Earth hasn’t always looked like the blue orb we know so well. The variety of contending creatures that have come and gone over billions of years, in a sense, paints a picture of the many planets Earth has been: a lava-covered rock with a poisonous atmosphere, an ocean world with the bare beginnings of microbial life, a steaming tropical riot of earth-shaking dinosaurs, or an Ice Age expanse where cave-dwelling humans hunted mammoths.

“We tend to talk about Earth-like planets as a planet like ours is today,” said Doug Hudgins, program scientist for NASA’s Exoplanet Exploration Program at NASA Headquarters in Washington. “The planet has been radically different at times in the past.”