They were back in the early Earth, before life appeared.

Something vaguely analogous to a microenvironment on the early Earth?

The atmosphere of the early Earth is not well understood.

There being no life on the early Earth, the carbon dioxide came from volcanoes.

There are several theories about the origin of small molecules that could lead to life in an early Earth.

We think extremophiles are very similar to organisms that existed on early Earth.

The atmospheric composition of early Earth (the first billion years) was drastically different than its current state.

Their work lead us closer to understanding ribozymes that could have existed in early Earth.

Titan has been described as having conditions similar to those of early Earth.

He knew enough to realize that the air of an earlier Earth would probably be too rich for him at first.