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.
But a new study by Daniel Mills of the University of Southern Denmark in Odense suggests this may not be the case. By studying modern breadcrumb sponges, Mills has threatened this assumption, while simultaneously strengthening another.
Sea sponges may not seem animal-like, but they are among the planet's earliest animals. They're always multicellular and they grow from an embryo. They've also got complex physiological structures, including a network of channels that help draw food and water through their bodies. And based on the paleontological evidence, modern sea sponges aren't too far removed from their ancient brethren.
For the study, Mills took sponges (Halichondria panicea) from an oxygenated fjord in Denmark. Then, after placing them in an aquarium, his team slowly removed the oxygen. Remarkably, the sponges were able to survive even with 200 times less oxygen that's currently found in our atmosphere. These sponges lived on just 0.5% of the oxygen available today. So, because oxygen makes up about 20% of our atmosphere, that suggests ancient sponges needed oxygen concentrations of just 0.1%.
This led the researchers to conclude that primitive animals likely didn't require a whole lot of oxygen. Consequently, O2 levels may not have been the limiting factor that delayed the rise of animal life.
So, if it wasn't an oxygen starved Earth that stalled the evolution of complex life, what did?
Remarkably, this new theory strengthens the idea that sea sponges may have played a crucial role in what was to follow. Early oceans may have been oxygen-poor because they were full of dead microbial matter, which sucks up oxygen as it decomposes. But sponges, who feed on this dead matter, may have cleared the water of it, allowing O2 levels to rise.
Alternatively, and in the words of Mills, "There must have been other ecological and evolutionary mechanisms at play. 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."
Read the entire study at PNAS: "The oxygen requirements of the earliest animals."