How did life on Earth begin?
We can never know for sure, but scientists are certain that it cannot have happened all at once. Instead, they believe that it developed very gradually, through a series of chemical reactions. Scientists test this idea by re-creating the conditions of the early Earth in laboratory experiments. In one experiment, they put a smal amount of water in a glass container, to imitate the sea. Next, they remove all the air above the water, and replace it with the gases they think were in the atmosphere 4 billion years ago. After this, they seal the container, and pass electric sparks through the gases, to imitate lightning flashing through the air. When they open the glass container, they find that the water contains some of the complex chemicals that up living thing. Experiments like these suggest that life could have started quite by acciddent, most probably in the sea.
If you could go back 4 billion years in time, you would find the Earth a very strange place. You would not be able to breathe because the air would be filled with poisonous gases, and you would not be able to eat because there would be no food, and no living things. There would be nothing flying in the sky and nothing swimming in the water. Most of the ground would be bare rock. But in the sea, somewhere, the beginnings of life would be stirring.