By the beginning of the Cambrian period 541 Ma, free oxygen concentrations had increased sufficiently to enable the evolution of multicellular organisms.
A 2012 study looked at the imprints made by falling raindrops onto freshly deposited volcanic ash, laid down in the Archean Eon 2,700 Ma in the Ventersdorp Supergroup, South Africa.
[4] A similar study in 2016 looked at the size distribution of gas bubbles in basaltic lava flows that solidified at sea level also during the Archean (~2,700 Ma).
[5] Both results contradict theories[citation needed] that suggest the Archean was kept warm during the Faint Young Sun period by extremely high levels of carbon dioxide or nitrogen.
A 2016 study performed mass spectrometry on air bubbles trapped inside rock salt deposited 813 Myr ago.