The Elder Species, not to be confused with the builders of the wormhole network, are the three sentient species who founded the Great Empire of Biran, Sol and Mu Arae. All independently developed advanced spaceflight and began to use the ancient wormhole network to expand throughout the galaxy. In some cases, they encountered and incorporated alien civilizations into themselves, though only a dozen or so species considered fully sentient were ever known except from ruins.
In the course of their respective efforts at colonization, the Elder Species eventually met each other. It was perhaps greatly fortuitous that their environmental requirements were very different, and they could utilize their systems more effectively together. The Biran and Humans could share many planets, but in vastly different niches (the Biran are cold-water reef dwelling aquatic organisms), and the Mu Araen live in the atmospheres of gas giants.
Their symbiosis allowing them to out-compete mono-specific contenders for galactic hegemony (including reluctant factions of their own individual species) economically, the Elder Species eventually grew ambitious and declared an Empire, which lasted for centuries before its unfortunate collapse.