A NEW study by Oxford scientists has provided the "strongest evidence yet" that oxygen levels were key to evolution of early animals.

It has long puzzled scientists why, after three billion years of nothing more complex than algae, complex animals suddenly started to appear on Earth.

Now, a team of researchers has put forward some of the strongest evidence yet to support the hypothesis that high levels of oxygen in the oceans were crucial for the emergence of skeletal animals 550 million years ago.

The new 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.

The research, based on fieldwork carried out in the Nama Group in Namibia, is published in the journal Nature Communications.

It was led by Dr Rosalie Tostevin, a postdoctoral researcher in the department of earth sciences at Oxford.

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, we 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."