The most rigorous attempt at a statistical analysis of the problem, a recent study led by scientists at Dalhousie University, concludes that there are about 8.7 million species on Earth. The team analyzed the numerical relationship between species, genus, family and order in well-studied life-forms and used that pattern to estimate the number of species in categories of life that haven’t been well studied. Some scientists argue that that almost surely underestimates some lesser-known classes of life.
Only some 1.25 million species have been described in the 253 years since Linnaeus devised the method we use to name them. This means that if there are, indeed, roughly 8.7 million species over all, nearly 90 percent of the species on Earth have not yet been discovered and described.