Life on Earth, thanks to the mechanisms of natural selection and evolutionary biology, is incredibly diverse. Darwin once waxed lyrical about “endless forms most beautiful,” and he wasn’t wrong: Life can be found everywhere, from the top of brand new volcanic islands to the dark depths of the planet’s crust.

Now, a pioneering study has attempted to take on the herculean task of estimating how many different species of life there are on our pale blue dot. Its conclusion is that, in a world dominated by microbes, there are more than a trillion. Incredibly, this means that only one-thousandth of 1 percent of species have actually been identified.

Previous total species estimates are decidedly arbitrary. However, this Proceedings of the National Academy of Sciences study showcases a universal mathematical law that has allowed its authors to come up with the most robust method to date of investigating biodiversity.

“Just like mapping the Milky Way and other galaxies helps us understand and appreciate our place in the universe and its history, understanding the immense diversity of microbial life helps us understand and appreciate our place in the evolution of life on Earth,” Kenneth J. Locey, a postdoctoral fellow at Indiana University (IU) and co-author of the paper, told IFLScience.

Microbial life rules the world. Tsvetkov Maxim/Shutterstock

Species databases for all kingdoms of life, from bacterial to animal, from archaea to plant, already exist, but they are incomplete. The IU team, initially wanting to see if the same biodiversity patterns existed in the microbial world as they do in the animal and plant realms, compiled the most up-to-date databases into one large compendium, the largest of its kind.

Their efforts revealed that at least 5.6 million species of life had been recorded, but this clearly wasn’t all of them. In particular, they felt the databases on microbial life represented a chasm of knowledge that needed to be addressed. As they note, with more adventurous searching methods and better equipment, swaths of new microbial life keep popping up in unexpected places.

“In a recent study, a sample of water from a pretty average stream contained 35 new phyla (major groups),” Jay T. Lennon, an associate professor at IU and the study’s other co-author, told IFLScience. “The tree of life completely changed in one fell swoop.”

In order to estimate how many microbial species there are on Earth, they turned to scaling laws, mathematical relationships that describe relationships between two quantities, like species and population size. By carefully picking apart the compendium they put together, the researchers realized that a scaling law that also exists in a wide range of fields, including economics, also applied accurately to all groups of lifeforms, including the microbiome.

By using this “universal scaling law,” they could not only predict what species of microorganism would be dominant in various environments, but that there are upward of a trillion different microbial species on Earth. This makes them far and away the most dominant lifeform when juxtaposed with the comparatively small variety of plants and animals.

Using the known data set (red pixels), the universal scaling law could be used to estimate how many species of life there were across various ecosystems on Earth. "Dominance" is a measure of how commonplace a species is in a set ecosystem, and it scales very clearly with species abundance, whether that's looking at microbial life or larger kinds of organisms. Locey & Lennon/PNAS

This National Science Foundation-funded study, rather than highlighting all we know, points out how much we still don’t know about the world we inhabit. Microorganisms drive Earth’s natural systems, so understanding everything about them is a paramount task – literally everything depends on them.

“Now we just have to hope other researchers start finding them all,” Locey said.