In 1837, Charles Darwin sketched a simple tree in one of his notebooks. Above it, he scrawled “I think.” That iconic image perfectly encapsulated Darwin’s big idea: that all living things share a common ancestor. Ever since then, scientists have been adding names to the tree of life. Last year, for example, one group compiled what they billed as a “comprehensive tree,” a garguantuan geneaology of some 2.3 million species that “encompasses all of life.”
Impressive work, but they should probably have said “all the life we have sequenced so far.” Existing genetic studies have been heavily biased towards the branches of life that we’re most familiar with, especially those we can see and study. It’s no coincidence that animals made up half of the “comprehensive tree of life,” and fungi, plants, and algae took up another third, and microscopic bacteria filled just a small wedge.
That’s not what the real tree of life looks like.
We visible organisms should be the small wedge. We’re latecomers to Earth’s story, and represent the smallest sliver of life’s diversity. Bacteria are the true lords of the world. They’ve been on the planet for billions of years and have irrevocably changed it, while diversifying into endless forms most wonderful and most beautiful. Many of these forms have never been seen, but we know they exist because of their genes. Using techniques that can extract DNA from environmental samples—scoops of mud or swabs of saliva—scientists have been able to piece together the full genomes of organisms whose existence is otherwise a mystery.
Using 1,011 of these genomes, Laura Hug, now at the University of Waterloo, and Jillian Banfield at the University of California, Berkeley have sketched out a radically different tree of life. All the creatures we’re familiar with—the animals, plants, and fungi—are crowded on one thin branch. The rest are largely filled with bacteria.
And around half of these bacterial branches belong to a supergroup, which was discovered very recently and still lacks a formal name. Informally, it’s known as the Candidate Phyla Radiation. Within its lineages, evolution has gone to town, producing countless species that we’re almost completely ignorant about. With a single exception, they’ve never been isolated or grown in a lab. In fact, this supergroup and “other lineages that lack isolated representatives clearly comprise the majority of life’s current diversity,” wrote Hug and Banfield.
“This is humbling,” says Jonathan Eisen from the University of California, Davis, “because holy **#$@#!, we know virtually nothing right now about the biology of most of the tree of life.” (…)