The first thing to remember about simple life forms is that there is nothing simple about them. The most basic cell has been compared to a factory carrying out as many functions as every type of factory on earth put together.
Therefore, it is still something of a mystery how living things were first formed, but many scientists believe that in Earth's early ages rain carried chemicals from the air and ground into the oceans, making a salty soup out of which life evolved.
Though the molecules that make up living things are much more complicated than those found in the primitive oceans, experiments have shown that when acted upon by certain sources of energy, chemicals found on the young Earth could have produced the amino acids and nucleotide bases that are the building blocks of life. Other theories suggest that these building blocks could have hooked together into more complicated proteins and nucleic acids to form "protocells," precursors to true living cells.
Some scientists theorize that after eons of reacting with each other and with other chemicals, some protocells developed skins that could absorb chemicals and grow larger without bursting. Gradually, over many generations, surviving protocells became more complex until some of them were able to grow and then split into two identical copies of the original cell, at which point they would be alive.
But other scientists, such as Nobel Prize winning biochemist Francis Crick, consider this chain of events so vastly improbably that they speculate early life forms may have drifted in from outer space.
Scientists are still searching for definite answers to the complex mystery of life.
