Animals today are so dependent on access to lots of oxygen that it's easy to think it was a prerequisite for our ancestors to appear. However, studies of basalts formed at the bottom of the ocean have challenged that idea.

While life appeared on Earth not that long after formation, there was nothing more than single-celled organisms for well over 3 billion years. Then, very suddenly, a profusion of more complex life appeared and spread.

The common story goes that multicellular life forms couldn't survive until the atmosphere (and consequently air that dissolved in the ocean) became rich with oxygen, but to confirm that we need to know when oxygen concentrations approached modern levels.

In Nature, Dr Daniel Stolper and Dr Brenhin Keller of the University of California, Berkeley, produce evidence that suggests the atmosphere only reached modern oxygen concentrations between 540 and 420 million years ago. Animals and other multicellular life date to between 700 and 800 million years ago.

"The oxygenation of the deep ocean and our interpretation of this as the result of a rise in atmospheric O₂ was a pretty late event in the context of Earth history," said Stolper in a statement. "This is significant because it provides new evidence that the origination of early animals, which required O₂ for their metabolisms, may have gone on in a world with an atmosphere that had relatively low oxygen levels compared to today."

Oxygen is highly reactive, so unless it's continuously released, it combines with rocks until the air is depleted. Consequently, the early atmosphere contained almost none of this crucial gas, limiting the energy options for early life forms.

When microbial life first started to release oxygen, this too reacted, rather than inducing any long-lasting increase in atmospheric levels. Around 2.5-2.3 billion years ago, however, photosynthetic organisms started releasing oxygen faster than it could react away, creating a substantial shift that we can read in the rocks of the era.

Nevertheless, atmospheric oxygen levels were still so low that effectively none reached the deep ocean. The timing of subsequent increases in oxygen concentrations (particularly the one that allowed functions such as fire) has been hard to establish, with considerable contradictory evidence emerging.

On land, oxygen at today's 21 percent of the atmosphere leaves a legacy that is hard to distinguish from something much lower. Stolper and Keller looked to the deep oceans, which models indicate only become oxygenated when atmospheric levels reach 3-10 percent.

They found that iron in rocks formed from submarine volcanic eruptions only started being oxidized 541-420 million years ago. Stolper considers this a more reliable measure of ocean oxidation than geochemical signatures used previously, leading the team to the conclusion that animals operated in a much less oxygen-rich environment than today's.