The Old Ones.

In the 40k background, the Old Ones were one of the first sentient races (other than the Necrontyr) in the galaxy. They seeded and terra formed Earth, thus creating humanity. They also created Eldar, the Orks and what is known as the lizardmen. The Eldar and Orks (who were then intelligence and very technologically advanced) spread to new worlds. It is possible that in the Fall the Eldar on Earth were isolated and devolved to become the Elves, retaining their mental prowess. The Orks who relied on a mushroom for their intelect quickly ran out and also devolved, but by this time had spread to several worlds. So there, Warhammer and 40k are exactly the same.