No one really knows for sure but one of the best theory asserts that life started in the ocean since several billion years ago, oceans were covering the major part of the earth surface. It is also thought that life came about from and assembly of molecules and atoms coming from outer space. These molecules formed the first living cells which then assembled to form more complex organisms such as plants, fish....

Again we were not there so it is only a scientific theory based on assumptions and a few facts.