This species developed along similar lines as Humans on Earth, and their planet was considered a prime example of Hodgkin's Law of Parallel Planetary Development. They used the English language and developed an architecture identical to that of Earth; the technological development was so similar, that a 1960s distress signal was virtually indistinguishable from the Human SOS.
Around 1960, the life prolongation project led to a virus, which wiped out all of the adult Earth Two Natives. Only the children survived, which started to call themselves Onlies. The Onlies had greatly increased life spans, aging very slowly with several surviving until 2266. When Onlies entered puberty, they began to suffer from the effects of the deadly virus, turning into Grups.
The surviving Earth Two Natives still lived on the planet when a landing party from the USS Enterprise arrived in 2266 and developed a cure for the virus. The Earth Two Natives were left under the care of a medical team, with Starfleet Command sending teachers and advisors. (TOS: "Miri")
- See: Earth Two natives