Did the First Animals Live in a World Without Oxygen?

A new study suggests the answer may be yes.

Animal life in the ediacaran era from, 635 to 541 million years ago, was strictly low energy

According to current scientific thinking, complex life on Earth needed oxygen to evolve.

Now Daniel Stolper and Brenhin Keller from the University of Berkeley, California, have called that thinking into question. In a new paper published in Nature, they report on their analysis of submarine lava flows to determine the oxidation state of iron in the rocks. Today the deep ocean contains a lot of oxygen, which circulates in fluids within the basaltic rock that oxidizes iron. The researchers found that this oxygenation effect dates back to only 540 to 420 million years ago, long after the first animals evolved on our planet. Before then, there must have been much less oxygen in the deep ocean, and by extension in the atmosphere and shallow water habitats.

This finding challenges the traditional idea, advanced by David Catling and others, that oxygen is a key requirement for the rise of complex life. After all, animals need oxygen to satisfy the high-energy metabolism that allows them to move, jump, fly, and hunt.

So how can these two contrasting ideas be reconciled?