Sebeceans are the human life form in farscape in the far part of the universe. The Peacekeepers are wholely made up of Sebeceans who travel on large spaceships and are militaristic in approach. When John Crichton travels through the wormhole, he attracts the attention of the Peacekeepers. Although identical in every form to humans from Earth, Bialar Crais deems them a new life form and that anyone such as Aeryn Sun who comes into contact with John Crichton is contaminated and must be killed.
In What Was Lost, Part 2, John discovers that Sebeceans and Humans share the same heritage. It is implied in Peacekeeper Wars that Sebeceans came from Earth brought to this part of galaxy by the Eidelons. There is no homeworld of the Sebeceans in the television series so it adds weight that the Sebeceans are from Earth.
Biologically speaking, Humans and Sebeceans can procreate with one another which is what John Crichton and Aeryn Sun did to create their son Ka Crichton.