Genomic analysis pinpoints early date for the evolution of photosynthesis

Photosynthesis may have evolved about half a billion years before oxygen became abundant in the Earth’s atmosphere, suggests a new study (Proc. R. Soc. B 2021, 10.1098/rspb.2021.0675). This discovery updates the timing of one of the most important events in the planet’s history.

The most common form of photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to fuel their metabolisms, producing oxygen along the way—and making the planet’s air breathable. The evolution of oxygen-generating photosynthesis “is probably the single most important event that ever occurred on Earth, aside from the origin of life itself,” says Gregory Fournier, a geobiologist at the Massachusetts Institute of Technology, who led the work.

Scientists agree that photosynthesis evolved in microbes called cyanobacteria but have proposed different hypotheses about when it happened. Pinpointing the timing “is extremely important for understanding the history of the Earth’s habitability and the history of the biosphere,” Fournier says. Some studies have suggested that photosynthesis evolved 1 billion years before what’s known as the Great Oxygenation Event (GOE), which occurred about 2.4 billion years ago when cyanobacteria drove a sharp increase in O₂ in Earth’s atmosphere. Others have proposed that cyanobacteria emerged only half a billion years before the GOE.

Researchers often rely on the fossil record to date events that occurred far back in geological time. But microbial fossils are few and far between, and it can be tough to tell from a fossil which type of bacteria it represents. So researchers combine scant data from the fossil record with molecular clocks—computer models that compare genome sequences of related species to estimate when they diverged. Molecular clocks, however, have limitations of their own. Without solid fossil data, the models contain biases, with no empirical means of choosing between them.

Fournier and his colleagues designed a molecular clock with more accurate timing by adding information drawn from additional genomic evidence. Their approach capitalizes on the fact that bacteria frequently pick up genes from their environment, a process called horizontal gene transfer. These genes are incorporated into the genome and are passed on to a bacterium’s descendants. The scientists reasoned that genes acquired through this process come from organisms that are evolutionarily older than the one those genes land in. By accurately tracing such gene transfers, researchers can use them to make molecular clock data more accurate.

The team analyzed tens of thousands of horizontal gene transfer events in cyanobacteria and found about 30 that they could use to narrow down their evolutionary history. This analysis led them to conclude that cyanobacteria—and their photosynthetic activity—evolved 2.9 billion years ago, half a billion years before the GOE.

“There’s a gap between the earliest bound we can put on when oxygenic photosynthesis is happening and the first evidence we see of oxygen in the atmosphere,” Fournier explains. That gap fits an emerging theme in the evolution of Earth and life, he says: The evolutionary origin of different processes and organisms precedes our first evidence of them in the geological record.

Tanai Cardona, an evolutionary biologist at Imperial College London who was not involved in the work, says the study “adds weight to the notion that the origin of oxygenic photosynthesis occurred during the very early history of life, well over three billion years ago.”

The team is now applying their molecular clock technique to a wider group of microbes in order to explore the evolutionary age of bacteria that consume oxygen, Fournier says. That approach could help them understand the sequence of events that occurred between the emergence of photosynthesis and the GOE.