Iron In the Early Oceans Accelerated The Evolution Of Life

Iron is biologically important. It is essential in many of the molecular engines in our cells that allow us to live. We know that iron existed in the oceans 2.4 billion years ago because of rocks called banded iron formations. These also show that oxygen appeared in the atmosphere 2.4 billion years ago. However a recent study shows that iron could have been present in the oceans for at least a billion years prior to this event.

A particularly colourful banded iron formation

It is possible that this helped give rise to early life and evolution until free oxygen appeared in the atmosphere. Classic models of the evolution of the early oceans suggested that, when free oxygen appeared in the atmosphere, it would have had a 'rusting' effect, stripping the oceans of the all important ferrous nutrient. The iron oxide compounds would have been laid down in banded iron formations. Another theory gained tractions recently.

Hydrogen sulphide produced by anaerobically respiring bacteria would have reacted with the iron in the oceans. All the iron would  have been 'scrubbed' from the waters, depriving life forms of this essential element, causing a biological catastrophe. Both events would have occurred around 1.8 billion years ago. Now both these prominent theories are being challenged by new research.


'The problem all along was a general lack of physical evidence in the oceans for the amounts of oxygen, iron, and sulfide in the Earth's middle history, particularly in a critical billion-year window between roughly 1.8 and 0.8 billion years ago,' said Noah Planavsky, a doctoral student in UC Riverside's Department of Earth Sciences and the lead author of the new study.'Some earlier work supported a return to an iron-rich ocean 0.8 billion years ago. Rather than a return, however, we predicted that iron may have dominated the deep ocean continuously right up to the oxygenation and concomitant rise of animals a mere half-billion years ago.' 

Snails dwelling on hydrothermal vents have made use of the
ejected minerals, in particular iron, to build a shell and armour
plated foot covered in solid metal

Planavsky and his team sampled rocks of different ages from  across the globe to try and fill this data gap. Their results were surprising. Their research pointed to a scenario where the oceans were rich in iron and poor in oxygen for 90 percent of the Earth's history in the upper layers of the ocean and along the continent shores respectively. Planavsky came to the conclusion that neither oxygen or hydrogen sulphide restricted iron levels in the oceans from 1.8 billion years ago. Instead, hydrothermal vents in the deep oceans would have been responsible for the distribution of elements and compounds, in particular iron. 


The team's new task was to work out whether hydrogen sulphide or other such compounds were the throttle that held back life for over a billion years. The oceans, while rich in iron and low in oxygen that would have poisoned the anaerobic bacteria, would still have been a harsh environment. Timothy Lyons, a biogeochemist from the University of California states that hydrogen sulphide could have severely restricted evolution in shallow waters by blocking nutrients.