Humans 

Versatility.
That's what it means to be human. You take the damn elves.  Sure they're 
quick and smart, but they're set in their ways.  You wouldn't find elves 
living in a hole in the tundra, would you?  Or living in a ship for months 
on end?  Adapting.  That's the key.  Once, on a battlefield against an army 
of orcs and ogres, I noticed that the filthy monsters were just chomping 
up my front lines. They would charge for the cavalry, every time.  They 
loved to eat horses, the sons-of-bitches.  But anyway, once I noticed that, 
I set 20 horses with straw men on them on the top of a hill, and had my men 
dig ditches around the hill.  Filled the hill with pits, spikes and traps, 
lacing them with anything that poke a dozen holes in each one of 'em.  The 
monsters charged the hill and died by the dozens. We flanked them, gutted 
them and smeared them.  And you know what was the damnedest thing?  We did 
the exact same thing the next day.  The same trick worked again and again, 
and we ran that filth off the battlefield.  Why?  They didn't change.

Your average human is just that, average.  Not as strong as a dwarf, but 
not as slow as one.  Not as bright as a gnome, but fitter.  He'll never 
outfly a sprite, but he can swat down and smush the bugger.  It's like a 
mold, y'know?  You can find a mold anywhere.  Anywhere where's there's air, 
water and land, and sometimes they can make do without those either.  Let 
the elves have the woods, we'll thrive in the desert.  Halflings in their 
neat little valleys, we'll mushroom in the mountaintops.  You wonder why 
there are more humans out there than any other race, more of us than any 
other race by far?  We aren't picky.  That's why its humanity's world.  
We'll take it from every other race. It's just a matter of time.

--Taken from "Interviews with Grand Inquisitor General Nohamstis, Conqueror 
for The Empire of Bright Star" T.A. 137

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Humans are the most common race.  They have no special talents like the 
other races, they are far more versatile. They can learn any profession. 
Humans also receive a bonus to their profession's primary attribute.