How many species are there on this planet? In 1691, the scientist John Ray estimated that there were 20,000 species of insects. His numbers were significantly off — so far, at least a million insect species have been described. But he reached that estimate the way most scientists still do, by extrapolating from the number of already known species. Three centuries later, there is still no scientific consensus on the total number of species. 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 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. According to the study, it would take another 1,200 years to provide a scientific description of them all at our current pace. (The study estimates that it would take 303,000 taxonomists working full tilt just to provide the most basic scientific description of all the unknown species.) At the rate we are losing species, a huge number currently alive will have gone extinct in that time. And that 8.7 million? It doesn’t include the species of bacteria, which may number in the millions. So, while we are learning more each day, it is still true to say that no matter how much we think we know about life on Earth, we know almost nothing; except, perhaps, that Life is varied and extraordinary beyond even our imaginings.
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