I've read that the cell nucleus was once an autonomous organism that was "enslaved" by the larger cell at the time of the Eukaryotic Revolution. Are there any organs in the human body that we know of that started out as autonomous organisms (of course they would be unrecognizable in their current form)?
Very short answer
It is not the nucleus who was an independent organism but the mitochdonrion (organelle) (One might discuss this point and say that both are correct, it is just a philosophical issue of the identiy of the eukaryotic cell). The plastids (the chloroplasts for example) were also independent organism. In consequence, in every cell, we have DNA in the nucleus and DNA in the mitochondria. The mitochondria is contained in every cell of our body and is transmitted via the ovule (and not the spermatozoid) to the offsprings.
Now let's think about your hypothesis that one of our organ (let's say, our liver) might have been an independent organism in the past. Therefore, this organ would contain different DNA than the rest of the cells of our body. How would you transfer the DNA of your liver to your offspring? Well, it is hardly imaginable.
Moreover, it has been argued that because we now have a complex and efficient immune system, a endosymbiose would be very unlikely.
But no, none of the cell in our body come from a previously different organism. But every cell of our body is some kind of chimera of two previous cells. Note: the vast majority of our DNA are present in the nucleus and not in the mitochondrion.
It seems much more easy to have two organisms that stay together during reproduction when reproduction is mediated by striking.
You may find some funny examples (border cases) of two organisms that don't separate during reproduction in corals (zooxanthella), leafcutter ants (and their fungus), parasitoid bees (and the virus that females create in their ovary) and lichens
Remi.b is perfectly correct if the question were restricted to humans (and yes, nucleus is not thought to be a result of symbiogenesis; actually some bacteria have similar structures).
But in the animal kingdom there several cases when certain organs "start(ed) out" as more or less separate organisms:
- Many parasites have dwarf males and giant females (which actually feed on the host). These females when reaching maturity recruit actively swimming males, which upon attachment to female genitalia functionally become testicles and get nourishment from the female. The resulting organism can be viewed as hermaphrodite (although "self-fertilization" results in crossing). This phenomenon was described for example for Rhizocephala (Crustacea).
- Similar cases are known for free-living organism: for example, in some species of the deep-sea anglerfishes males are dwarf and after attachment to females become "parasitic".
- Siphonophorans (colonial Cnidaria) have several distinct organs (swimming bubble, locomotive organs, hunting tentacles and so on), but this differentiation is achieved via specialization of individual members of the colony - zooids. One can argue, that these organism were never separate, but for example one can still tell, which organs are medusae and which are polyps, each one of them "is homologous to a free-living solitary animal". In other colonial hydroids one can find something reminiscent, but the degree of integrity of the whole colony never reaches that of Siphonophorans.
- In some species of bryozoans (which are colonial as well), several zooids (they are called "heterozooids" in this case) devote their lives to defend or move the colony, becoming highly specialized organs of the colonial organism.
- There are probably other systems fitting in the question, can't remember right now.