The definition of an organism Wikipedia gives is

An organism is any contiguous living system.

I understand that these are not organisms according to this definition:

  • A bird and a tree it's perching on are living and contiguous, but they don't form a system.
  • All humans are living and form a system, but they are not contiguous.

But some things are unclear to me. My veins and my skin are contiguous and living. Do they form a system? I think so, at least according to what I read about systems in Wikipedia. They are definitely interconnected; they have behavior; and I believe they have structure, though I'm not really sure what structure is. Perhaps if we carefully define what we want "structure" to mean, we can save the definition from allowing (skin + veins) to be an organism?

Is (mother + her unborn child) an organism? I think it definitely is according to the definition. Is the unborn child an organism?

Is an apple that's just fallen off a tree an organism? Was it an organism before it fell?

Is (human + their microbiome) an organism? Is a human without the microbiome an organism?

I think the part of the definition that the least clear to me is the word "system", and in the definition of the word "system", it's the word "structure".


2 Answers 2


I think the definition in Wikipedia is simply bad because it depends on another debatable definition.

I prefer something which follows from an observation made by Ricard Dawkins in The Extended Phenotype (the following is my definition, but I think Dawkins had something similar in mind):

An organism is any system of components which depend on each other for survival and cooperate in proliferating potentially unboundedly.

A single-celled organism is by this definition an organism since it proliferates by coordination of its components.

A single-celled organism with organelles deriving from endosymbiosis (think mitochondria) is an organism: even though mitochondria do proliferate on their own inside the cell, they don’t do so outside of the cell, hence have a bounded proliferation only; and they depend on the cell as a host. On the other hand, they cooperate with the cell division.

Endosymbionts, on the other hand, are organisms because even though they depend on a host (and the host on them), they may theoretically switch host or even survive a limited time without a host.

Similarly, the cells in a multi-cellular organism can sometimes divide independently (in organs) but they cannot survive without the rest of the organism, and the unbounded reproduction of any organ in fact requires going via the germ line of said organism.

A virus which depends on a host to proliferate does not form an organism with the host since it doesn’t use the host’s germ line; if, on the other hand, it does use the host’s germ line (consider virally derived pseudo-genes on our DNA) then, yes, it forms a common organism with its host.

Is (mother + her unborn child) an organism?

I wouldn’t say so, since the child is only temporally part of the host.

Is an apple that's just fallen off a tree an organism?

Interesting; I’d say yes, as long as it retains the potential ability to proliferate (it’s a separate organism from the tree though).

Is (human + their microbiome) an organism?

No. Humans can survive (albeit only shortly) without a microbiome; in the same vein, the bacteria of the microbiome can change their host.

Unfortunately, this definition also has a drawback: there are organisms which cannot reproduce because they are sterile, yet they are clearly still organisms.

Note that other definitions (e.g. “an organism is a collection of components sharing common genetic material”) also don’t work universally since they also break down – the example of genetic commonality would preclude mosaicism and organelles derived from endosymbiosis.

  • $\begingroup$ Thank you. I'm not sure I understand the part on mitochondria. Do you consider a mitochondrion an organism or not? You say "endosymbionts are organisms", but also "mitochondria . . . have a bounded proliferation only". Aren't mitochondria endosymbionts? $\endgroup$
    – ymar
    Aug 27, 2012 at 15:03
  • $\begingroup$ @ymar No. They form an organism with their host cell. Endosymbionts are organisms but mitochondria are derived from endosymbionts. They are no longer endosymbionts (according to the common definition of the term). $\endgroup$ Aug 27, 2012 at 15:05
  • $\begingroup$ OK, I see. Could you explain what you mean by potential unbounded proliferation? $\endgroup$
    – ymar
    Aug 27, 2012 at 15:11
  • $\begingroup$ @ymar Literally, proliferation without (theoretical) limit. Mitochondria can try as hard as they want, they cannot proliferate beyond their host cells. On the other hand, humans can theoretically proliferate to the bounds of the biosphere, which, itself, has no fixed bounds (except the edge of the Universe if you’re so inclined). $\endgroup$ Aug 27, 2012 at 15:35
  • $\begingroup$ I don't understand. Let's forget about humans for a moment because technology makes things complicated. But a cod cannot live on the desert unless there is a caring human that supplies water and food for it. There surely are limits to where cods can live and proliferate by themselves, right? $\endgroup$
    – ymar
    Aug 27, 2012 at 15:43

I believe you are using the incorrect definition of "Contigous." The most common usage is basically "sharing a border," but it can also be:

-touching or connected throughout in an unbroken sequence contiguous row of houses


You also need to include the part about what defines "living":

"... can react to stimuli, reproduce, grow, and maintain homeostasis."

Your organs form a system, which results in you. A mother and her unborn child could also be considered an organism under this definition. The apple would not, nor would the tree and bird combination.

  • $\begingroup$ I think that's the definition of "contiguous" I'm using. Why do you think it's not? I know all of my organs together form a system. What I'm not sure about is whether my skin together with my veins forms a system. Is being made of living cells enough for being living? I think it should be if a colony of bacteria is considered an organism. $\endgroup$
    – ymar
    Aug 27, 2012 at 9:52

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