In the history of life on this planet, it is one of the most important events. But evidence for when photosynthesis first occurred is incredibly limited. In general, scientists look for an increase of oxygen in the geological record as an indicator that living organisms had started to photosynthesize. After examining rock laid down at the bottom of a shallow sea around 3.2 billion years ago, a new study claims to have found evidence that microorganisms were producing oxygen in enough quantities at this time to have left a mark in the rocks.
“Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen,” explains Clark Johnson, who co-authored the paper published in Earth and Planetary Science Letters. “Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old, and quite well preserved, and we believe they show definite signs for oxygen in the oceans much earlier than previous discoveries.”
The 3.4-billion-year-old core of rock was drilled in a geologically stable region of eastern South Africa. David Tenenbaum/University of Wisconsin-Madison
Obtained from South Africa, the rock cores the scientists looked at are made from iron oxide and quartz, and were formed under the ocean. The researchers used a technique called mass spectrometry to determine the amounts of different types of iron, or isotopes, that were present in the rocks, which could be used to figure out exactly how much oxygen was needed to form the iron oxides. They found that the iron oxide that formed in the deep sediment, below the level of wave disturbance, was created in water with very little oxygen.
But the rock that formed in shallower water, which was subjected to mixing by the waves, contained iron oxide that needed much more oxygen to form. The researchers claim that the amount of free oxygen necessary for this to have happened is only explained by the presence of photosynthesizing cyanobacteria.
In fact, this research seems to back up previous reports of a 3.4-billion-year-old fossil that appears to show that the oldest photosynthetic organisms were early forms of cyanobacteria. This fits in with another study that found that the molecular machinery found in modern cyanobacteria predates those found in other types of photosynthetic bacteria, suggesting that cyanobacteria were the first to start photosynthesizing early on in the history of life on Earth.
The researchers are quick to point out that this evidence comes from only a single rock core from one location. So while they think it likely that life was photosynthesizing in this particular shallow sea that gave rise to these rocks, they can’t be certain it was happening elsewhere at the same time. However, it's probable that it did spread: “Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live,” says Johnson.
The point at which the levels of oxygen being produced finally became enough to start accumulating in the atmosphere is known as the Great Oxygenation Event, and is generally accepted to have happened around 2.3 billion years ago. This research adds to our understanding of the build-up to this tipping point, and the origins of photosynthesis in the first place.