for strength, speed and a lack of natural weapons. We also altered the world around us to increase our survival rate.

So, what on that world would put that kind of pressure the ancestors of the sentient bed sheets? Also, how would those beings use that intelligence to manipulate the world around them to increase their survival.

I don't know how they could get past the bang two rocks together to get a sharp rock phase when they can't bang two rocks together. I'd say tht tool using is pretty much out.

Could they become as intelligent as octopi? Probably, if they need to search for food and hide from predators. Could they become as intelligent as cetaceans? Maybe but not likely. I don't see that they would have the mobility to form highly evolved packs for hunting and defense.

  • $\begingroup$ I was thinking more along the lines of the physical mechanisms that result, not the need for intelligence in the first place. $\endgroup$ – JDługosz Feb 24 '17 at 23:03
  • $\begingroup$ Well, if you don't know why/how they became intelligent, then you risk them becoming just TV aliens with forehead appliances. Now, if they were designed to be intelligent and didn't need to evolve, that's a totally different story. $\endgroup$ – ShadoCat Feb 25 '17 at 0:38

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