A New Branch Of The Tree Of Life

It is not every day that a new branch of organism at a level even higher than that of kingdoms is discovered.

The tree of life is a useful diagram for understanding the relationships between different forms of life, present and extinct. The trunks are made up of three broad groups called domains – Bacteria, Archaea and Eukaryota – which then branch into kingdoms such as animals and fungi. From there the branches become more and more specific until you reach individual species.

The new discovery adds quite a major bough to the tree – Provora. These lifeforms make up a category informally called a “supergroup,” which sits below domains and can contain multiple kingdoms.

“This is an ancient branch of the tree of life that is roughly as diverse as the animal and fungi kingdoms combined, and no one knew it was there,” said Dr. Patrick Keeling, senior author of the study.

Members of the Provora supergroup are tiny organisms that the team describes as the “lions of the microbial world.” That’s because they prey upon other microbes, and within their ecosystem they’re relatively rare. The supergroup is further divided into two clades – the “nibblerids,” which use tooth-like structures to nibble chunks off their prey, and the “nebulids,” which engulf their prey whole.

The team discovered this new kind of life in samples taken from around the world, including the coral reefs in Curaçao, sediment from the Black and Red seas and water from the Pacific and Arctic oceans. . . .

“In the taxonomy of living organisms, we often use the gene ‘18S rRNA’ to describe genetic difference,” said Dr. Denis Tikhonenkov, first author of the study. “For example, humans differ from guinea pigs in this gene by only six nucleotides. We were surprised to find that these predatory microbes differ by 170 to 180 nucleotides in the 18S rRNA gene from every other living thing on Earth. It became clear that we had discovered something completely new and amazing.”

From New Atlas.

Molecular phylogenetics of microbial eukaryotes has reshaped the tree of life by establishing broad taxonomic divisions, termed supergroups, that supersede the traditional kingdoms of animals, fungi and plants, and encompass a much greater breadth of eukaryotic diversity. The vast majority of newly discovered species fall into a small number of known supergroups. 

Recently, however, a handful of species with no clear relationship to other supergroups have been described, raising questions about the nature and degree of undiscovered diversity, and exposing the limitations of strictly molecular-based exploration. 

Here we report ten previously undescribed strains of microbial predators isolated through culture that collectively form a diverse new supergroup of eukaryotes, termed Provora. The Provora supergroup is genetically, morphologically and behaviourally distinct from other eukaryotes, and comprises two divergent clades of predators—Nebulidia and Nibbleridia—that are superficially similar to each other, but differ fundamentally in ultrastructure, behaviour and gene content. 

These predators are globally distributed in marine and freshwater environments, but are numerically rare and have consequently been overlooked by molecular-diversity surveys. In the age of high-throughput analyses, investigation of eukaryotic diversity through culture remains indispensable for the discovery of rare but ecologically and evolutionarily important eukaryotes.

Denis V. Tikhonenkov, et al., "Microbial predators form a new supergroup of eukaryotes" Nature (December 7, 2022).