I'm always on the lookout for fascinating, provocative topics for Skeptophilia, but even so, it's seldom that I read a scientific paper with my jaw hanging open. But that was the reaction I had to a paper from a couple of months ago in Nature that I just stumbled across yesterday.
First, a bit of background.
Based on the same kind of genetic evidence I described in yesterday's post, biologists have divided all living things into three domains: Eukarya, Bacteria, and Archaea. Eukarya contains eukaryotes -- organisms with true nuclei and complex systems of organelles -- and are broken down into four kingdoms: protists, plants, fungi, and animals. Bacteria contains, well, bacteria; all the familiar groups of single-celled organisms that lack nuclei and most of the other membrane-bound organelles. Archaea are superficially bacteria-like; they're mostly known from environments most other living things would consider hostile, like extremely salty water, anaerobic mud, and acidic hot springs. In fact, they used to be called archaebacteria (and lumped together with Bacteria into "Kingdom Monera") until it was discovered in 1977 by Carl Woese that Archaea are more genetically similar to eukaryotes like ourselves than they are to ordinary bacteria, and forced a complete revision of how taxonomy is done.
So things have stood since 1977: three domains (Bacteria, Archaea, and Eukarya), and within Eukarya four kingdoms (Protista, Plantae, Fungi, and Animalia).
But now a team led by Denis Tikhonenkov, of the Russian Academy of Scientists, has published a paper called "Microbial Predators Form a New Supergroup of Eukaryotes" that looks like it's going to force another overhaul of the tree of life.
Rather than trying to summarize, I'm going to quote directly from the Tikhonenkov et al. paper so you get the full impact:
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.
The members of Provora are distinguished not only genetically but by their behavior; to my eye they look a bit like a basketball with tentacles, using weird little tooth-like structures to nibble their way forward as they creep along. (Thus "nibblerid," which is their actual name, despite the fact that it sounds like a comical monster species from Doctor Who.) The first one discovered (in 2017), the euphoniously-named Ancoracysta twista, is a predator on tropical coral, and was found in (of all places) a home aquarium. Since then, they've been found all over the place, although they're not common anywhere; the only place they've never been seen is on land. But just about every aquatic environment, fresh or marine, has provorans of some kind.
The provorans appear to be closely related to no other eukaryote, and Tikhonenkov et al. are proposing that they warrant placement in their own supergroup (usually known as a "kingdom"). But it raises questions of how many more outlier supergroups there are. A 2022 analysis by Sijia Liu et al. estimated the number of microbial species on Earth at somewhere around three million, of which only twenty percent have been classified. It's easy to overlook them, given that they're microscopic -- but that means there could be dozens of other branches of the tree of life out there about which we know nothing.
It's amazing how much more sophisticated our understanding of evolutionary descent has become. When I was a kid (back in medieval times), we learned in science class that there were three divisions; animals, plants, and microbes. (I even had a Golden Guide called Non-Flowering Plants -- which included mushrooms.) Then it was found that fungi and animals were more closely related than fungi and plants, and that microbes with nuclei and organelles (like amoebas) were vastly different from those without (like bacteria). There it stood till Woese came along in 1977 and told us that the bacteria weren't a single group, either.
And now we've got another new branch to add to the tree. The nibblers. Further illustrating that we don't have to look into outer space to find new and astonishing things to study; there is a ton we don't know about what's right here on Earth.