End Of An Era: A Palaeontological Whodunnit?

A stromatolite colony. Colonies even larger than this 50 tonne
behemoth existed during the days of the slimeworld

For hundreds of millions of years the Earth was lifeless; spinning through space under siege from a meteorite bombardment, derived from the remnants of the nascent solar system. Then came the slimeworld.

For billions of years bacteria and archaea were masters of the planet, forming vast colonies, the stromatolites. Their rule was unchecked, unchallenged, across the Earth. These simple cells were joined between one and two billion years ago by complex cells, the eukaryotes, but still the stromatolites outnumbered the newcomers.

Yet their dominion was eventually challenged. 550 million years ago there evolved a group of organisms, the Ediacara biota, so named after the location in Australia where their fossils were first discovered. Biological machines built from complex cells, they were the first animals on the planet and many of them fed on the thick mats of bacteria covering the sea bed. Yet as soon as they appeared they quickly vanished: the result of an extinction event, the first to affect the animal kingdom. The causes of this extinction, however, have been the subject of some debate, yet a recent theory from researchers working at the University of Cambridge could resolve this.

'We know that rangeomorphs lived too deep in the ocean for them to get their energy through photosynthesis as plants do," said Dr Jennifer Hoyal Cuthill from Cambridge's Department of Earth Sciences. 'It's more likely that they absorbed nutrients directly from the sea water through the surface of their body. It would be difficult in the modern world for such large animals to survive only on dissolved nutrients.' Their ability to directly absorb nutrients from the water was due to the way they constructed their bodies. Ediacarans used what is known as fractal geometry, building up a large surface area from a series of self similar modules which branched off from one another, each branch smaller than the last. A system which is quick, effective and simple to program genetically.

Mathematical models of Ediacarans created  a computer using fractal geometry

In this low pressure environment, such body plans were ideal due to their simplicity and the abundance of organic matter dissolved in the water. 'The oceans during the Ediacaran period were more like a weak soup -- full of nutrients such as organic carbon, whereas today suspended food particles are swiftly harvested by a myriad of animals,' said Professor Simon Conway Morris. During the Cambrian Explosion when complex, highly motile grazers and predators evolved, the soup-like state of the ocean was replaced by the modern, less nutrient rich version. Out-competed by this new wave of animals, the Ediacara biota struggled to obtain food, leading to their decline.

The Cambrian predators would also have fed upon the Ediacarans themselves, increasing the environmental pressure. 'As the Cambrian began, these Ediacaran specialists could no longer survive, and nothing quite like them has been seen again,' said Dr Hoyal Cuthill. The reason for their initial success was their fractal body plan offered the optimum mathematical solution for filling physical space with an absorbent surface area. Yet when the terms of their environment changed during the Cambrian Explosion, a mathematical space filling model was unable to keep up with a world where complex body parts offered a myriad of solutions to the same problem of obtaining food, and in ways which were ultimately more effective. The Ediacarans began to disappear and by the end of the Cambrian period, had vanished entirely.