Humans are highly social and extremely adaptable, their civilizations having survival countless upheavals over the course of history. "Midworlders" is the term used by the High People to describe any non-Yevim inhabitant of the earth.
Of Human Origins
The Yevim say that human beings are the result of their mistake—that after the defeat of Asag and a epoch of relative peace, a population of Yevim left Muraius to reside on the surface world. Because they became disconnected from the presence of the God Tree, their immortality was lost. Thus humans came to roam the earth in tribes, forced to rebuild civilization and eke out an existence against the children of Asag, who sought to reclaim the surface world for themselves.
Whether this is true or not is a matter of scholarly debate among sages of all stripes. Human beings remain the most resilient among all the races on the earth. Their lifespans vary between a hundred and a hundred and fifty years, and they come of age rather quickly, reaching adulthood in a mere sixteen.