There Is an Organism That Is Both Plant and Animal

In the microscopic world of a pond, the strict line between plant and animal completely dissolves. Here you can find *Euglena*, a fascinating single-celled organism that thrives by embracing the best of both kingdoms. When sunlight is plentiful, it uses its internal chloroplasts—the same green structures found in plant cells—to perform photosynthesis and create its own energy. It actively seeks out light using a primitive red eyespot, which guides its movement as it swims with a whip-like appendage called a flagellum.

This dual identity is a window into a dramatic evolutionary past. Scientists theorize that an ancient ancestor of *Euglena* was a predator that once swallowed a green alga. Instead of being digested, the alga became a permanent resident inside the host cell, a process called endosymbiosis. This event gave *Euglena* its photosynthetic powers. When deprived of light, however, it reverts to its ancestral predatory behavior, absorbing nutrients from the water (Review) or consuming bacteria. For over a century, this unique mix of traits made *Euglena* a puzzle for classification, ultimately helping biologists recognize the need for a separate kingdom, Protista, to categorize complex life that fits neither the plant nor animal mold.