Schoolroom kingdoms are taking a backseat to life’s supergroups

By Susan Milius 3:00pm, July 29, 2015

STRANGE RELATIONS People, pandas, and even mushrooms perch as closer cousins to single-celled choanoflagellates than to other multicellular organisms.

Magazine issue: Vol. 188, No. 3, August 8, 2015, p. 22

The tree of life might seem like a stable design, appropriate for indelible ink. Plenty of people think so. An Internet search for “phylogenetic tattoos” turns up some showy skin art.

But the branches are shifting. Since a radial diagram based on 1990s genetics inspired a rush for tree-of-life tattoos, technical diagrams of life’s ancestral connections have been redrawn. And the simplified version of the tree of life memorized by schoolchildren for decades lags far behind what researchers depict today.

When Patrick Keeling at the University of British Columbia in Vancouver unveils a working scientist’s current diagram for his students, most have never heard the names of the major branches. “Kind of fun,” Keeling says.

In the new vision — based on increasingly sophisticated genetic analyses — people and other animals are closer cousins to single-celled choanoflagellates than to other multi­cellular organisms. Giant kelp that grow as wavering undersea forests off the California coast are closer relatives to single­-celled plankton called diatoms than to multicelled red seaweeds or plants.

Genetics-based versions of the treetop of life have inspired a new genre of tattoo, often with symbolic organisms rather than multisyllabic labels.

Science Ink: Tattoos of the Science Obsessed, Carl Zimmer (Sterling). © 2011 Carl Zimmer

The old tree isn’t exactly wrong. The kingdoms that used to crown its top — plants, fungi and animals — still exist. But they’ve moved. In the new diagram, the tree’s former crowning glories shrink to mere side branches, three among hundreds, crowded by the vast diversity of complex single cells.

Biologists analyzing this treetop rarely use the word kingdom anymore. They talk of five or maybe seven bigger branches called supergroups. And the story of demoting kingdoms and introducing supergroups is far from over. A 2014 review noted five proposals for designating the most ancient stretch of supergroup branches, the bit that goes lowest on the new tree. A paper appearing in the Proceedings of the National Academy of Sciences in February described a new method for resolving this debate, and discussions continue.

Even as a work in progress, the new groupings offer benefits, such as help in searching for new medicines and insight into evolutionary puzzles such as the supposed wastefulness of sex. As the tree continues to morph, it will no doubt inspire new tattoos.

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