However, the existence of species, and of sexual reproduction, is not a universal feature of life. In their book Origins of Sex, Lynn Margulis and Dorion Sagan argue that ``The fact that we are descendants of that minority of organisms that is sexually reproducing has led us to a very unbalanced and unrealistic view of biological sexuality'' [Margulis & Sagan 86] (p.15). Indeed, the very existence of sexual reproduction in nature is a great puzzle from a neo-Darwinian point of view, as organisms which engage in it (and therefore only pass on half of their genes to their offspring) would appear to be at an evolutionary disadvantage to those which reproduce asexually. The question of how sexual reproduction evolved, and the related, but separate, question of how, once it appeared, it has been maintained in the face of apparent selection pressure to remove it, constitute major research topics of modern evolutionary biology (see, for example, [Maynard Smith 86], [Margulis & Sagan 86]).
The reason for mentioning this subject here is that our common notions of what it is to be alive, through our experience of biological life, may include some aspects which are by no means universal laws of biology, but rather contingent features of the particular way in which life has evolved on our planet. This raises the question of whether we would be able to recognise artificial life if we ever succeeded in creating it, and to what extent we might want to try to replicate the course of biological evolution in our artificial systems. These issues will be discussed in Chapter 3.