The first time the Anu manipulated human DNA, they intended primarily to speed up the development of sentience. But the humans turned out to be very smart and breed very fast, and even though the Anu could put leaders in charge, there was a chance that the humans would rebel.
Increasingly paranoid, the Anu went back and re-engineered humanity using a method somewhat similar to a retrovirus, this time without Ninhursag, who was horrified at the results of the first manipulation and refused to be part of this. They re-coded human DNA to shorten their lifespan, to lower their intelligence, to make them more sexually obsessed, more violent and more submissive.
These new humans could be much more easily controlled.