From ScienceDaily:
Oxygen is crucial for the existence of animals on Earth. But, an increase in oxygen did not apparently lead to the rise of the first animals. New research shows that 1.4 billion years ago there was enough oxygen for animals — and yet over 800 million years went by before the first animals appeared on Earth.
Animals evolved by about 600 million years ago, which was late in Earth’s history. The late evolution of animals, and the fact that oxygen is central for animal respiration, has led to the widely promoted idea that animal evolution corresponded with a late a rise in atmospheric oxygen concentrations.
“But sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies,” say postdoc Emma Hammarlund and Professor Don Canfield, Nordic Center for Earth Evolution, University of Southern Denmark.
…
Apparently sponges and worms can manage with less oxygen than that.
“The sudden diversification of animals probably was a result of many factors. Maybe the oxygen rise had less to do with the animal revolution than we previously assumed,” says Hammarlund. More.
In case anyone thinks we yet have much of an idea what happened, see also:
Life exploded after slow O2 rise? Not because of a change in animal behaviour?
Ocean toxicity slowed rapid evolution of complex life? Didn’t slow it much according to these folks above
Cyanobacteria responsible for Earth’s early oxygen
Earth’s boring billion now hot again (“1.8 billion years ago, low oxygen may not have hindered life after all”)
and
What we know and don’t, know about the origin of life
Of possible interest: The appearance of very complex life forms 540 million years ago and 600 million years ago. Also an animal that stages light display is 600 million years old. Anyway, see Darwin’s Doubt.
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Here’s the Significance:
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. (public access) – Shuichang Zhang, Xiaomei Wang, Huajian Wang, Christian J. Bjerrum, Emma U. Hammarlund, M. Mafalda Costa, James N. Connelly, Baomin Zhang, Jin Su, Donald E. Canfield. Sufficient oxygen for animal respiration 1,400 million years ago. Proceedings of the National Academy of Sciences, 2016; 201523449 DOI: 10.1073/pnas.1523449113