Oxygen is crucial for the existence of animals on Earth, but an increase in the gas did not lead to the rise of the first animals on our planet, a new study has found.
Researchers showed 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 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," said Emma Hammarlund and Don Canfield from the University of Southern Denmark.
The researchers analysed sediment samples from the Xiamaling Formation in China. Their analysis shows that a deep ocean 1.4 billion years ago contained at least 4 per cent of modern oxygen concentrations.
The research uses trace metal distributions to show that the bottom waters where the Xiamaling Formation sediments deposited contain oxygen.
The distribution of biomarkers, molecules derived from ancient organisms, demonstrate that waters of intermediate depth contain no oxygen.
Therefore, the Xiamaling Formation deposited in an ancient oxygen-minimum zone, similar to (but also different) from those found off the present coasts of Chile and Peru.
The scientists used a simple ocean model to estimate the minimum concentrations to atmospheric oxygen required to reproduce the distribution of water-column oxygen in the Xiamaling Formation.
"The water column had an oxygen concentration at least 4 per cent of present atmospheric levels (PAL). That should be sufficient for animals to exist and evolve," said Canfield.
"Researchers know of simple animals, such as sponges and worms, that today are capable of managing with less than 4 per cent PAL, even much less," said Hammarlund.
"Sponges probably resemble some of the first animals on Earth. If they manage with less than 4 per cent today's oxygen levels, it is likely that the first animals could do with these concentrations or less," Canfield added.
The sudden diversification of animals might have been a result of many factors and the oxygen rise may have had less to do with the animal revolution than previously assumed, researchers said.
The findings were published in the journal Proceedings of the National Academy of Sciences.
