Researchers have caught their best glimpse yet into the origins of photosynthesis, one of nature’s most momentous innovations. By taking near-atomic, high-resolution X-ray images of proteins from primitive bacteria, investigators at Arizona State University and Pennsylvania State University have extrapolated what the earliest version of photosynthesis might have looked like nearly 3.5 billion years ago. If they are right, their findings could rewrite the evolutionary history of the process that life uses to convert sunlight into chemical energy.

Photosynthesis directly or indirectly powers and sustains almost every organism on Earth. It is responsible for the composition of our atmosphere and forms the foundation of the planet’s many interwoven ecosystems. Moreover, as Wolfgang Nitschke, a biologist at the French National Center for Scientific Research in Paris, noted, photosynthesis liberated cells to grow and evolve boundlessly by letting them derive energy from a new, inexhaustible, nonterrestrial source. “When photosynthesis entered the picture, life connected up to the cosmos,” he said.

Scientists want to figure out what made that possible. In its current form, the machinery that converts light energy to chemical energy in photosynthesis—a protein complex called a reaction center—is incredibly sophisticated. The evidence suggests, however, that its design, which stretches back almost to the root of the tree of life, was once very simple. Researchers have been trying for decades to fill that enormous gap in their understanding of how (and why) photosynthesis evolved.

To that end, they have turned their attention to existing organisms. By studying the molecular details of the reactions that green plants, algae and some bacteria use to photosynthesize, and by analyzing the evolutionary relationships among them, scientists are trying to piece together a cogent historical narrative for the process.

The muddy soils around geothermal hot springs in Iceland, like the Geysir spring pictured here, are the natural habitat for primitive photosynthetic heliobacteria. Scientists are now studying those organisms for insights into the early evolution of photosynthesis. Arctic-Images/Getty Images

The latest important clue comes from Heliobacterium modesticaldum, which has the distinction of being the simplest known photosynthetic bacterium. Its reaction center, researchers think, is the closest thing available to the original complex. Ever since the biologists Kevin Redding, Raimund Fromme and Christopher Gisriel of Arizona State University, in collaboration with their colleagues at Penn State, published the crystallographic structure of that protein complex in a July edition of Science, experts have been unpacking exactly what it means for the evolution of photosynthesis. “It’s really a window into the past,” Gisriel said.

“This is something we’ve been waiting for for 15 years,” Nitschke said.

In Search of a Common Ancestor

At first, most scientists did not believe that all the reaction centers found in photosynthetic organisms today could possibly have a single common ancestor. True, all reaction centers harvest energy from light and lock it into compounds in a form that’s chemically useful to cells. To do this, the proteins pass electrons along a transfer chain of molecules in a membrane, as though skipping along a series of stepping stones. Each step releases energy that’s ultimately used down the line to make energy-carrier molecules for the cell.

But in terms of function and structure, the photosystem reaction centers fall into two categories that differ in almost every way. Photosystem I serves mainly to produce the energy carrier NADPH, whereas photosystem II makes ATP and splits water molecules. Their reaction centers use different light-absorbing pigments and soak up different portions of the spectrum. Electrons flow through their reaction centers differently. And the protein sequences for the reaction centers don’t seem to bear any relation to each other.

Both types of photosystem come together in green plants, algae and cyanobacteria to perform a particularly complex form of photosynthesis—oxygenic photosynthesis—that produces energy (in the form of ATP and carbohydrates) as well as oxygen, a byproduct toxic to many cells. The remaining photosynthetic organisms, all of which are bacteria, use only one type of reaction center or the other.