Classroom tree

J. Hirschfeld; Source: R. Whittaker/ Science 1969

The outline of five kingdoms of life drawn from an influential 1969 Science paper was based largely on nutritional habits (bottom right) instead of pure evolutionary relationships. This system glorified multicellular kingdoms over single-celled Protista and Monera.

Whittaker crowned his tree of life with three kingdoms of primarily multicellular eukaryotes sorted in large part by nutritional style: plants (capturing light energy), fungi (absorbing nutrients by contact) and animals (ingesting their food). He recognized a fourth kingdom of eukaryotes: the Protista. It was a hodgepodge of single-celled organisms, differing in forms and lifestyles but lumped together largely for convenience. He said as much, acknowledging that none of the classification systems he discussed “can be wholly satisfactory.”

Today many scientists hope for biological categories that consist of a single ancestral lineage and all its evolutionary descendants. A tree of life built this way “actually changes how you interpret a lot of stuff,” Keeling says. Much of biology rests on information gleaned from one organism and hypothesized to be true for its close kin. Identifying relatives is important. “If you’ve got the tree wrong, then you could be wrong about a lot of things,” he says.

The challenge of getting that tree right looms sequoia-high, since so much of the diversity of life lies in complex microbes still unknown to science. Out of about 150,000 kinds of marine plankton detected during the Tara oceans expeditions, as reported May 22 in Science, researchers could genetically identify about a third only as some eukaryote. They couldn’t place them in any known group.

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