Evolution?
Devolution.
Draenei were once known as the Eredar. Sargeras came to their home world of Argos to recruit the Eredar. Two groups followed Sargeras and become known as the Man'ari (to the Draenei at least), led by Archimonde and Kil'jaeden. The other group become known as the Draenei, the exiled, led by Valen.
The Draenei in WC3 are the result of the fel magic from the Burning Legion. Basically, the Burning Legion found the Draenei after they fled, gave fel magic to the Orcs (turning them green), and had them ravage their homeworld and almost destroy the Draenei. The land, now tainted with fel magic, slowly turned some of the Draenei. Some became Broken, and others completely turned into the Lost Ones.
Yay lore. :D