When Istara created her celestial anchor upon the Prime in the form of the world, she realized that she would need a hardy cadre of beings dedicated to her aim of creating an extension of the Realm of Dawn upon the Prime. She also realized that these beings would need to prosper under the most adverse conditions and climates without her direct intervention.
This is the legend of the creation of the Human race, and whatever arguments may be raised against it, it is widely accepted that the Humans were indeed one of the first races to enter Istaria.
Several other cultures choose to emphasize this point as little as possible.
Physically the weakest of the Istarian peoples (okay, Dryads aren’t exactly weightlifters), Humans have had to become by far the most adaptable.
No other race has (or needs to have) quite the Human incentive to master whatever skill or discipline becomes necessary - and then to work out another if that one should fail.
Born adventurers, shapeshifters in all but the body, outclassed and outgunned in so many ways, they can be killed, beaten, tricked, and even turned against each other - but they do keep coming...
They call themselves "the Children of Istara" - though not every other folk refer to them thus - and they are in many ways the most unknowable and unpredictable of the Living Races.
It was a single act of Human self-sacrifice that gave the peoples of today a fighting chance against the assaults of the Withered Aegis; it was also a single Human’s lust for power that brought the entire world to the brink of oblivion.
Outgoing, social and generally amicable as they are, in their time they have represented the extremes of Istarian civilization.
The Sslik will never trust them; the Elves and Half-Giants have well-known proverbs about Human unreliability. Their closest allies are the Gnomes, and after those the Dwarves. Fiends despise all Humans - or say they do - but some are strangely fascinated with them as well.