Let there be photosynthesis: three billion years of fresh air

Long ago, the Earth was filled with unbreathable gases that prohibited the existence of life as we know. But a moment in history caused a dramatic shift as oxygen was breathed into the world. It accumulated in the atmosphere and oceans, kickstarting a domino effect of diversification that led to the uniquely habitable planet we live on today.

What triggered this event in the first place?

Everything changed when bacteria struck a genetic trade deal over 3 billion years ago and evolution said: “Let there be photosynthesis.”

Photosynthesis is the process plants and some bacteria use to convert light and water into energy and release oxygen in the process. It first evolved in cyanobacteria and eventually became the standard breathing mechanism for all plants.

Photosynthesis is the most important way of releasing oxygen into the atmosphere for humans and animals to breathe.

A slow start still changes the world

Because cyanobacteria were the first to breathe out oxygen, the origin of this enigmatic phylum also marks the origin of photosynthesis.

Recently, Massachusetts Institute of Technology (MIT) researchers used molecular techniques to precisely measure the rise of photosynthesising cyanobacteria. They concluded the life-changing event spanned half a billion years during the Archeon Eon, between 2.9 and 3.4 billion years ago.

Interestingly, this measurement vastly predates the Great Oxygenation Event – the period in which Earth’s atmosphere rapidly became filled with oxygen 2–2.4 billion years ago – which means photosynthesis had evolved well before the world began to change.

“In evolution, things always start small,” says lead author Greg Fournier, associate professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“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.”

Some genes are meant to be traded

The evolution of photosynthesising bacteria was not at all regular. Instead of passing genes from parent to offspring, the cyanobacteria that eventually caused photosynthesis instead shared genes through a ‘handshake’.

This process – called horizontal transfer – meant that genes leaped from one genome to another when two organisms came in close contact.

To track this, the researchers used a molecular clock. This technique relies on the hypothesis that genes evolve at a uniform rate over time, so time is measured based on the number of mutations in one organism compared to others. The most useful part of this is that it can be applied to trace the evolutionary history of living bacteria – no fossils needed.

In this case, the researchers found evidence of genes that were older than the organism they were found in. This means an older organism “gave” the genes to other bacteria via horizontal transfer at some stage in history.

In fact, the researchers found 34 clear instances of horizontal transfer. They then slotted the genes into a model and estimated the age of cyanobacteria as a group to confirm when they first evolved.

They found cyanobacteria arose around 3.4 billion years ago, but oxygen-producing cyanobacteria only fully established themselves 500 million years later.

Photosynthesis evolution predates the Great Oxygenation Event

This data strongly suggests photosynthesis was already happening before the Great Oxygenation Event, but it took a little while to become fully established as the species evolved and diversified.

“This work shows that molecular clocks incorporating horizontal gene transfers promise to reliably provide the ages of groups across the entire tree of life, even for ancient microbes that have left no fossil record … something that was previously impossible,” Fournier says.