Of all the species living on Earth, perhaps two-thirds of them live in the dirt under your feet, according to a group of Swiss scientists who set out to make an educated guess on the matter.
Obviously, it isn’t possible to count everything in the soil everywhere on Earth. The scientists’ estimate is built on multiple estimates made by other groups of researchers. The Swiss trio looked at the entire tree of life—they wanted to know how many bacteria and viruses lived in the soil as well as how many plants, worms, bugs and fungi.
Viruses were particularly hard to measure, because viruses aren’t categorised as species, like plants and animals are. Instead, the researchers decided that viruses with sufficiently different DNA or RNA would count as different species.
What’s the point of making such a guess, if it’s just a guess? We know that dirt is the basis of life on Earth, and that it even has an impact on human health—previous research has shown that the soil microbiome and human microbiome are connected. That means it’s useful to have a big-picture idea of what lives down below.