|Trilobites: one of the many heralds |
of the Cambrian Explosion
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.|
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
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.