Oxygen did not lead to rise of first animals

Oxygen is crucial for the existence of animals on the Earth. But an increase in oxygen levels did not apparently lead to the rise of the first animals on our planet, according to researchers.

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, said a team of researchers from the University of Southern Denmark, China's National Petroleum Corporation and the University of Copenhagen, Hammarlund and Canfield.

"Sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies", Emma Hammarlund and professor Don Canfield of Nordic Centre for Earth Evolution, University of Southern Denmark, were quoted as saying in a statement.

The researchers reached this conclusion after analysing sediment samples from the Xiamaling Formation in China. Their analyses revealed that a deep ocean 1.4 billion years ago contained at least 4 percent of modern oxygen concentrations.

The study used trace metal distributions to show that the bottom waters where the Xiamaling Formation sediments deposited contained oxygen.

The distribution of biomarkers, molecules derived from ancient organisms, demonstrated that waters of intermediate depth contained 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.

With this backdrop, the researchers 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 four percent of present atmospheric levels (PAL). That should be sufficient for animals to exist and evolve," Canfield said.

"Researchers know of simple animals, such as sponges and worms, that today are capable of managing with less than 4 percent PAL, even much less," Hammarlund added.

"Sponges probably resemble some of the first animals on Earth. If they manage with less than 4 percent today's oxygen levels, it is likely that the first animals could do with these concentrations or less," Canfield  noted.

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, according to the study authors.

The new study was published in the latest issue of journal Proceedings of National Academy of Sciences.