the main theory is that living things developed from molecules that could replicate, or copy, themselves, rather like DNA does
there are two possible origins for these replicating molecules
-they were produced by the conditions on Earth at the time
-they came from somewhere else, such as another planet in our Solar System, or further out in space
over many millions of years these molecules joined with other molecules, becoming gradually more complex and dependent on each other.
eventually led to all of the different living things that we see on Earth today