When the last Apollo mission was on its way to the Moon four decades ago, one of the astronauts took a snapshot that is among the most famous in NASA history. It is known as the “blue marble” photograph because it shows Earth, from about 28,000 miles away, as a bright, swirling and mostly blue sphere. The dominant color wasn’t surprising—it’s the color of the oceans, which cover nearly three-quarters of the planet.
But Earth is hardly unique in having water. It is everywhere in the universe; even that dusty neighbor Mars, it is now apparent, was once awash.
What sets Earth apart isn’t colored blue but green, a green that is best appreciated not from space, but up close—in a fresh-cut suburban lawn, in lily pads on a frog pond, in a stand of firs on a mountainside. It’s the green of chlorophyll, and of photosynthesis.
Photosynthesis is nature’s take on solar power, its way of making use of all that light energy that comes from the Sun. Modern solar cells do this with semiconductors, and the harvest consists of electrons, which flow after they are excited by photons of light. In nature the electrons are excited in the pigment chlorophyll, but that’s only a first step. The energy is ultimately stored in the chemical bonds of the sugars that, along with oxygen, are the products of photosynthesis.
Those products transformed Earth, the oxygen sweetening the atmosphere and the sugars providing food. Together, they allowed for a long and slow blooming of life that eventually included many organisms—humans among them—that cannot photosynthesize.
Plants have been using light in this primal way for a large chunk of Earth’s existence. But just how did they gain the ability to photosynthesize?
The short answer is they stole it, about a billion and a half years ago, when single-celled organisms called protists engulfed photosynthesizing bacteria. Over time, through the transfer of genes aided by a parasite, the absorbed bacteria became a functional part of the protist, enabling it to transform sunlight into nourishment. “The three of them made it happen,” says Rutgers University evolutionary biologist Debashish Bhattacharya. “The tree of life involves a lot of invention and stealing.” A version of this sunlight-driven, chlorophyll-containing little machine exists to this day in plant cells. It is called a chloroplast.
Scientists are still learning about the complex process, called endosymbiosis, by which a cell, like a protist, for some reason absorbs other living things to create something quite new in biology.
Genetic analyses of algae conducted by Bhattacharya suggest that the pivotal endosymbiotic event that endowed plants with the engine of photosynthesis happened just once in our planet’s early history, in a common ancestor—a single microscopic protist that made green the most important color on Earth.
This latest finding satisfies a basic principle of science: The simplest explanation is usually the best one. The idea that endosymbiosis would have occurred once—before the protists diverged and evolved into different species— is far more sensible than the alternative: that endosymbiosis reoccurred with each new emerging species.