A planet that is a candidate for being inhabited will no doubt experience catastrophes, collisions, and extinction-level events on it. If life is to survive and thrive on a world, it must possess the right intrinsic and environmental conditions to allow it to persist. Here, an illustration of early Earth’s environment may look fearsome, but life somehow still found a way. (NASA GODDARD SPACE FLIGHT CENTER)

What Was It Like When Life Began On Earth?

The planet has had life on it, in some form or another, for nearly as long as Earth has existed.

If you came to our Solar System right after it formed, you would have seen a completely foreign-looking sight. Our Sun would have been about the same mass it is today, but only about 80% as luminous, as stars heat up as they age. The four inner, rocky worlds would still be there, but three of them would look extremely similar. Venus, Earth, and Mars all had thin atmospheres, liquid water on their surface, and the organic ingredients that could give rise to life.

While we still don’t know whether life ever took hold on Venus or Mars, we know that by the time Earth was only 100 million years old, there were organisms living on its surface. After billions of years of cosmic evolution gave rise to the elements, molecules, and conditions from which life could exist, our planet became the one where it not only did, but where it thrived. To the best of our scientific knowledge, here’s what those first steps were like.

A micron-scale view of very primitive organisms. Whether the first organisms formed on Earth or predate the formation of our planet is still an open question, but evidence favors the scenarios where life arises on our world. (ERIC ERBE, DIGITAL COLORIZATION BY CHRISTOPHER POOLEY, BOTH OF USDA, ARS, EMU)

Life as we know it has a few properties that everyone agrees on. While life on Earth…