'Rotten-egg smell delayed the evolution of animals'

The smell of rotten eggs is off-putting at the best of times, but now scientists believe the same chemical responsible for the awful smell may have held back the evolution of life.

Oceans polluted by hydrogen sulphide could explain why it took so long for life to develop on Earth. Around two billion years ago, deeper layers of the oceans contained high levels of the rotten egg chemical and little oxygen. The smelly poison would have placed a big obstacle in the path of biological evolution, say scientists.

Study leader Dr Simon Poulton, from the University of Newcastle, said: "It has traditionally been assumed that the first rise in atmospheric oxygen eventually led to oxygenation of the deep ocean around 1.8 billion years ago.

"This assumption has been called into question over recent years, and here we show that the ocean remained oxygen-free but became rich in toxic hydrogen sulphide over an area that extended more than 100 kilometres from the continents.

"It took a second major rise in atmospheric oxygen, around 580 million years ago, to oxygenate the deep ocean. This has major implications as it would have potentially restricted the evolution of higher life forms that require oxygen, explaining why animals appear so suddenly relatively late in the geological record."

Oxygen began to fill the Earth's atmosphere for the first time between 2.4 billion and 1.8 billion years ago, rising from nothing to about 5 per cent of present levels. But scientists have not been clear about what happened to oxygen in the oceans.

Early life first emerged in the oceans almost four billion years ago but did not progress beyond the stage of primitive single cells for nearly three billion years. More complex multi-cellular life evolved just over a billion years ago. Around 500 million years ago the "Cambrian Explosion" saw the creation of most of the animal groups that exist today. Soon after, dinosaurs and mammals appeared.

The new findings, reported in the journal Nature Geoscience, are based on an analysis of sedimentary rock samples from a region around Lake Superior in North America. They showed that while the ocean surface 1.8 billion years ago was oxygenated, middle depths extending a large distance from the shoreline were sulphide-rich and starved of oxygen.

Dr Poulton said: "What we have done with this study is to provide the first detailed evaluation of changes in ocean chemistry with water depth in the global ocean at this critical time. Earth scientists will need to consider the consequences of this oceanic structure when trying to piece together the co-evolution of life and the environment."