What A Newfound Kingdom Means For The Tree Of Life

The tree of life just got another major branch. Researchers recently found a certain rare and mysterious microbe called a hemimastigote in a clump of Nova Scotian soil. Their subsequent analysis of its DNA revealed that it was neither animal, plant, fungus nor any recognized type of protozoan — that it in fact fell far outside any of the known large categories for classifying complex forms of life (eukaryotes).

Instead, this flagella-waving oddball stands as the first member of its own “supra-kingdom” group, which probably peeled away from the other big branches of life at least a billion years ago.

Impressive as this finding about hemimastigotes is on its own, what matters more is that it’s just the latest (and most profound) of a quietly and steadily growing number of major taxonomic additions. Researchers keep uncovering not just new species or classes but entirely new kingdoms of life — raising questions about how they have stayed hidden for so long and how close we are to finding them all.

Late one evening, something odd in the sample caught her eye. An elongated cell radiating whiplike flagella was “awkwardly swimming, as though it didn’t realize it had all these flagella that could help it move,” Eglit said. Under a more powerful scope, she saw it fit the description of a hemimastigote, a rare kind of protist that was notoriously hard to cultivate. The next morning, the lab was abuzz with excitement over the opportunity to describe and sequence the specimen. “We dropped everything,” she recalled.

Hemimastigotes represent one of a handful of Rumsfeldian “known unknown” protist lineages — moderately well-described groups whose positions on the tree of life are not precisely known because they are difficult to culture in a lab and sequence. Protistologists have used peculiarities of hemimastigotes’ structure to infer their close relatives, but their guesses were “‘shotgunned’ all over the phylogeny,” Simpson said. Without molecular data, lineages like hemimastigotes remain orphans of unknown ancestry.

The team sequenced more than 300 genes, and Laura Eme, now a postdoctoral researcher at Uppsala University, modeled how those genes evolved to infer a classification for hemimastigotes. “We were fully expecting them to fall within one of the existing supergroups,” she explained. Lab members were instead stunned to find that hemimastigotes fit nowhere on the tree. They represented their own distinct lineage apart from the other half-dozen super groups.

Tree of Life USA