Oxygen increase and the pacing of early animal evolution

Abstract

While it is widely accepted that early animals originated and primarily evolved during the Neoproterozoic to Cambrian period, there remains ongoing debate over how fluctuations in marine-atmospheric oxygen levels influenced their evolution and diversification. To investigate this, we analyzed pristane/phytane ratios—a redox-proxy based on organic geochemistry—in surface sediments from the Cryogenian to Cambrian successions in South China, Oman, and Australia. The temporal changes in this proxy exhibited consistent patterns across all sites, revealing five cycles of anoxic to oxic conditions in the ocean between 660 and 510 Ma. By examining the average pristane/phytane ratios, we identified three events when oxygen levels increased. This represents a transition from anoxic conditions to anoxic-dysoxic boundary at 630–600 Ma, followed by a shift to dysoxic conditions at 570 Ma (Shuram event), and finally reaching oxic conditions at 520 Ma (Cambrian explosion). Significantly, these oxygenation events align with eumetazoan evolution. An inverse relationship was observed between oceanic redox events and positive/negative shifts of δ13Ccarb, occurring between the first and second oxygenation events, suggesting an increase in atmospheric oxygen levels. Based on these findings, we propose that there were global increases in oceanic and atmospheric oxygen levels, at least during the early Ediacaran period. These shifts in oxygen levels likely played a role in influencing the rate of evolution among early animals.