Despite its amazing morphological diversity, life as we know it on Earth today is remarkably similar in its basic molecular architecture and biochemistry. The assumption that all life on Earth today shares these molecular and biochemical features is part of the paradigm of modern biology. This paper examines the possibility that this assumption is false, more specifically, that the contemporary Earth contains as yet unrecognized alternative forms of microbial life. The possibility that more than one form of life arose on Earth is consistent with our current understanding of conditions on the early Earth and the biochemical and molecular possibilities for life. Arguments that microbial descendents of an alternative origin of life could not co-exist with familiar life are belied by what we know of the complexity and diversity of microbial communities. Furthermore, the tools that are currently used to explore the microbial world – microscopy (with the aid of techniques such as DAPI staining and fluorescence in situ hybridization), cultivation and PCR amplification of rRNA genes – could not detect such organisms if they existed. Thus, the fact that we have not discovered any alternative life forms cannot be taken as evidence that they do not exist.