The Cambrian Substrate Revolution May Have Played A Role In Stabilising Oxygen Levels During The Explosion Of Life

A pantheon of the weird and wonderful from the Cambrian Explosion

The Cambrian Explosion is one of the most important events in evolutionary history, responsible for producing the diversity of animal phyla on Earth today. The effects have been the subject of intense study, but its causes are equally contentious. Some believe it was the result of new predators sparking an evolutionary arms race, others the development of an exoskeleton which enabled animals to diversify into previously unoccupied morphospace. Another theory is the increase in the levels of atmospheric oxygen led to the Cambrian Explosion. Increases in atmospheric oxygen throughout geological history have been associated with increases in species diversity and the Cambrian geological record does record an increase to coincide with this.

During the Precambrian, extensive algal mats covered the seabed, allowing anaerobically-respiring bacteria to thrive in the anoxic sediment, perpetuating its stagnant, toxic state. Then came the Cambrian substrate revolution. Organisms capable of feeding on these algal mats evolved. The mats subsequently declined in size and number, gradually revealing the sediment beneath, allowing oxygen to reach the previously anoxic ocean bed. Eventually animals began to burrow into the seabed itself thanks to the newly oxygenated conditions. Yet the Cambrian Substrate Revolution may also have stabilised the level of atmospheric oxygen by altering the geochemical interplay between the atmosphere and the seabed.

Oxygen levels rose during the Cambrian but they plateaued at 12.5%. Now researchers believe that the Cambrian substrate revolution may have been responsible for this. 'Our research is an attempt to place the spread of animal life in the context of wider biogeochemical cycles, and we conclude that animal activity had a decreasing impact on the global oxygen reservoir and introduced a stabilizing effect on the connection between the oxygen and phosphorus cycles,' said Dr. Richard Boyle from the Nordic Centre for Earth Evolution at the University of Southern Denmark. Their model is related to the interplay between oxygen, carbon and phosphate.

Burrowing trails from 530 million year old rocks:
bioturbation from the Cambrian Explosion

Carbon in the form of organic matter is buried in ocean sediment. This returns oxygen from the atmosphere. However, the rate of carbon burial is related to the amount of phosphate in the sediment, which in turn controls the rate of oxygen depletion. The researchers suggest that the evolution of burrowing organisms during the Cambrian substrate revolution oxygenated the deep ocean sediments.

The response of bacteria living in the sediment to the newly oxygenated conditions was to uptake phosphate into their cells, a behaviour seen in anaerobic bacteria today. In turn biological activity on the ocean bed would have decreased due to the lack of phosphate, a mineral required by all cells in order to live, causing a decrease in carbon burial and putting the breaks on the release of oxygen into the atmosphere.

Today the action of burrowing organisms, a process known as bioturbation, is an incredibly important part of the feedback cycles which keep the Earth's weather, climate and geochemistry ticking over. What this research shows is how animals may have revolutionised not just the biological world but also the planet itself. The Cambrian Explosion continues to surprise us, nearly 100 years after the fossil treasure trove from the Burgess Shale was discovered.