In this chapter I shall consider the place of microbes in the evolutionary sequence of living things and attempt to assess what importance they had in influencing the directions which biological evolution has taken. Since I shall be dealing with questions that cannot usually be verified experimentally – for I shall be mainly concerned with events that took place in the darkest recesses of prehistory, even before recognizable fossils were formed – I must recall to readers the warning I gave about scientific fact early in this book. Even in everyday matters, laboratory science contains elements of uncertainty, particularly in interpretation of experimental findings. When one is concerned with retrospective deduction from today's knowledge about the state of our planet during its juvenile millennia, interpretation is so uncertain a process that it amounts to informed speculation. The surprising thing, really, is that one can say anything at all about the biology of those distant eras. Yet, as the reader will see, if one accepts geologists' views about the broad outlines of this planet's geological history, one can put together a coherent and reasonably plausible account of how the earliest living things developed. Whether it bears any relation to the truth is another matter, but it is a form of speculation that widens our understanding of life and its potentialities, as well as exercising the imagination. So, for this chapter, I shall relax scientific puritanism and see what sort of theoretical picture can be built up about the infancy of terrestrial life and the way in which today's microbes arose.