It was perhaps simply a question of a number of small molecules floating around in the primeval sea ("primeval soup") connecting up together (perhaps energised by the ultraviolet light from the sun) into a stable much larger molecule, called a replicator. Other small molecules floating by with an affinity for the replicator joined up by attaching themselves to it. Successively added molecules would also become stable by mimmicking the pattern of the original replicator, then eventually split off from it forming a second replicator. Once this happened replication gradually spread.

As the replicators in the primeval sea increased, the number of smaller molecules decreased. Replicators, as protection against being cannibalised by other replicators, started to build protective protein coatings around themselves. These coatings became increasingly more elaborate and complex survival vehicles. Through a process that continued (and still does) over the span of more than 3,000 million years, the replicators are the present DNA strings located within each cell of our body, and all plant, animal and human life are the very sophisticaled protective vehicles.

We in this role therefore ensure the propagation of the DNA and through this the continued evolution of our species.   This evolution process is driven by the principle of natural selection : survival of the fittest, or as Dawkins puts it the survival of the stable.

Think of the following metaphore to gain a better idea of this proces of natural selection
Imagine a DNA string to be the plan and building instructions for a house. There are two types of strings, one for building a house of straw, the other for building a house of stone. Inside each house (in a cupboard) are also a complete copy of the building plans ("DNA"). A number of both house types are being built and completed.

Sooner or later there will be a few storms or bush fires. The houses of straw will of course not survive those conditions and will blow to pieces or burn down together with everything in it.
The houses of stone on the other hand are doing fine. They withstand all weather conditions and don't burn down in a bush fire. In due course there will only be stone houses in the region and all straw houses have dissapeared. Natural selection has taken place.