Animals Contributed To The Oxygen Spike 700 Million Years Ago

The evolution of nitrogen-fixing genes in cyanobacteria removed constraints on growth and so injected vast quantities of oxygen into the atmosphere as a by-product of their metabolic pathways. Understanding this great leap forward is important. Yet new evidence shows that the causes are more complex than a simply cyanobacterial proliferation: animals themselves may have contributed to the oxygenation of Earth's environments, specifically the oceans.

The remains of sponges contained within Neoproterozoic
(one billion to 542 million years ago) rocks

A team of researchers from Denmark had already shown that sponges can exist in environments with an almost negligible concentration of the gas; while experiments and fossil evidence back this up.

The idea that sponges could have evolved in a previously oxygen-deficient ocean, allowed researchers from the University of Exeter to create their own theory as to the origin of complex, multi-cellular animals.

'There had been enough oxygen in ocean surface waters for over 1.5 billion years before the first animals evolved, but the dark depths of the ocean remained devoid of oxygen,' said Professor Tim Lenton. 'We argue that the evolution of the first animals could have played a key role in the widespread oxygenation of the deep oceans. This in turn may have facilitated the evolution of more complex, mobile animals.' The balance between the supply and demand of oxygen is an important aspect of oceanic chemistry. One factor which affects concentration is the size of populations of aerobically respiring micro-organisms in the water. The smaller the population, the lower the demand.

Controls on population will ultimately impact on oxygen concentration at different depths in the ocean. Lenton's team suggests that the filter-feeding activities of the first sponges removed smaller and more numerous species of plankton from the continental shelves. This left behind larger specimens which sank into the deeper layers of the ocean, due to the greater weight, reducing the oxygen demand and consumption.

Sponges, the simplest form of animal, which
feed by filtering nutrients from sea water

Cyanobacteria would have increased the level of oxygen in the waters where the sponges lived. As a result large, complex animals with the ability to move and hunt, requiring more oxygen to survive, would have evolved, together with the formation of environments capable of sustaining them.

'The effects we predict suggest that the first animals, far from being a passive response to rising atmospheric oxygen, were the active agents that oxygenated the ocean around 600 million years ago,' Lenton concluded.

Ecosystems are naturally complex. Even the tiniest changes are amplified by the links between organisms. On a human time-scale they manifest as a decline or an increase in populations, and species diversity. Yet over geological time, its evolutionary effects are massive. It is incredible to think that the appearance of tiny bags of filter-feeding cells could have changed the geochemistry of the Earth, resulting in the evolution of the first complex animals and life's rich diversity.