Smallest living organism with a brain

I’ve thought about asking this several times, but hesitated because I’m not quiet sure how to articulate my thoughts. But, I’ll give it a try anyway.

What is the smallest creature/organism to have a brain? Of course, somebody’s going to ask, what do you mean by brain? OK, maybe brain isn’t the right term. I’ll try to explain what I mean by brain. I’m talking about a thinking process. At a bare minimum, something that allows an organism to react to it’s environment based on instinct.

For example, plant and animal cells react to their environment, but not based on any sort of thought, instinct, or emotion. They just, react. Whereas a microscopic flatworm has a basic nervous system, and brain, which allow it to do things, like search for food and use its ocelli to detect, and avoid light. So, even though it has no emotion or reasoning capabilities, I’m thinking there must be some active thinking process, however primitive, going on here, right?

Well, even if that wasn’t a very good example, I hope you get the gist of my question. Thank you.

I believe you’re asking for the smallest organism with a central nervous system. I nominate the fairy fly (see section 9) which is the smallest insect measuring in at 0.2 millimeters.


Oh, sorry, you said smallest brain. I was thinking of smallest soul.

For someone with over 11,000 posts at the SDMB, you should know better than to post that in GQ.

I dont know the answer, but this site has some cool movies of mites working on a construction project.

The smallest vertebrate with a brain is the stout infantfish measuring 7 mm.

Yeah, central nervous system is probably an accurate description. After I posted the OP, I realized that the way I should have described my question is, what is the smallest organism that can input data about the world around it, and make decisions based on that data, even if all the decisions are preprogrammed (instinct).

There are literally millions of species of roundworms that are microscopic. These creatures have an anterior nerve ganglion of some sort complete with nerves leading to a battery of environmental receptors in the head and three different but distinct nerve cords running for the enterior ganglion to aling the body. IOW microscopic creatures with a brain and CNS. There’s no doubt that these microscopic creatures can process data about the world and make decisions. They’ve even shown primitive learning behaviour.

If you want somehitng a little more advanced there is an entire phylum known as the tardigrades. These things have a distinc thead, limbs etc,. in addition to a brain and CNS and are still microscopic, although the giants of the phylum can be seen by the naked eye under ideal conditions.

TRY to refrain from remarks that disparage individuals or a group in GQ. I know it’s hard.

samclem GQ moderator

Is this place so PC that soimeone can’t even make what is an obvious joke? A not very funny one at that.


Not at all. I am not speaking for the Moderators/SDMB but my take on it is they wish to maintain the integrity of the various aspects of the SDMB. Many here could tell of other forums they have participated in that became unbearable due to poor moderation. The GQ section is clearly described as a place to give factual answers (or near as can be) to questions asked and they want to keep it on that track. If you wish to slam attorneys head over to the BBQ Pit on this same forum and have at it.

In short it is not Political Correctness run amok…just everything in its proper place.

Yeaaah… maybe. The hard scientific data is great, but even in GQ I like the bits of humanity around the edges. That’s what makes the SDMB what it is, otherwise we’d just be like Wikipedia or something.

This is a rather generic description of living things. Pretty much all organisms - plants, fungi, amoebae, bacteria - respond to their environment in some way.

Yes, I but specifically said I was asking about organisms that make decisions.

You’ll have to define what you mean by “decision”:

[li]Plant seeds generally exist in a dormant state. If you water them, some may “decide” to immediately germinate and grow; others may “decide” to wait.[/li]
[li]The slime mold Dictyostelium generally lives as single amoeba-like cells. Then, in response to some change in the environment such as lack of food, the individual cells “decide” to aggregate together, form a fruiting body, and produce spores.[/li]
[li]A single bacterial cell will tumble in its liquid medium. Then it “decides” to move off in another direction, hopefully where there’s food. Finding nothing, it “decides” to tumble some more, since active movement takes too much energy.[/li][/ul]

The “central nervous system” qualification suggested by previous posters seems to be a good way of limiting your parameters.

I get so sick of people who play semantics games and ask “What do you mean by X?” It should be pretty damn obvious what I’m talking about. Actually, since this isn’t the pit, I guess I’d better stop now before I go too far and tick off a mod or admin.

If you got the impression that I was just playing semantic games, then I apologize. I was just trying to clarify the limits of your question. Actually, your OP seemed to be relatively well-defined, which is why other posters quickly settled on the “nervous system” qualification. Your subsequent post appeared to expand the parameters, which is why I thought I would attempt to clarify, if not for you then for the benefit of others who may be reading this thread.

OK, in that case I take it back. Yes, the smallest organism with a nervous system is what I’m looking for.

Then how about all of those microscopic roundworms Blake mentioned? They not only make decisions, they can learn from their environment.

To head off what I think your reply will be, we cannot read minds. What is ‘pretty damned obvious’ in your head is utterly unknown to us. If you want to specify your topic down, do it explicitly instead of accusing people of playing word games.

Blake’s post answers my question. This thread, as far as I’m concerned is over.

I think that part of the point is that as you try to refine your search to “smallest thing that does such and such” is that as you zoom in on that line, the line just stays fuzzy. There might be no clear dividing line in the animal kingdom, really, where you just stop going from “reacting” to “thinking”.

Just as the line around plant/animal can get pretty fuzzy at times.

Even Blake’s example was of a thing that basically sounds like a clump of nerves attached to a string of nerves. There are probably things below that that are just clumps of nerves.

Defining “making a decision” is probably the crux of the matter, not just a “semantic game”. You could even construe our thinking as just a more complicated form of the bacteria tumbling around based on external stimuli.

Trying to do what you’re saying is a lot different than saying “what’s the smallest vertebrate”?