THE ANCIENT EARTH WAS AN ALIEN PLANET

Strange as it may sound, we can and do study the ancient Earth to help us understand the possibility of life on other planets. This is because the Earth was so fundamentally different in the first half of its history compared to the second half that we can think of it as an alien planet. From about 4.6 billion years ago, when the Earth formed, to about 2.4 billion years ago, there was essentially no oxygen in our atmosphere, no ozone layer, likely higher carbon dioxide, oceans with lots of dissolved iron and silicon, and fewer and smaller continents. After 2.4 billion years ago, the world began to look more like the world we are familiar with. Yet life has existed and persisted on Earth for at least 3.5 billion years.

How do we know? Earth is a dynamic planet where ocean basins form when the crust cracks and lava pours forth, only to later be shoved back down into the Earth or crumpled and thrust upward into mountains, and those mountains are later eroded to the sea. This constant recycling continually produces new sedimentary rocks that preserve the history of life. But in an ironic twist, this recycling also destroys most of these very same rocks that record our history. But all hope is not lost because by chance there are a few places on Earth that preserve rocks from ancient times (back to about 3.5 billion years ago) where we can find evidence of what the planet was like during this “alien” period.

In this talk, I will discuss where and how we study the conditions of the ancient Earth, what those conditions were, and how we use that evidence to inform our search for life on other planets, particularly on Mars. The ability for life to thrive under the “alien” conditions of the early Earth, and to have evolved so easily and so early in our planet’s history gives us hope that life could have evolved elsewhere in our solar system.