Scientists Pinpoint the Birth of Oxygen on Earth

Photosynthesis: the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water, generating oxygen as a byproduct.

Plants consider oxygen to be a byproduct of eating – a ‘belch’ of gas it has no use for. Humans and most other living, breathing creatures can’t live without oxygen, which would lead one to think we should take better care of Earth’s plants. That’s a subject for another day. Today, we’re looking for the first time oxygen entered the atmosphere – an event scientists from MIT have recently traced to a common cyanobacteria that evolved around 2.9 billion years ago into the first oxygen photosynthesizer. Doesn’t that deserve some kind of award or a plaque or at least a laurel and hardy handshake?

“In evolution, things always start small. Even though there’s evidence for early oxygenic photosynthesis — which is the single most important and really amazing evolutionary innovation on Earth — it still took hundreds of millions of years for it to take off.”

Photosynthesis is “the single most important and really amazing evolutionary innovation on Earth”? That’s what Greg Fournier, associate professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences claims in a university press release touting the paper he co-authored that was published recently in the Proceedings of the Royal Society B – and who’s going to argue with an MIT professor? While many scientists agree Earth had a so-called Great Oxygenation Event (GOE) when enough oxygen appeared in the planet’s biosphere to support oxygen-breathing life, its date-of-birth was a mystery, along with whether it was a slow process or a big bang of oxygen. Do you look for the first fossil with a lung? The first plant?

Fournier had a better idea. He and his colleagues used molecular clock dating to look for instances of horizontal gene transfer where genes jump between species. The organism getting the horizontal gene transfer becomes the first of a new species and the molecular clock could then date the event. They applied this technique to cyanobacteria using new cultures of modern cyanobacteria and found 34 clear instances of horizontal gene transfer. One out of six molecular clock models gave a consistent match, which identified the “crown” group of oxygenic photosynthesizers and determined that it originated during the Archean eon around 2.9 billion years ago, while cyanobacteria as a whole branched off from other bacteria around 3.4 billion years ago. That means oxygenic photosynthesis was already happening 500 million years before the Great Oxidation Event and cyanobacteria were producing oxygen slowly before the atmosphere had enough for oxygen-breathing life.

“This evolutionary moment made it possible for oxygen to eventually accumulate in the atmosphere and oceans, setting off a domino effect of diversification and shaping the uniquely habitable planet we know today.”

That sounds like a trophy-worthy event. It also sounds like a signature for astronomers to look for in the search for life on other planets. In fact, that’s exactly what scientists are looking for on Venus, Mars and Europa.

Not only is this a significant discovery, perhaps the “single most important and really amazing evolutionary innovation on Earth,” the Oxygenic Photosynthesizers would be a great name for a band.