Abstract : Despite its amazing morphological diversity, life as we know it on Earth today is remarkablysimilar in its basic molecular architecture and biochemistry. The assumption that all life on Earth todayshares these molecular and biochemical features is part of the paradigm of modern biology. This paperexamines the possibility that this assumption is false, more specifically, that the contemporary Earthcontains as yet unrecognized alternative forms of microbial life. The possibility that more than oneform of life arose on Earth is consistent with our current understanding of conditions on the earlyEarth and the biochemical and molecular possibilities for life. Arguments that microbial descendentsof an alternative origin of life could not co-exist with familiar life are belied by what we know of thecomplexity and diversity of microbial communities. Furthermore, the tools that are currently used toexplore the microbial world – microscopy (with the aid of techniques such as DAPI staining andfluorescence in situ hybridization), cultivation and PCR amplification of rRNA genes – could not detectsuch organisms if they existed. Thus, the fact that we have not discovered any alternative life formscannot be taken as evidence that they do not exist.