Firstly, DF currently requires a dwarven civ for the game to be playable in fortress mode. It's possible to mod the raws so humans are "playable", but they'd more or less be funny looking giant dwarves as the game doesn't yet support other race civs in fortress mode (set to be changed during the Myth & Magic arc). Some workshops and crafting recipes might be missing, for instance. You might want to look at the mod sub forum to see if there is anything about playable human modding to be found.
Secondly, it's possible to get worlds where all civs except the human ones are dead (although DF might not recognize them being dead, due to a bug in that department), while the remainders of those races have either joined the human conquerors or live as refugees without any civ allegiance. Unmodded such a world would be possible to play in adventure mode only.
Note, however, that you'd likely have to generate a huge number of worlds for DF to naturally generate a world where human civs are the only ones left standing, in particular if you want reasonable number of other races to be present.
There might be raw tweaks that would weaken non human civs to conquest, although I don't know what they'd be.
While titans and megabeasts are good at wiping out civs at their infancy, they're not good at leaving the civ members alive, so you'd probably want to keep their numbers at reasonable levels.
Finally, this doesn't contradict what PlumpHelmetMan said, but rather complements it with a different approach.
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Topic: Human civs only (Read 986 times)