Note for Chapter V: On Greenlife · 7:43am Jun 17th, 2016
(This will make much more sense if you read the chapter and in particular footnote 3 first.)
The biosphere of Cordelia's homeworld, Eliéra, is a mixture of three different ecologies: bluelife, greenlife, and silverlife, as detailed in this article on the Eldraeverse blog. The short version is that the Precursors transported life from at least couple of different worlds there, and these are the ecologies that survived and adapted to the world and each other's presence.
"Greenlife" is basically Earth-descended life, a subset. The eldrae themselves, while a species manufactured by the Precursors from various different biological elements, trace a lot of their ancestry to greenlife, and in particular a species they call Pseudoeldrae archaea, which is an ancient hominin offshoot species. Of course, none of this is known, as in the standard Eldraeverse, Earth is somewhere well beyond the rim of the Worlds, undiscovered.
However, for the purposes of the SufficientlyAdvancedverse in which this fusion fic is set, Equus is the source of greenlife, and there ain't no Earth or humans anywhere. (Occam's Razor for SF Writers: Planets shall not be multiplied beyond necessity.) The role of Pseudoeldrae archaea is played by the fossils of an obscure Zebrican plains ape, whose extinction along with the rest of its genus is the reason that this alternate world wound up firmly dominated by the Equidae, not the Hominidae.
So now you know.
(Incidentally, if you were wondering what some of those other mentioned species look like, you can see some descriptions here.)
Which of course leads to even more questions, mostly regarding the origin of Equus as a Big Dumb Object and whether it got its greenlife from somewhere else even farther up the chain. And how it ended up with nearly as many sophont races as a multi-lightyear star empire.
Cordelia et al. certainly have their scholarly work cut out for them on multiple fronts.