Decades of research on Earth’s atmosphere have left scientists puzzled about the timeline of breathable oxygen for early animals. With new clues from Spain’s Rio Tinto waters, researchers found evidence that ancient atmospheric oxygen levels were sufficient for animal evolution nearly 2 billion years before their appearance. Conflicting data on past oxygen levels complicates understanding oxygen’s role in animal evolution. Recent studies suggest that Proterozoic oxygen levels were potentially high enough for animals to prosper early on, challenging the idea that lack of oxygen delayed animal evolution. The history of Earth’s breathable oxygen, as deciphered from geochemical estimates, reveals a complex interplay with the evolution of animal life.
https://eos.org/science-updates/how-great-was-the-great-oxidation-event