Board Thread:Warhammer 40k General Discussion/@comment-14745711-20140911102114/@comment-25177957-20140915181549

Without the human race, Chaos would be unable to thrive off of it's emotions, the majority of the followers of Chaos in the 41st millenium are Humans. The Necrons have no souls and are unconvertable to Chaos, the Tyranids are unconvertable, The Tau are and will be for a very long time, the Eldar of the Craftworlds are enemies of Chaos, and the Dark Eldar only feed Slaanesh. Without human psykers, Daemons would be unable to enter realspace on a regular basis and conquer planets. The Chaos Gods would be starving. So they very much do need Humanity. And that's why they're not ignorant enough to go back and "wipe it all out". If Earth was simply subjucated as a Daemon World, Humanity wouldn't expand in number and would eventually kill itself off.

No Chaos Gods existed during the time of the Old Ones, Or the pre- War in Heaven Necrontyr. It seems as if many posting in this thread are having a difficult time figuring out when each Chaos god was birthed. The lore only states when Khorne and Slaanesh were born. However it's safe to assume they were born in this order.

Nurgle: Birthed first out of the Chaos Gods, a few million years after the Necrons entered stasis and the Old Ones disapeared.

Khorne: Came about during the middle/dark ages of Terra due to the bloodshed and violence (Remember that Chaos mostly feeds off of human emotions and the Eldar were not at war with eachother and were likely building their empire in peace)

Tzeentch: Likely came about when the first non-Shaman or Emperor psyker came to exist in the human race, and began plotting slowly, the downfall of Humanity.

Slaanesh: Came about after the fall of the Eldar.

It's mostly uncertain how Nurgle was born, but he could be traced to the enslaver plague.