Good question. Way back when, two species: Homo anthropicus and Simius furrius, coexisted in harmony, but everything changed when the Fire Nation attacked. The two species, through breeding with each other, created the first human baby. This baby later grew up to become the Avatar and defeat the Fire Nation, but it was too late. By that time, the Fire Nation had wiped out Homo anthropicus, leaving only Simius furrius, who eventually evolved into the monkeys and apes we know today. Without anyone to breed with, the first human ever could not procreate, so the species died out. In memoriam, the Simius furrius designed the Matrix, so all humans currently alive are projections of a computer program created by genius monkeys.