I'm a big fan of history in the same way I am a lot of things; I tend to skim the basics and skip to the good parts, not totally investing in it like a real scholar or academic might. Regardless, I do manage to hang onto a more then a few important bits, especially if the subject interests me or has a very broad and/or useful application. I recall one article discussing exactly why we grow old and die, and why this has actually helped our species out; our ability to survive, learn and create are assisted by the fact that a new generation comes along to replace the old.
I'm also working my way through a book discussing why certain cultures and peoples managed to climb to the top of the heap. While the immediate reasons (technology, for one) are fairly simple, the indirect factors, the things that lead to a scenario in which a wayward asshole from Spain and relative handful of soldiers can bamboozle and capture a King are much more complex. Really, there is a lot to be said on that one but lots of people smarter and more qualified have already done so; I'm hear to talk about how it all relates to fantasy stuff - or at least how it does in my head.
Lets take your basic vanilla D&D world. Elves are immortal masters of both mystical and martial pursuits. Dwarves are tough little buggers who are expert craftsmen within a human lifetime, when they can expect to live several centuries more then that. It is generally assumed that these two species did rule the world at one point, but are now in decline because of orcs, dark lords, civil war or some combination thereof. If you're world is really D&Dish, there will be crazy things like Rakshasa and Illithid running around too. Humanity is a late comer whose time has come only due to the twilight of other, more powerful races.
At least, that's how it seems to go in most of the vanilla stuff I read. (I make the distinction because a lot of settings seem to stray away from this and do their own thing, which is ducky) But really, would it be that way? Would Human beings be doomed if they had been contemporaries with the elder races in their heyday?
Lets assume not.
We'll say elves live forever, but they live differently then we do as a result. They need less food and no sleep. Bearing children takes a lot longer and is a pretty large commitment for an elf, as their childhood is correspondingly more lengthy then a human's would be. They don't farm; as stated they need less food, and probably can make an easy living hunting and gathering. Government is probably sparse if existent at all, as elves would have little need to conglomerate. War between each other is anathema - small, personal feuds probably sprout up here and there, but for the most part the elves are willing to let each other be and get along. They live to learn, experiment and to experience.
Dwarves reside in their mountains, away from the rest of the world. They aren't expert craftsman; they are their craft. Their long lives are devoted to stone and gems, iron and gold, the very things that they were born from. They are sturdy folk, and they know how to use the things they make, including weapons. After all, it is towards the dark they must venture if they are to claim materials for their craft and there is certainly no shortage of dark beasts that await in the shadows. While they band together to fight monsters they are much less likely to make war against anyone. What do those who live away from the mountains have that they could ever want, anyway?
Humans do not live a long time - they grow up hard and fast. They consume voraciously. Conflict over resources and land are near constant - the desire to farm and band together is a strong one. Intensive food production means not everyone in the society has to work to feed themselves, and can instead explore other venues. This leads to things like religion, an organized military and various technologies. In spite of their short lives, humans breed quite a bit, and their kingdoms grow more and more with the population.
In this scenario, it seems likely that humanity will inherit the land. Elves and dwarves could put up a showy fight, but in the ends Mankind's superior numbers, experience with mass combat and more varied technology would ensure their victory.
Among the monsters, there isn't much competition. Rakshasa and other fey need a society to feed off of, and would quickly find themselves over-whelmed if humanity turned on them. Giants are too few and to hostile towards each other to accomplish much beyond pointless destruction, usually. Dragons are content with their hordes of gold and treasure and care very little for their own kin. True abominations, the beholders, the mind flayers and the like, would make enemies just by being widely known, and more then likely wiped out fairly quickly. If mankind has any real competitor for dominance in this situation, it would be the Orks, whose lives are nastier, more brutal, and even shorter then even human ones.
I really do love Orks actually. I'll probably write about them another time.