All of the life that is known, all organisms that exist on Earth today or are known to have existed on Earth in the past, are part of the same life form: a life form based on DNA and protein. it does not necessarily have to be that way. Why not have two competing life forms on this planet? Why not have biology as we know it and some other biology that occupies its own distinct niche? Yet that is now how evolution has played out. From microbes living on the surface of antarctic ice to tube worms lying near the deep-sea hydrothermal vents, all known organisms on this planet are of the same biology.

Looking at the single known biology on Earth, it is clear that this biology could not have simply sprung forth from the primordial soup. The biological system that is the basis for all known life is far too complicated to have arisen spontaneously. This brings us to the notion that something else, something simpler, must have preceded life based on DNA and protein. One suggestion that has gained considerable acceptance over the past decade is that DNA and protein-based life was preceded by RNA-based life in a period referred to as the 'RNA world'.

Even an RNA-based life form would have been fairly complicated - not as complicated as our own DNA- and protein-based life form - but far too complicated, according to prevailing scientific thinking, to have arisen spontaneously from the primordial soup. Thus, it has been argued that something else must have preceded RNA-based life, or even that there was a succession of life forms leading from the primordial soup to RNA-based life. The experimental evidence to support this conjecture is not strong because, after all, the origin of life was a historical even that left no direct physical record. However, based on indirect evidence in both the geologic record and the phylogenetic record of evolutionary history on earth, it is possible to reconstruct a rough picture of what life was like before DNA and protein.