Any textbook will tell you that oxygen is essential for advanced life to evolve. But why did life not explode when oxygen levels rose dramatically 2.1 billion years ago? This is the big question after a Danish/Swedish/French research team, led by University of Southern Denmark, has shown that the oxygen content 2.1 billion years ago was probably the same as when life exploded 500 million years ago.

Oxygen and advanced life are inextricably linked. Some simple organisms like bacteria can survive without oxygen, but all higher organisms need oxygen and the Earth's biology would probably be a poor sight, if the atmosphere did not contain the 21 pct. oxygen, which is essential for eg the human brain to function.

The development of life exploded around 542 million years ago during the so-called Cambrian explosion, where oxygen levels rose to up to 10 pct. Before that life consisted of small and simple, typically single-celled life forms, and science has long thought that there was not enough oxygen for it to evolve into something bigger.

But now a Danish/Swedish/French research team shows that there was actually plenty of oxygen long before the Cambrian explosion.