Oxygen levels were key to early animal evolution: Study

London: High level of oxygen in ancient oceans were crucial for the emergence of skeletal animals 550 million years ago, a new study suggests. The study is the first to distinguish between bodies of water with low and high levels of oxygen.

It shows that poorly oxygenated waters did not support the complex life that evolved immediately prior to the Cambrian period, suggesting the presence of oxygen was a key factor in the appearance of these animals.

It has long been a mystery why after three billion years of nothing more advanced than algae, complex animals suddenly started to appear on Earth.

"The question of why it took so long for complex animal life to appear on Earth has puzzled scientists for a long time," said lead author Rosalie Tostevin, who was with University College London (UCL) in the UK, at the time of the study.

"One argument has been that evolution simply does not happen very quickly, but another popular hypothesis suggests that a rise in the level of oxygen in the oceans gave simple life-forms the fuel they needed to evolve skeletons, mobility and other typical features of modern animals," said Tostevin, who is now with the Oxford University.

"Although there is geochemical evidence for a rise in oxygen in the oceans around the time of the appearance of more complex animals, it has been really difficult to prove a causal link," she said.

By teasing apart waters with high and low levels of oxygen, and demonstrating that early skeletal animals were restricted to well-oxygenated waters, researchers have provided strong evidence that the availability of oxygen was a key requirement for the development of these animals.

However, these well-oxygenated environments may have been in short supply, limiting habitat space in the ocean for the earliest animals.

Researchers analysed the chemical elemental composition of rock samples from the ancient seafloor in the Nama Group - a group of extremely well-preserved rocks in Namibia that are abundant with fossils of early Cloudina, Namacalathus and Namapoikia animals.

The researchers found that levels of elements such as cerium and iron detected in the rocks showed that low-oxygen conditions occurred between well-oxygenated surface waters and fully 'anoxic' deep waters.

Although abundant in well-oxygenated environments, early skeletal animals did not occupy oxygen-impoverished regions of the shelf, demonstrating that oxygen availability was a key requirement for the development of early animal-based ecosystems.