Contrary to the long-held belief that a rise in oxygen triggered complex life forms on earth, a new discovery claims the evolution of the first animals may have oxygenated atmosphere and oceans.
The research builds on the recent work of scientists in Denmark who found that sponges - the first animals to evolve - require only small amounts of oxygen.
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.
"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," said professor Tim Lenton from University of Exeter in Britain.
The researchers considered mechanisms by which the deep ocean could have been oxygenated during the Neo-proterozoic Era (from 1,000 to 542 million years ago) without requiring an increase in atmospheric oxygen.
Crucial to determining oxygen levels in the deep ocean is the balance of oxygen supply and demand.
Demand for oxygen is created by the sinking of dead organic material into the deep ocean.
The new study argues that the first animals reduced this supply of organic matter - both directly and indirectly.
Sponges feed by pumping water through their bodies, filtering out tiny particles of organic matter from the water, and thus helping oxygenate the shelf seas that they live in.
This naturally selects for larger phytoplankton - the tiny plants of the ocean - which sink faster, also reducing oxygen demand in the water.
By oxygenating more of the bottom waters of shelf seas, the first filter-feeding animals inadvertently increased the removal of the essential nutrient phosphorus in the ocean.
This, in turn, reduced the productivity of the whole ocean ecosystem, suppressing oxygen demand and thus oxygenating the deep ocean, the research showed.
The more oxygen-rich ocean created ideal conditions for more mobile animals to evolve, because they have a higher requirement for oxygen.
"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. They created a world in which more complex animals could evolve, including our very distant ancestors," explained professor Lenton.
This study, published in the leading journal Nature Geoscience, provides a plausible mechanism for ocean oxygenation without the requirement for a rise in atmospheric oxygen.
