The Methuselah are not just mortal beings, they are altered human beings--former Earthlings from the 2080 colonizing mission to Mars known as the Red Mars Project. One of the colonists' first discoveries on Mars was an ancient alien artifact containing very advanced technology, which they called simply "Bacillus" (it would later be named "Kudlak" to distinguish it from the more powerful version, "Krusnik"). Studies showed that the Bacillus is actually a nanomachine, which can be integrated into the body, altering the host to adapt to the harsher conditions of Mars--greater strength and endurance, and extended life span being its original gifts. The colonies decided to use the Bacillus to help them survive the Martian environment. When word of the discovery got back to Earth, there was universal demand for its export back to the mother planet, and much dispute as to its ownership, which escalated into the devastating world war called the Armageddon.

It was after this war that the first wave of Martian colonists, called Returners, went back to Earth to see what could be salvaged from the ruins. They helped the remaining Terrans to rebuild society, but found that exposure to the Terran environment and especially the greater ultraviolet radiation caused the Bacillus to mutate. It consumed the host's red blood cells--causing the need for blood--and made them especially UV-sensitive. Thus they gained the vampiric traits they had not had on Mars, and the Methuselah came to be. Now forced to attack the humans they had aided, they helplessly watched the goodwill between them and the Terrans evaporate.

When a second wave of Returners came from Mars, its intention was not to help but to take over the still-weakened Earth. The original Returners were forced to take sides, and many cast their lot with the Terrans. (This is the war we see in the first episode's flashback.) The alliance of Terrans and Good Returners, as the pro-Terran Methuselah are called, succeeded in driving the Rebel Returners into the East, which is how things stand at the beginning of the series.

This is why the Methuselah Empire is also called the New Human Empire--they regard themselves as "New Humans", an improved version in effect--and why their term for ordinary Earthlings is not "Humans" but "Terrans".