It All Started in the Ocean
1. The first life on Earth was probably bacteria. 2. Some organisms, like dolphins, moved to land but later returned to the oceans.
Our planet is made up of five great oceans — the Atlantic, the Pacific, the Indian, the Arctic, and the Southern. They're all linked together, creating a huge body of saltwater called the World Ocean that surrounds the continents and islands and covers about two-thirds of the Earth's surface.

Scientists know — from studying tiny fossils — that life on Earth probably started in the oceans nearly 4 billion years ago. For most of Earth's history, life stayed and thrived in the oceans. About 500 million years ago, some living things, like our ancestors, moved out of the water and on to land, but most life stayed in the oceans.