Complex sentence: Haas argues that the density of sites in such a small area is globally unique for a nascent civilization, and that during the third millennium BC Norte Chico may have been the most densely populated area of the world (excepting, possibly, northern China).
Split into simpler sentences: Haas argues that the density of sites in such a small area is globally unique for a nascent civilization. During the third millennium BC, Norte Chico may have been the most densely populated area of the world (excepting, possibly, northern China).