How I Stopped Worrying and Learned to Love Eberron
Humans may be the most populous civilized race on Eberron. These people fall neatly in the middle of most of the races that exist. While they possess less arcane and physical power and live shorter than creatures like dragons or rakshasas, they are adept enough to create civilizations, master magic and psionics, and spread by the millions all over the globe.
This inbetween nature of the race means humanity has its fingers in many pies. Human genes and spirits show the most propensity for mixing with others; half elves and the kalashtar being the best examples. Also, while many races have an affinity for magic or psionics, mankind have shown ability with both energies.
Seven of the dragonmarked houses have human or hybrid members.