Atmospheric oxygen really took off on our planet about 2.4 billion years ago during the Great Oxygenation Event. At this key juncture of our planet's evolution, species had either to learn to cope with this poison that was produced by photosynthesizing cyanobacteria or they went extinct. It now seems strange to think that the gas that sustains much of modern life had such a distasteful beginning.

So how and when did the ability to produce oxygen by harnessing sunlight enter the eukaryotic domain, that includes humans, plants, and most recognizable, multicellular life forms? One of the fundamental steps in the evolution of our planet was the development of photosynthesis in eukaryotes through the process of endosymbiosis.

This crucial step forward occurred about 1.6 billion years ago when a single-celled protist captured and retained a formerly free-living cyanobacterium. This process, termed primary endosymbiosis, gave rise to the plastid, which is the specialized compartment where photosynthesis takes place in cells.

Endosymbiosis is now a well substantiated theory that explains how cells gained their great complexity and was made famous most recently by the work of the late biologist Lynn Margulis, best known for her theory on the origin of eukaryotic organelles.

In a paper "Cyanophora paradoxa genome elucidates origin of photosynthesis in algae and plants" that appeared this week in the journal Science, an international team led by evolutionary biologist and Rutgers University professor Debashish Bhattacharya has shed light on the early events leading to photosynthesis, the result of the sequencing of 70 million base pair nuclear genome of the one-celled alga Cyanophora.

In the world of plants, "Cyanophora is the equivalent to the lung fish, in that it maintains some primitive characteristics that make it an ideal candidate for genome sequencing," said Bhattacharya.

Bhattacharya and colleagues consider this study "the final piece of the puzzle to understand the origin of photosynthesis in eukaryotes." Basic understanding of much of the subsequent evolution of eukaryotes, including the rise of plants and animals, is emerging from the sequencing of the Cyanophora paradoxa genome, a function-rich species that retains much of the ancestral gene diversity shared by algae and plants.

For those unfamiliar with algae, they include the ubiquitious diatoms that are some of the most prodigious primary producers on our planet, accounting for up to 40% of the annual fixed carbon in the marine environment.

Bhattacharya leads the Rutgers Genome Cooperative that has spread the use of genome methods among university faculty. Using data generated by the Illumina Genome Analyzer IIx in his lab, Bhattacharya, his lab members Dana C. Price, Cheong Xin Chan, Jeferson Gross, Divino Rajah and collaborators from the U.S., Europe and Canada provided conclusive evidence that all plastids trace their origin to a single primary endosymbiosis.

Now that the blueprint of eukaryotic photosynthesis has come more clearly in sight, researchers will be able to figure out not only what unites all algae as plants but also what key features make them different from each other and the genes underlying these functions.