A brief history of Earth

"Our planet is thought to be about 4.5 billion years old, but the oldest fossils are about 3.5 billion years old. That doesn't necessarily mean that no life existed before that time, but no direct evidence has been found yet.

The first rocks that have been found deposited by water are about 3.8 billion years old, Valley said. But very little is known about the first 600 million years or so of the planet's history, known as the "Hadean Eon" because it was thought to be "hell-like," Valley said.

The leading theory is that Earth was bombarded by meteors in its early history. It took a big hit from an object the size of Mars about 4.5 billion years ago, leading to the formation of the moon. These impacts vaporized the Earth's crust and formed a super-hot magma ocean.

Evidence including this zircon suggests that within the first 100 million to 200 million years of its existence, our planet cooled enough to make crust. Steam from the atmosphere condensed to make oceans.

"Once you know that there were oceans, it's very reasonable that there would have been life that early" -- even when it was only 200 million years old, Valley said. ~ Read the full article.