[egypturnash]

Pohl's "Heechee Saga" books (”Gateway" and its sequels) end up with humans getting involved in wars between a couple different God-Like Ancient Aliens who wanted to change various cosmological constants in ways that would make intelligent life more or less likely to happen.

They don't show up until like book 3 of the series though.

[pavel_lishin]

The Xeelee Sequence of books addresses similar ideas, except I'd argue that the Xeelee and the Photino Birds are several orders of magnitude above the Heechee and The Foe.