The third book describes the visit of the two Lunar colonists to the 27th century Earth.

On 21st century Earth, astronauts find artifacts left by this civilisation and wonder who they were.

He is capable of flight, cognitive thinking, and voice projection, which is all considered highly advanced for 21st century Earth.

The level of technological development on the planet is similar to mid-20th century Earth.

As a result, the world created is similar to that of twenty-first century Earth, but with a greater likelihood of a violent death.

Although tattered and torn, their clothing revealed them to be citizens of late twentieth century Earth.

The game is set in the mid-22nd century Earth, and a handful of immense corporations control virtually all business, politics, and entertainment.

The Machine uses "traditional" technology, though far advanced from 21st century Earth.

H'lven technology is roughly equivalent to that of early 21st century Earth.

The first hypothesis would indicate that the characters of the book are the original humans of the 20th century Earth.