Evolutionary theory generally suggests that for life forms of higher complexity to evolve, both the atmosphere and the ocean had to develop higher oxygen levels over time, and that the higher levels in the atmosphere eventually led to the oxygenation of the ocean without assistance from the earliest animals. This theory is being partially challenged by the work of researchers from the University of Exeter and the University of Leeds, in a study that was recently published in Nature Geoscience.
Their premise is that the first animals were not passive bystanders benefitting from the collective rise of oxygen, but that instead they were actively involved in creating the oxygenated environment that allowed evolution of higher species.
Current theories presume that oxygenation of the surface waters of the ocean took place more than 1.5 billion years before the first appearance of animals, but the lower levels of the ocean were basically oxygen-free until a relatively recent 600 million to 1 billion years ago.
Building on a Danish study on sponges that determined that sponges need very little oxygen to survive, the team considered the effect of sponges and similar life forms of the early Earth.
Oxygen levels in deeper parts of the ocean are decreased by the breakdown process of dead organic matter, which sinks to the bottom of the ocean. The typical degradation processes from this organic matter involve the consumption of oxygen.
Sponges in particular have an oxygenation effect through their feeding mechanisms-water is pumped through their bodies, bits of organic matter are filtered out, and the overall oxygen demand caused by the organic material is reduced. Since the larger phytoplankton sink faster, they are disproportionately removed, increasing the overall effect.
The sponges effectively act as a filter, particularly affecting the amount of the phosphorous, which is one of the major nutrients promoting oxygen-consuming activities. The overall efficiency of oxygen consumption in the ocean is then reduced, gradually allowing oxygenation at all levels of the ocean.
With greater oxygen content available to them, more complex creatures were able to evolve and thrive - thus according to this theory, the early animals of the Earth facilitated the evolution of the more complex animals.
This theory also allows for the oxygenation of the deep ocean without a corresponding rise in atmospheric oxygen levels. Perhaps our understanding of the timing is not correct, and there was no large-scale increase in atmospheric oxygen correlating to the effect on the oceans? Without a time machine, we must depend on geologists and geochemists to find other clever methods of estimating the oxygen patterns in the early years of Earth.