If your only association of sponges is our animated friend who lives in a pineapple, you might be a little disappointed with the real thing. Sponges outside of the cartoon world aren’t terribly exciting at first glance. No eyes, no brains, and certainly no pants, square or otherwise – in fact, sponges aren’t much more than towers of filtering organs peacefully pulling nutrients out of the water. Despite their simplicity, these creatures may have played a larger role in the evolution of modern life than anyone would have assumed. In the latest edition of Nature Geoscience, Tim Lenton of the University of Exeter argues that it was these supposedly lowly life forms that allowed the ocean to become oxygenated to the point that it could support a much wider variety of life, including the life forms that would eventually make their way to land.
The logic of this theory lies in the sponge’s feeding process. When sponges first emerged, much of the ocean’s oxygen was being consumed by rotting microbial matter. Luckily, sponges evolved to eat this microbial matter, thus decreasing how much of it was floating around in the water. Lower levels of rotting microbial matter would have meant higher levels of oxygen. These higher levels of oxygen allowed for more complex forms of life to arise, such as the more mobile and active species we associate with sea life today such as crustaceans, fish, and yes, Patrick the Starfish.
What we know about the ocean’s oxygen content over time seems to support this theory. We know that between 700 and 600 million years ago, the ocean became more oxygenated. We also know that it was at about 700 million years ago that sponges came onto the scene. It appears that these super-simple animals could have been responsible for this shift in the ocean’s composition.
Filtering out drifting organic matter, sponges are essentially the ocean’s living pool filters. (Missouri Botanical Gardens)
Lenton says that “the effects we predict suggest that the first animals, far from being a passive response to rising atmospheric oxygen, were the active agents that oxygenated the ocean around 600 million years ago … they created a world in which more complex animals could evolve, including our very distant ancestors.”