The fact that vast majority of species are split into two opposite sexes has been known to us humans since the earliest times. When we grow up we get much pleasure from this fact not so much because we give it careful scientific thought, but for a normal vertebrate or a plant there is simply no other way of life. There wouldn't be a next generation if humans were always absolutely accurate while doing that (I mean enjoyng the bisexuality). The other species just don't care, actually the opposite: they catch every possible chance to produce progeny  .

I have browsed much special literature, mainly in genetics, to find that there are NO full and complete definitions of the term sex (could it be because sex is an applied science and the practitioners aren't back from the field yet? Actually, look here:  ). Anyway here I give few suggestions on the books dealing with this exiting topic. It would seem that the beautiful castle of scientific speculation is mainly built on our everyday knowledge of the differences between boys and girls [illustrations skipped: you can find many on the internet, but I don't want this page to be X-rated ;-), but a lot of discussion is here:   ]. But I personally think that the advantages of having sex are generally accepted by scientific community after many profs did their best writing papers proving that. See for example this: or this (great!)

 So let's ask ourselves the question: why only two sexes?     Well, first of all, there are many animals and plants that have only one. They can be considered both as a single "person" or none at all. Actually, the problem of what is personality outside from biological point of view is as appalling, but is to be mocked later. Sometimes unisex species are quite successful, and have "properly behaving" close relatives. How can this be?? And what to call them? If bisexual species could talk as expressively as we can, what words would they chose? 

Let's imagine the following situation: The last pack of Neanderthal men (or women, which is less likely (why? Have you never been to the summer sales in the shops and fought over the best bargain with the other women? You'll learn what handbags are really used for!)) are scattered in a forest after a final assault by successful and merciless Cro-Magnon men . No way ahead, but instead to eventually die, leaving no progeny. Suddenly there is a creature amongst them that's a bit smarter, and in addition is able to do it with itself and have a baby . Will the rest blame it? Be jealous? Not sure. They may watch the process with deep interest, but that will give the poor things one weird but last hope to survive. The funny thing here is that (f+m)other can enjoy a little dual game with the remaining members of his+her group, with or without babies. This has no important what so ever. However, the Cro-Magnons are different story. They are successfull anyway, they can afford to be arrogant for the moment . And they're already vengeful. But it would take a lot for them to realise that they are facing deadly danger from the cornered and desperate enemy. Well... arrogance and stupidity is still the main driving force of modern social life. No! Look at what NATO has been up to recently... We here are not on that.

Back to the pack of our hipothetical Neandertals... What will they call this creature? Man? Woman? They will need another word. All that is a fantasy. But it has HAPPENED before and will happen again and again in evolution. The only important difference is that snails, clams and mosses do not need words. We do. Here comes the third sex. (f+m)other. Now, if we follow the possible history of the unfortunate hairy beings cornered by those skinny cruel , we find that although average Neanderthals could be more stupid than their enemies, the clone progeny of that lucky (f+m)other may be fitter then average Cro-Magnons. Therefore, they have a good chance to survive and proliferate. Perhaps they would eventually discover some America or other place where the Cro-Magnons would not touch them, or simply wipe out their enemies.

Can they return to bisexuality and enjoy it again? A tricky question. As in the opposite case, there should be forerunners leaving no progeny. It can still be possible to have fun and practisize mating efforts (nice term! I like it ) with some weirdos of the normal (f+m)others. And suddenly in meantime such a guy meets a proper partner. Imagine his or her joy! But how will they and their progeny be treated by the rest of their people? All sorts of conflicts are possible, even as far as the hopeless religious ones. They might fall into the same sort of trouble as the first successfull (f+m)other, they might smoothly integrate into the tolerant society, they might even eventually exchange a gene or two with (f+m)others to spread the nice habit and understandable advantages of bisexuality.

And in the meantime they will cross the ocean to find their conservative distant relatives that keep mating between themselves to evolve gradually into another species. So what, you may ask. That is exactly how a different species could occur. A very, very fundamental biological act.

But it never happened in reality, although some remote human tribes have spent more then enough time in isolation from the evolutionary point of view to become a new species. Is this because due to the human "lack of creativity" when it comes to making kids that's the reason? Perhaps.

This example is given here simply to avoid diving into theory right from the beginning. But let us look at what does it illustrate. We just went through the scenario of speciation where speciation went through a transient state where third sex appeared for some time.
So, in Darwin's terms it was formally origin of species, but we saw that in reality it was origin of sexes. This is my point here.

In the example above we saw that two bitterly split and competing communities were on the brink of the "final solution" for one of them. But the weaker one developed a third sex  which allowed them to produce serially more able individuals and thus escape extinction. However, in the end they found themselves in the unstable situation once again: they had to forget completely about the joys and advantages of bisexuality or  come back to it generating yet another species. Just look at the picture jumping below:
 

In general, animal evolution only knows two stable situations. One is robust minority of uni-sex species. They are rare, conservative, but can be very successful. And then there is the lucky alternative, a bi-sexual majority. Enough said about them. But for this to happen there has to be unstable three-sex communities, otherwise no transition would ever occur! It is reminiscent of modern politics if we substitute the sexes with political parties

(true, politicians today do remind one of sex and its variations). Here we have two poles of stability: the two-party system, and the rare but sometimes powerful one-party societies, or just simply dictatorships. Multiparty states are less stable and parties there tend to form coalitions. Those small groups who don't join the big guys remain marginal. Marginal? Hehe. See what I mean?

Now we feel, what (f+m)others may do.  Let us try to imagine what would initially look absolutely weird thing: a community with the nuber of sexes more then three. Actually I came across this idea after talking in a pub to a lady working as doctor at police station. She kept cursing her job saying that there were 7 sexes in human beings: males, females, gay males of the both kinds, gay females of the both kinds and cops... By her wording I could conclude that she had concrete reasons to claim that.  Next picture  illustrates what  would look an animal community with 4 sexes:

Actually we see here nothing but just 2 species perfectly reproductively isolated! In other words, if we assume evolution through third sex, there is not necessary at all to use term species. We may instead of speciation speak about generation of new sexes. Especially that is true if we assume, that 2 other species may be simultaneously generated.

Therefore, we may treat two relative species as simply the four sexes of only one species! They just never or almost never mate. Pursuing this line of argument to the end, we could conclude that there are no species at all! There are almost twice as much sexes, the only problem is to determine who of them sleeps with whom. Sounds great, doesn't it? (except if you had been watching the gay pride parade on Saturday. Group sex was definitely on minds of many of those taking part).