As the fundamental chemical process that sustains life on Earth, photosynthesis has long been the subject of intense scientific investigation. Researchers have known for years that the process originated in bacteria, yet details of its origin and evolution remained elusive. Now new molecular research reported in the journal Science is shedding light on the story and pushing back the origin of this pivotal innovation.

By sequencing photosynthesis genes in two types of green bacteria and comparing those with data from the other lineages of photosynthetic bacteria, the scientists generated a photosynthesis phylogeny, or family tree. The results revealed that those bacteria containing photosystems in which oxygen is not produced, such as purple bacteria, evolved first. Bacteria containing oxygen-producing photosystems evolved later, in cyanobacteria, which eventually gave rise to the chloroplasts found in today's green plants.

The molecular data contradict the conventional scenario for the emergence of this energy-harnessing mechanism. Purple bacteria were thought to have been one of the last bacterial groups to evolve, because bacteriochlorophyll--the bacterial version of the light-capturing molecule--is more complex than the chlorophyll used by green plants. The new data, however, suggest that chlorophyll derived from bacteriochlorophyll, not the other way around.