Evolution: Small Oxygen Jump Caused Animals to Evolve to Breathe Air

First Posted: Jul 26, 2015 02:28 PM EDT

How did animals first start to breathe air? That's exactly what scientists are looking at a bit more closely. Researchers have found that only a slight bump in oxygen levels facilitated animal evolution to breathing air.

A long held theory states that a dramatic change in oxygen levels may have been responsible for the appearance of complicated life forms like whales, sharks, and squids evolving from less complicated life forms, such as microorganisms, algae and sponges. This latest study, though, may call this theory into question.

"We suggest that about 635 million to 542 million years ago, Earth passed some low, but critical, threshold in oxygenation for animals," said Benjamin Gill, one of the researchers, in a news release. "That threshold was in the range of a 10 to 40 percent increase, and was the second time in Earth's history that oxygen levels significantly rose."

The researchers estimated oxygen levels by analyzing iron found in shale rock, which was once mud on ancient seafloors. The location and amounts of iron in the rock gave important clues about ancient ocean water chemistries over time.

The researchers found that oxygen levels did rise in the water and atmosphere, but at lower levels than was thought necessary to trigger life changes.

"Going forward we will need much more precise constraints on the magnitude of oxygenation and the physiological requirements of early animals to continue testing the impact of oxygenation on Cambrian animal life," said Eri Sperling, one of the researchers.

The findings are published in the journal Nature.