We were discussing the geological evidence of liquid water on Earth and Mars three to four billion years ago.

我们讨论了30到40亿年前地球和火星上液态水存在的地质证据。

youdao

In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today's levels.

事实上，研究表明40亿年前大气中的二氧化碳含量远远高于今天的水平。

youdao

Four billion years ago was a hellish time on planet Earth.

四十亿年前的地球如同地狱一般。

youdao

Over 1, 500 minerals are thought to have formed on Earth before the beginnings of life some four billion years ago.

ECONOMIST: Minerals

The test was designed to simulate the type of impact that would have been frequent during Earth's early history, some four billion years ago, when rocky, icy debris in our Solar System accumulated to form planets.

BBC: Comets could have seeded life on Earth

Like fraternal twins, they were born at the same time, about four and a half billion years ago, and took roughly the same shape.

NEWYORKER: The Martian Chroniclers