Complex life on Earth requires oxygen, though carbon dioxide and methane choked the atmosphere during our planet’s early years. Geological samples have yielded evidence of microorganisms that appear to have contributed to the oxygenation of our planet 3 billion years ago. This pushes the first appearance of oxygen-producing lifeforms on Earth back by 60 million years. The research was led by Quentin Crowley of Trinity College Dublin and the paper was published in Geology.
It is important to clarify up top that this refers to microorganisms evidently capable of photosynthesis, forming oxygen as a byproduct. This does not mean it's the origin of all life on Earth. So far, microorganisms that existed prior to this point do not appear to have been able to generate oxygen.
"It is the rare examples from the rock record that provide glimpses of how rocks weathered," Crowley stated in a press release. "The chemical changes which occur during this weathering tell us something about the composition of the atmosphere at that time. Very few of these 'paleosols' have been documented from a period of Earth's history prior to 2.5 billion years ago. The one we worked on is at least 3.02 billion years old, and it shows chemical evidence that weathering took place in an atmosphere with elevated O2 levels."
The clues were discovered in paleosol—essentially ancient soil—from a deposit that was once shallow water in eastern India. Isotope decay analysis dated the samples at 3.02 billion years old. The samples appeared to have undergone chemical weathering (which isn’t the same as physical stress or erosion) due to high levels of oxygen. Because oxygen levels on Earth were pretty dismal at the time, the most logical explanation for this weathering pattern is that it was created by photosynthetic microorganisms.
Sample of 3.02 billion year old paleosol that has undergone chemical weathering due to heightened concentrations of oxygen. Image credit: Quentin Crowley
The Great Oxidation Event is believed to have been brought about largely by multicellular cyanobacteria who generated their energy through photosynthesis. They created large amounts free oxygen, which made it possible for aerobic organisms to emerge and prosper. Evidence has suggested that oxygen levels on Earth slightly began to climb about 2.96 billion years ago, but hadn't climbed significantly until about 2.4 billion years ago. However, this new research suggests that a brief period of oxygenation existed long before these multicellular cyanobacteria emerged.
"Our research gives further credence to the notion of early and short-lived atmospheric oxygenation,” Crowly continued. “This particular example is the oldest known example of oxidative weathering from a terrestrial environment, occurring about 600 million years before the Great Oxidation Event that laid the foundations for the evolution of complex life."