How have environmental constraints influenced the timing of animal evolution? It is often argued that oxygen first increased to sufficient levels for animal respiration during the Neoproterozoic Eon, 1,000 million to 542 million years ago, thus explaining the timing of animal evolution. We report geochemical evidence for deep-water oxygenation below an ancient oxygen minimum zone 1,400 million years ago. Oceanographic modeling constrains atmospheric oxygen to a minimum of ∼4% of today’s values, sufficient oxygen to have fueled early-evolved animal clades. Therefore, we suggest that there was sufficient atmospheric oxygen for animals long before the evolution of animals themselves, and that rising levels of Neoproterozoic oxygen did not contribute to the relatively late appearance of animal life on Earth.

Abstract

The Mesoproterozoic Eon [1,600–1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were <0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels >4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.