Life on Earth
No one knows how or why life began. We do know that life on Earth started at least 3.8 billion (3 800 000 000) years ago. By this time, the young planet had cooled and formed a rocky crust. Some scientists believe life began in a rock pool or in the ocean. Others think it may have arrived from space with comets and asteroids.
As far as we know, Earth is the only place where life is found. Everywhere we look on our planet, bacteria and other simple life forms have made a home.
Plants and animals need sunlight to grow. Until the 1970s, scientists thought that life only existed near Earth's surface.
Since then, bacteria have been discovered several kilometres under ground inside solid rock. They live on gases and minerals in the rock. Bacteria have also been found in strong acids, in the freezing cold of Antarctica, and near hot springs in the dark depths of the ocean. Some can even survive in highly radioactive places. As long as water is available, life is able to grab a foothold.
If life is so tough, could it start and survive on other worlds? The most likely place in our Solar System is Mars.