Sea sponges can survive on astonishingly small amounts of oxygen. The discovery raises questions as to whether the first complex lifeforms could do the same, and if so why they didn't evolve earlier.

Life appeared on Earth within a few hundred million years, but for billions of years it was restricted to single celled organisms. When complex life forms emerged the timing was shortly (geologically speaking) after the dramatic rise in oxygen levels in the atmosphere (shortly followed by the oceans) 630-635 million years ago. The conclusion was obvious: the rise in oxygen was necessary to fuel the complex organisms that eventually became us.

The connection has been challenged before. Last year researchers at the University of Southern Denmark produced evidence that 2.1 billion years ago atmospheric oxygen was similar to that when the complex life forms appeared, raising questions as to why, if so, we can't find evidence of advanced life forms from that time.

Now Daniel Mills, a PhD student at the same university has gone further. He tested how little oxygen was needed by sea sponges, considered some of the closest surviving relatives of the first multicellular creatures. There have been reports of sponges living in low oxygen waters near under sea volcanoes, but Mills didn't need to go to such exotic locations – he plucked Halichondria panicea from Kerteminde Fjord, just meters from the university's Biological Research Center and raised them in seawater from the fjord with both normal and drastically reduced oxygen levels.

“The sponges were able to both respire and feed to oxygen levels cycling between 0.5% and 4% PAL [Present Atmospheric Levels],” Mills reports in Proceedings of the National Academy of Sciences . In one experiment a sponge survived and even appeared to grow for 24 days in such conditions, although four hours of complete oxygen loss caused cell death in another sponge.

“Therefore, it is possible that the oxygen content of the atmosphere was completely permissive to the origin and early evolution of sponge-grade metazoans well before their evolutionary first appearance,” the paper notes. If this is the case, the big question is why complex lifeforms didn't appear earlier – how much more evolved might we be with an extra few hundred million years headstart?

Mills has yet to test the possibility that sponges need more oxygen to make it out of their larval stage. However, should the sponges survive this challenge attention will shift to questions of what else could have stopped complex life appearing earlier. The paper speculates that greater turbidity in the early oceans meant conditions in shallow waters often became completely oxygen depleted. Alternatively, Mills suggests,“Maybe life remained microbial for so long because it took a while to develop the biological machinery required to construct an animal. Perhaps the ancient Earth lacked animals because complex, many-celled bodies are simply hard to evolve.”