Earth's Ancient Toxic Ocean May Have Delayed Evolution

Hundreds of millions of years ago, the Earth's oceans were a toxic soup. Oxygen-poor waters flipped between an iron-rich state and a toxic hydrogen-sulphide-rich state. Now, new research shows that these inhospitable, ancient seas probably delayed the spread of complex life forms and, in consequence, evolution.

The study, published online in the journal Nature Communications, examined data from ancient rocks in order to find out exactly what Earth's ancient oceans were like. Researchers then developed a model in order to examine the sensitivity of the early oceans to the global nitrogen cycle. In particular, they examined the availability of nitrate, and the feedbacks within the global nitrogen cycle.

The rocks indicated that the deep oceans of our young planet contained little oxygen, and that they were mainly inhabited by bacteria. The bacteria needed to survive in low nitrate conditions, though the study itself shows how the bacteria using nitrate in their metabolism would have displaced the less energetically efficient bacteria that produce sulphide. As the one form of bacteria displaced the other, the presence of nitrate in the oceans prevented the build-up of the toxic sulphide state, causing the ocean waters to flip to another state.

The model that the researchers constructed revealed that the global nitrogen cycle would have had a vast impact on the Earth's oceans. It could have controlled the shifting of the oceans between the two oxygen-free states, and could have potentially restricted the spread of early complex life.

Richard Boyle, one of the researchers, said in a press release, "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. 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."

So how come life exists today? An abundance of nitrate and an oxygenated ocean prevents the ocean from reversing to a form which inhibited early life. The research could help scientists understand better how modern oceans interact with life, and how a changing composition--such as what may occur during ocean acidification during global warming--could impact species.