There's a longstanding theory which says oxygen-rich oceans were a key requirement for complex life to emerge on Earth. But a new study involving sea sponges upsets this notion, showing that primitive animals may have been able to survive with hardly any oxygen at all.
The first microbes appeared on Earth about 3.6 billion years ago, but it took an exceptionally long time for complex multicellular life to emerge — another three billion years. Perhaps not coincidentally, this also happened to be the time when levels of oxygen in the atmosphere escalated to present day concentrations of about 20%. Many scientists have thus concluded that animals needed the higher levels to survive, thrive, and evolve.