Did snowball Earth's melting let oxygen fuel life?

We may owe our green Earth to a big freeze that covered the entire planet in thick sheets of ice 2.3 billion years ago, researchers say.
As this “snowball” Earth thawed, the new theory goes, it released strong oxidants into the oceans and atmosphere for the first time, setting off the chain of events that led to oxygen-tolerant marine organisms and photosynthesis as we know it today.
The evolution of efficient, oxygen-based photosynthesis has been hard to explain. Primitive life forms garnered energy from sunlight, using it to free electrons from sulphur and iron in an oxygen-free environment.
Oxygenic photosynthesis, which involves freeing electrons from water to produce oxygen, requires more energy. But oxygen was deadly to most primitive life on Earth. “The first organisms to do this would die,” says Hyman Hartman at MIT in Boston, US.
Glacial surfaces
So how did organisms evolve oxygen tolerance? Hartman and colleagues decided to focus on the role of hydrogen peroxide (H2O2). Ultraviolet light from the Sun produces H2O2 when it hits water molecules.
Today, sunlight destroys the peroxide as it forms. However, if UV light penetrated the surface of a glacier, small amounts of peroxide would have been trapped in the glacial ice for long periods, they say.
Indeed, H2O2 has been spotted on Jupiter’s icy moon Europa. The surface of a terrestrial ice sheet would have been very similar on a primitive Earth that lacked an oxygen-rich atmosphere and a protective ozone layer.
Primitive organisms
A thaw would have dissolved the peroxide into the oceans and atmosphere, releasing oxygen gradually, the researchers calculate. The water would have diluted the harmful oxidants enough for primitive organisms to survive. These organisms would have then gone on to evolve the enzymes that could protect them from oxidants, the team says.
Once they had those enzymes, cells could evolve oxygen-based photosynthesis, which is more efficient than its anaerobic form. Such organisms would then have generated more oxygen for the atmosphere.
But Jim Kasting of Penn State University at University Park, US, a specialist in the origin and evolution of Earth’s atmosphere, is not convinced by this scenario. One key sticking point for him is the presence of sterols in 2.7 billion year old rocks – these compounds are only made by living cells in the presence of oxygen.
If the sterol results are right, and that is by no means certain, then these rocks pre-date the snowball Earth by 400 million years, and Hartman’s theory is wrong.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0608839103)