Cyanobacteria Started Creating Oxygen, So Now Humans Exist

All life on Earth owes its existence to a single class of Cyanobacteria that took it upon itself to learn photosynthesis. A new study shows that around 2.5 billion years ago, a certain enterprising bacteria diverged from its non-oxygen-producing forebears to become the first oxygen-producing one, a split which might have taken place multiple times in Earth’s early history. It was only after this divergence that modern life emerged.

First, here’s a little background to better understand this discovery: There are three known classes of Cyanobacteria. Oxyphotobacteria, which produces oxygen, and Melainabacteria and the memorably named ML635J-21, which do not. The researchers analyzed 28 known Melainabacteria genomes and 10 additional Melainabacteria genomes that had previously been misclassified. Finally, they examined three ML635J-21 genomes, also misclassified.

The big finding: All 41 genomes were closely related to Oxyphotobacteria, but couldn’t create oxygen. This means that their last common ancestor probably couldn’t either, and that photosynthesis was an ability gained much later in the timeline than scientists had generally believed.

What’s really cool is that scientists estimate that the rise of oxygen in the atmosphere about 2.3 billion years ago was a direct result of this then sort-of new oxygenic photosynthesis. Humans breathe oxygen, of course, so this oxygen-creating Cyanobacteria played a major role in the evolution the humanity’s earliest ancestors.

A paper detailing the research was published Thursday in the journal Science.

“Cyanobacteria are really special”

“Cyanobacteria are really special from a planetary perspective, because they’re the ones that figured out how to do the photosynthesis everyone knows and loves,” co-author Woodward Fischer tells Inverse. “Everything we breathe, we owe in some way to these guys. That they figured out photosynthesis relatively recent in their history … we didn’t necessarily expect that. That’s a big deal.”

Based on when the classes diverged from their common ancestor, the researchers believe that the rise of oxygen on Earth began about 2.3 billion years ago, a direct result of the newly evolved ability to photosynthesize. Despite their importance, we never really knew where these Cyanobacteria came from, how they evolved, when some began producing oxygen, or what their closest relatives were. We do now.

The researchers were able to analyze the genomes via metagenomics, a relatively new sequencing technology by which scientists can extract DNA from a given environment without the need to culture the organisms in a petri dish. They can sequence the genome directly. In the case of the anaerobic classes the researchers studied here, that environment is the gut — animal guts, but the human gut, too, especially if that human eats a lot of vegetables.

“Even though they don’t do this rad metabolism, they’re just as successful in their own way,” Fischer says.

Like other profound discoveries about the early evolution of life on our planet, this new information has implications for how life might evolve on other planets. As we ramp up our study of exoplanets, we can use the new timeline as a potential benchmark for when we might expect young planets to develop an oxygenated atmosphere.

Abstract

The origin of oxygenic photosynthesis in Cyanobacteria led to the rise of oxygen on Earth ~2.3 billion years ago, profoundly altering the course of evolution by facilitating the development of aerobic respiration and complex multicellular life. Here we report the genomes of 41 uncultured organisms related to the photosynthetic Cyanobacteria (class Oxyphotobacteria), including members of the class Melainabacteria and a new class of Cyanobacteria (class Sericytochromatia) that is basal to the Melainabacteria and Oxyphotobacteria. All members of the Melainabacteria and Sericytochromatia lack photosynthetic machinery, indicating that phototrophy was not an ancestral feature of the Cyanobacteria and that Oxyphotobacteria acquired the genes for photosynthesis relatively late in cyanobacterial evolution. We show that all three classes independently acquired aerobic respiratory complexes, supporting the hypothesis that aerobic respiration evolved after oxygenic photosynthesis.