The researchers focused on the different levels of chromium isotopes within their samples. Isotopes are variants of elements; all isotopes of an element have the same number of protons in their atoms, but each has a different number of neutrons - for instance, each atom of chromium-52 has 28 neutrons, while atoms of chromium-53 have 29.
When atmospheric oxygen reacts with rock, a process known as weathering, heavier chromium isotopes, such as chromium-53, often get washed out to sea by rivers. This means heavier chromium isotopes are often depleted from soils on land and enriched in sediments in the ocean when oxygen is around. These proportions of heavier chromium were just what were seen in the South African samples. Similar results were seen with other metals, such as uranium and iron, that hint at the presence of oxygen in the atmosphere.
"We now have the chemical tools to detect trace atmospheric gases billions of years ago," Crowe told LiveScience.
'Almost certainly biological'
All in all, the researchers suggest atmospheric oxygen levels 3 billion years ago were about 100,000 times higher than what can be explained by regular chemical reactions in Earth's atmosphere. "That suggests the source of this oxygen was almost certainly biological," Crowe said.