Figure 2. Timeline of the changing taxonomic position of microsporidia, from their discovery in 1857 to the present.

When first described in 1857, they were classified as schizomycete fungi. Later they were consisdered sporozoan protists (and more specifically members of the subgroup Cnidosporidia), a position favoured for over 100 years. In 1983 a new hypothesis radically departed from this idea, suggesting they were an ancient, primitive lineage that evolved before the origin of mitochondria. Molecular data originally supported this possibility, but as data accumulated, it became clear that they were in reality highly reduced fungi, a conclusion broadly supported by the genomic data now available, although their exact relationship to fungi remains contentious.