Breathing Life into the Ocean
How the simplest life forms may have given rise to legions of marine animals
Our current model for Earth’s evolution goes something like this: In the beginning, Earth was a collection of granules. As larger collections coalesced, planetesimals formed. As they collided with each other, heating occurred. Gases, including water vapor, were released with each impact. Thus, as the Earth grew, or accreted, so did its oceans. Then, somehow, the oceans became full of oxygen: enough to support vast growth and differentiation of countless animal species. The traditional view has been that a rise in atmospheric oxygen somehow led to a rise in ocean oxygen, and thus the modern marine biosphere. There was a gap in the model, however. Oxygen first arose in the atmosphere 2.4 billion years ago, but according the geological record, the deep ocean was oxygen-free until around 600 million years ago.
Researchers at the University of Exeter in England challenged the notion that a well-oxygenated atmosphere must pre-date a well-oxygenated ocean. Simultaneously, their theory explains the sudden rise of deep-ocean oxygen and the explosion of ocean life in the Cambrian era.
Drs. Tim Lenton and Simon Poulton suggested that the ocean gave rise to the majority of its own oxygen supply somewhere between one billion and 500 million years ago. “Shallow shelf waters are believed to have become oxygenated much earlier in Earth’s history,” said Poulton, “it turns out that simple benthic filter feeders such as sponges may not require particularly high levels of oxygen, and so these organisms may have evolved into shallower water environments. ”
In their paper in Nature Geoscience, Poulton and Lenton suggest that simple life forms like sponges, living in shallow water, aided in the rapid oxygenation of deep oceans by participating in a positive feedback process that removes organic material containing phosphorus from shallow water.
Normally, tiny organic materials sink through the water column and settle on the ocean floor. There, the fallen organics consume oxygen in the process of decaying. A similar system exists in many modern lakes and oceans, where an ensuing lack of oxygen kills off marine life.
In the primordial ocean, removal of organics and phosphorus by sponges would have resulted in spare oxygen available for use by other evolving species.
“[Sponges] trigger removal of phosphorus, which reduces productivity and hence oxygen demand in deep waters,” said Poulton said. “The idea is that more complex life required higher oxygen levels, so once you start to increase oxygen through the processes outlined above, then more complex life can evolve.”
More complex life did evolve, and rapidly, during what is known as the Cambrian explosion around 543 million years ago. “More complex animal life is encouraged by more oxygen being available in the water, because once animals start to move around and eat each other this requires more oxygen,” said Lenton, “but the first simple animals such as sponges don’t need nearly as much.” This hypothesis suggests that in a substantial way the world as we know it today may have started with a sponge.
If Lenton and Poulton are correct, then similar processes may have taken place in low-oxygen oceans on other planetary bodies, like Europa. In this way, simple organisms can quite literally clear a path for higher life forms to evolve.