The Long Fuse To The Cambrian Explosion

Trilobites: one of the many heralds
of the Cambrian Explosion

The Cambrian Explosion is the reason for the diversity of life around us today. Had it not been for this single event, the oceans would be almost empty and the invasion of land may never have happened. The only visible life would have been immobile sacks of cells, the first primitive animals, and towering pillars of slime built by trillions of bacteria. Yet as the planet spun into the Phanerozoic eon, life diversified, giving rise to almost all of the existing animal phyla today.

One question baffled palaeontologists: why did the Cambrian Explosion not occur sooner? About 20 million years prior to the base of the Cambrian, there was an event which is known as the Avalon Explosion which produced those strange creatures known as the Ediacara biota. The transition between the two events was punctuated by a period of little evolutionary change, resulting in a delay between the origins of the fist animals and the Cambrian Explosion.

A discovery made in 2011 showed that the oceans which the Ediacarans inhabited were lacking in oxygen, yet that did not explain the exact conditions that early animals endured. Now a model has been created which gives an explanation for the long fuse to the Cambrian Explosion. The model, created in collaboration between the University of Leeds, Exeter and South Denmark, University College London and Plymouth Marine Laboratory has shown that the reason for the delay was due to the oceanic nitrogen cycle between 700 and 550 million years ago.

A simple version of the nitrogen cycle. 

Nitrogen is vital to life as it is needed to make proteins and DNA. However, it can only enter the global food chain through the action of bacteria. Upset either the nitrogen cycle or a food chain and it spells disaster for global biodiversity. This is what the research team hypothesized as to the cause of the long fuse. The exact details suggest that interruption was due to fluctuations between a hydrogen sulphide-rich and toxic ocean and an iron-rich, anoxic one.

The iron rich and anoxic oceans favoured anaerobically respiring bacteria. As they thrived, their metabolic processes produced hydrogen sulphide which built up in the water and sediment, creating an inhospitable environment for animals, stalling their evolutionary advance. Animals use nitrate from their food to build their bodies. Many bacteria on the other hand can extract nitrogen directly from the molecule itself or from more volatile compounds such as ammonia.

As a result, nitrate would have built up in the oceans during the hydrogen sulphide phases to the point that it was in abundance, leading to a sudden increase in the number of bacteria which used it. Nitrate-consuming bacteria can produce more energy than anaerobically respiring forms, and so would have out-competed other species of microorganisms. The upshot of this is that hydrogen sulphide-producing species would have been a minority in oceanic ecosystems, preventing the build-up of hydrogen sulphide.

Bacterial blooms in the oceans. Sites like this
would have dominated the Precambrian Earth

'Data from the modern ocean suggests that even in an oxygen-poor ocean, this apparent global-scale interchange between sulphidic and non-sulphidic conditions is difficult to achieve,' said Dr Richard Boyle from the University of Exeter. 'We've shown here how feedbacks arising from the fact that life uses nitrate as both a nutrient, and in respiration, controlled the interchange between two ocean states. For as long as sulphidic conditions remained frequent, Earth's oceans were inhospitable towards complex life.'

It was only due to the build-up of nitrate that complex life was able to advance further, leading to the Cambrian Explosion 520 million years ago. This was perhaps the most important event in the history of the planet. In hospitable worlds, life is a near inevitability. The Cambrian Explosion, however, is unique to the Earth. It is responsible for the world we see today. Yet understanding its mechanics is vital to our understanding of the plant and its history.