When Life Figured Out How To Use Oxygen.

• Position: GREAT OXYGENATION EVENT

Around 2,500,000,000 years ago, our planet experienced what possibly was the greatest change in its history: according to the geological record, molecular oxygen suddenly went from nonexistent to becoming freely available everywhere. Evidence for what is called the Great Oxygenation Event (GOE) is clearly visible--for instance, in banded iron formations containing oxidized iron.

The GOE is, of course, what allowed oxygen-using organisms--including, eventually, us--to evolve, but was it indeed a "great event" in the sense that the change was radical and sudden, or were organisms that were alive at the time already using free oxygen, just at lower levels?

Dan Tawfik, professor in the Department of Biomolecular Sciences at the Weizmann Institute of Science, Rehovot, Israel, explains that the dating of the GOE is indisputable, as is the fact that the molecular oxygen was produced by photosynthetic microorganisms. Chemically speaking, energy taken from light split water into protons (hydrogen ions) and oxygen. The electrons produced in this process were used to form energy-storing compounds (sugars), and the oxygen, a by-product, initially was released into the surroundings.

The question that has not been resolved, however, is: Did the production of oxygen coincide with the GOE, or did living organisms have access to it earlier?

One side of this debate states that molecular oxygen would not have been available before the GOE, as the chemistry of the atmosphere and oceans prior to that time would have ensured that any oxygen released by photosynthesis immediately would have reacted chemically.

A second side of the debate, however, suggests that some of the oxygen produced by the photosynthetic microorganisms may have remained free long enough for nonphotosynthetic organisms to snap it up for their own use, even before the GOE.

Several conjectures in between these two have proposed "oases," or short-lived "waves," of atmospheric oxygenation.

Jagoda Jablonska, a doctoral student in Prof. Tawfik's group, thought that the lab's focus--protein evolution--could help resolve the issue. That is, using methods of tracing how and when various proteins have evolved, she and Tawfik might find out when living organisms began to process oxygen. Such phylogenetic trees are widely used to unravel the history of species, such as human families, but also of protein families--and Jablonska decided to use a similar approach to unearth the evolution of...