A long-held theory about why it took so long for animals to evolve has been called into question by a new study on sea sponges, some of the most primitive animals alive today.

The first microbes evolved around 3.6 billion years ago, but complex life involving many cells that cooperate for the good of a single organism didn't show up for around another 3 billion years.

That was also about the time that levels of oxygen in the atmosphere rose dramatically toward present day concentrations of about 20 per cent. And many scientists thought that animals needed the higher oxygen levels to survive and evolve.

However, a new study shows that some of the most primitive living complex animals, sponges — likely among the earliest animals to evolve — can survive with almost no oxygen.

Sponges may not look much like the animals familiar to most of us, but they are considered animals. That's because they are always multicellular and grow from an embryo. 

They have a somewhat organized structure, including a network of channels that help draw food and water through their bodies. They also have cells specialized for different functions, such as eating, that work together, said CarriAyne Jones, a postdoctoral researcher at the University of British Columbia who helped carry out the study when she was at the Nordic Centre for Earth Evolution at the University of Southern Denmark.

Jones and her colleagues found that sponges raised in the lab could survive on 0.5 per cent of the oxygen in the atmosphere today. Since oxygen makes up roughly 20 per cent of our atmosphere, that means the sponges needed oxygen concentrations of just 0.1 per cent.

"What we're showing is that actually, basal [primitive] animals don't need that much oxygen," Jones said in a phone interview.

She added that the results, published Monday in the Proceedings of the National Academy of Sciences, suggest that oxygen levels might not have been the limiting factor that delayed the rise of animal life.

"We have to look for some other reason why they waited until 800 million years ago to evolve."

Daniel Mills, a Ph.D. student at the University of Southern Denmark and the lead author of the paper, said in a video interview that while the idea of oxygen triggering the rise of animals was popular, "it was never really clear how much oxygen early animals needed."

Accustomed to normal oxygen levels

He decided to test that with sponges collected in the sea nearby, where oxygen levels are normal for the present day.

He, Jones and their colleagues grew the sponges in tanks with low oxygen, while monitoring their health and behaviour.

Jones said she was surprised by how well the sponges did at really low oxygen levels.

"I expected them to maybe not feed as much or respire as well," she said. "They did just fine. They grew. They respired. They fed."

Occasionally, the oxygen in the tanks would drop to zero. At that point, some sponges survived, but others died, showing they do need some oxygen to survive.

But so long as the levels stayed above 0.5 per cent of present day levels, the sponges thrived.

Jones said some sponges do live in low-oxygen environments today, such as Vancouver Island's Saanich Inlet in B.C.

But the fact that a sponge from a normal oxygen environment was fine at low oxygen suggests "this is just sort of inherent in their physiology."

Mills said in a Skype interview that being able to survive on low oxygen may be the result of how sponges' bodies are organized, and in fact theoretical calculations have suggested that a sponge body type wouldn't need much oxygen.

Still, it is also possible that the ability to survive in low-oxygen environments may be something that evolved more recently in sponges, he acknowledged. But if it is something found in most or all sponges, it is more likely to be an ancestral trait, so Mills plans to test other kinds of sponges soon.

In the meantime, he said the results suggest that animals didn't evolve until long after there was more than enough oxygen for them.

"There's probably something else in the environment or these ecosystems that wasn't supportive of the existence of animals" — possibly a missing nutrient or the absence of the right pool of genes, he said. "So now we have to start looking for other things."