From Serenity : The Wiki
Little by little, the tribes used the Earth up. Barren, she had little left to offer them.
Swollen of her, they left. And for the first time since the Great Burn that birthed her, she was alone.
- - Shadow play narration in "Heart of Gold"
The third planet in the Sol system, the birthplace of mankind and one mankind treated poorly, resulting in the mass exodus to space.
Durran Haymer has a collection of rare artifacts from Earth-That-Was.