Early in the Furons' first forays into space, they discovered the Sol system. After a conquest of Mars they visited Earth and mated with the "nubile" native species "to let off a little steam", mixing their DNA into it. Soon afterwards they revisited Mars and, for reasons that are unclear, launched a brutal war against the Blisk. The ineffectiveness of their traditional weapons led them to invent deadlier ones and they ultimately changed Mars from a radioactive ocean world into a dry desert one.
The DNA of the entire Furon race was mutated during this war and only cloning technology allowed it to survive. The technology is so advanced that a Furon's memories and personality can be moved into each successive clone, granting an immortality of sorts. It proved faulty, however, and each new clone was increasingly defective. In AD 1959 Cryptosporidium 136 and 137 were sent to Earth to extract Furon DNA from Earthlings.