Life on Earth
Studies of fossils in ancient rocks tell us that life probably began about 4 billion years ago, when the Earth was very young. No one knows how life began on Earth. Most scientists think that it began in liquid water. But was this in pools on the surface, or underground, or at the bottom of oceans where volcanic activity forms hot springs?
Experiments show that the chemical building blocks of life can form in space or on Earth. Many of them must have come to Earth with falling meteorites and comets. Somehow, these simple 'organic' chemicals came together to form more advanced molecules. Then they began to copy themselves and grow.
What we do know is that – once it gets under way – life is incredibly tough. Living bacteria have been found in frozen Antarctica, in boiling hot water and inside rocks up to five km underground. We also know that such organisms can survive for years in the harsh environment of space.
Once plants became widespread, large amounts of oxygen were released into the air. Earth became the only planet in our Solar System with an oxygen-rich atmosphere. By searching for oxygen on other worlds, we may one day find evidence of alien life.