Oxygen's second rise
The deep ocean gets air — and animals can grow large
See it on the timeline →In depth
The Neoproterozoic oxygenation is traced through trace elements, iron speciation and sulphur isotopes in sediments worldwide. Atmospheric levels probably rose from a few per cent towards something approaching today's. The direction of causation with animal origins is debated: more oxygen may have enabled large animals, but burrowing and filter-feeding animals also change sea-floor chemistry and may have contributed to the oxygenation. Either way the physical constraint is real — a body without a circulatory system can only get so thick before oxygen fails to reach the middle.
For children
After the Great Oxidation two billion years ago, only the top layers of water had oxygen. Now it filled all the way down into the deep. It is no coincidence that large animals appear just then: the bigger the body, the more oxygen it takes to run it.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org