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I understand that the human brain consists of many cells working together but could a single celled life form evolve something like a brain with similar intelligence to the human brain? I was thinking of a situation in which a planet has a highly advanced civilization of single celled organisms.

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  • $\begingroup$ Read Greg Bear's Blood Music. Short story is better than nothing, but he later wrote an entire novella. $\endgroup$
    – John O
    Commented Apr 25 at 13:44

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I don't think there's enough to a single Earth-model cell to encode the amount of information that intelligence rests on. Although there are the oddities we know as slime moulds. One might call them single cell organisms but that would be rather perverse. They're rather a form of life that has lots of cell nuclei floating around in a quantity of protoplasm bounded only by an external membrane. So elsewhere, life might not involve cells in the same way it does here.

Another model is a colony of eusocial creatures like bees or termites. The creatures are not particularly smart and may not be capable of life separated from the colony. The colony should be viewed as one individual far smarter than its ( non sentient?) members. SF of note: Orson Scott Card's Ender's Game and Charles Stross' Missile Gap.

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Swarms of honey bees can make good decisions despite the fact that individual bees are rather stupid (Honeybee Democracy, Thomas D. Seeley). Single celled organisms can, in theory, develop a similar type of collective intelligence without actually merging into a multicellular organism. This idea has been used in this book.

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The only real requirement for this to happen is the development of a sub-cellular equivalent of a neuron. Once this happens, the organism can become smarter without necessarily becoming multicellular. And could conceivably reach intelligence.

Note that it might be microscopic, or somewhat larger. (While there are macroscopic cells on this planet, I wouldn't expect anything too large due to food delivery and heat dissipation issues.)

I suspect that the hardest part in evolving a single cellular intelligence would be having some way to manipulate its environment. Without that, there really isn't a survival benefit to intelligence.

Another thing to consider is the organism reproduction method. If it reproduces by cell division like Earthly single cellular organism, then the question arises of what happens to the "brain" when it divides? It could stay in one, be split between the two, or be cloned like DNA. All three allow for different story possibilities.

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Yes, we have good evidence that monocellular slime mold can achieve rudimentary intelligence, including problem-solving skills. It can navigate mazes, learn, etc.

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  • $\begingroup$ Don't slime molds "solve" mazes the same way flowing water or a fire would? I.e there's no intelligence being displayed, its just expanding randomly and following energy gradients. $\endgroup$
    – M S
    Commented Apr 25 at 13:32
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    $\begingroup$ @MS no, slime molds actually use backtracking algorhitms. That's actually a pretty simple thing to do, but way more complex than flowing like water. $\endgroup$ Commented Apr 26 at 5:57
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I once had some thoughts about a society of intelligent unicellular animals (I never got far enough to decide if they were more like bacteria or amoebae). The idea was that a great number could link themselves together and act as neurons, enabling some sort of thinking. They would then attract more unicellulars to act as muscles, and build (in my case a city which baffled the explorers who found it -- sophisticated and well-maintained but apparently empty). When they were finished, they went back to being unicellulars.

The major problems I encountered were (1) why/how they decided to combine -- in other words, which unicellular decided to start a brain; (2) how they could have evolved (once you've mastered the trick of multicellular life, why give it up?); and (3) how they could transmit knowledge to their successors.

But if it helps you in building your world, you're welcome.

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In his Children of Time trilogy, Adrian Tchaikovsky introduces one of the most interesting alien species ever: thr nodan parasites. They are colonies of single celled organisms that are able to store more information in a cell than a human brain can contain. They are extremely intelligent, and have learned to copy the memories of sentient, multicellular organisms.

The nodan parasites saga for something resembling humanity is one of the most beautiful sci-fi stories I have ever read. The nodan parasites are introduced in the second book of the trilogy, Children of Ruin.

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The only plausible answer I can think of is networking AKA something like what is described in the novel and the TV series based on it called 'the Swarm'.

In that story (which takes place on Earth) there is a species of single cell ocean dwelling bacteria which 'evolved' intelligence long before mankind even existed. The thing is the 'intelligence is a collective agency based on colonies of the single celled organisms linking together and networking. The larger the mass of bacteria in a colony the greater that colonies thinking or processing power.

Basically its a form of biological computer in which any single isolated cell is not intelligent until in merges with or divides into a mass of bacteria of sufficient size to form a working 'brain'. Presumably 'messages' can be sent between individual brains/colonies via messenger cells containing packets of DNA encoded with information. So I guess you could call those motile bacteria 'messenger cells'. From memory in the book the bacteria can also infect and take over/control the central nervous systems of other marine species in order to have them perform tasks for the group mind. So I guess those bacteria would count as 'worker' cells.

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