The emergence of oxygen in the Earth’s atmosphere and in its oceans bears directly on the enigma of how complex life arose. Of particular interest to scientists is the disparity between atmospheric oxygen and oceanic oxygen levels. Several models assume that during the Proterozoic Eon (2.5 billion to 542 million years ago), the concentration of oxygen in the ocean was substantially lower than atmospheric oxygen levels.
Complex life arose and diversified approximately 542 million years ago in the Cambrian period — the subject, of course, of Stephen C. Meyer’s forthcoming book Darwin’s Doubt (available this month for pre-order, including free shipping plus 4 free digital books). Less complex, but still eukaryotic (referring to organisms with cells that have a nucleus and other organelles), red algae can be dated to 1.2 billion years ago. There is some evidence from compounds found in Australian shale that eukaryotes may even be as old as 2.7 billion years, although this early date is still debated. However multi-cellular eukaryotes, such as plants and animals, are generally dated back to the 500 million year mark, which is also about the time that oxygen levels in the ocean increased to essentially what they are today. This is contrasted with atmospheric oxygen levels which started increasing around the time of the early eukaryotes, some 1 billion years before, if not earlier.

There are differing views on the evolution of eukaryotes. One view assumes that increasing oxygen levels drove the evolution of mitochondria, which lead to multi-cellular, complex organisms. However, most origin of life scenarios have evolution occurring in the ocean, and the late date for increasing oceanic oxygen levels thus poses a problem. Others believe that examples of organisms with mitochondria that do not use oxygen provide evidence for an earlier emergence of eukaryotic life. (See Mental and Martin’s article “Energy metabolism among eukaryotic anaerobes in light of Proterozoic ocean chemistry” for some background.)

Scientists are interested in whether the dearth of oxygen in the ocean compared to atmospheric oxygen had an impact on the formation of life. Also of particular interest is whether the anoxic (or “no oxygen”) oceans contained hydrogen sulfide (H₂S), a poisonous compound for organisms that breathe oxygen.

One way to determine what the oxygen and hydrogen sulfide concentrations were in the early ocean is to look at the abundance of certain metals. Transition metals have an interesting chemistry: under certain conditions they will “oxidize” and under other conditions they will “reduce.” We see this process in action whenever a metal object is left outside and rusts. Rust is just the product of iron metal oxidizing into iron oxide. Importantly, some compounds cause metals to oxidize, like oxygen, and other compounds cause metals to reduce, like H₂S.

A group of biogeochemists at UC Riverside investigated oxygen and hydrogen sulfide levels in the deep ocean by looking at sedimentary rock deposits of two metals, molybdenum (Mo) and chromium (Cr). Mo is reduced under anoxic conditions when hydrogen sulfide is present, and will react with sulfur. The team looked at shale with Mo deposits known to have been enriched from the anoxic/sulfide conditions, and compared this to other Mo deposits from marine shell. The relative abundance of this reduced Mo indicates that the oceans did not have widespread hydrogen sulfide concentrations, contrary to other hypotheses that assume the concentration of H₂S was more abundant during this time.

Additionally, Cr is reduced under anoxic conditions, without hydrogen sulfide being present. The authors reason that by “comparing Mo enrichments in independently constrained euxinic [i.e. no oxygen, contains H₂S] shales and Cr enrichments in independently constrained anoxic shales can offer a unique and complementary perspective on the global redox landscape of the ocean.”

Based on extensive studies of Mo and Cr concentrations dating back to the Proterozoic, the authors found that the Proterozoic ocean was characterized by very low oxygen levels, and while there was some hydrogen sulfide present, it was in much lower levels than was once thought. (See this Nature article for a slightly different take on the level of hydrogen sulfide in the Proteozoic. Also see this Royal Society article.) Their models predict that about 1-10% of the ocean seafloor was both anoxic and contained hydrogen sulfide, compared to 0.1% today. This concentration may still have had an effect on biological life, but it is not as widespread as was thought.

In considering origin of life scenarios, the presence of atmospheric oxygen may very well be dated as far back as 4 billion years ago, given studies with another metal, cerium (ENV reported on it here), and photosynthetic life may date back to as early as 2.7 billion years ago (late Achaean). However, if the anoxic ocean theory is right, then oceanic oxygen levels were very low until about 500 million years ago. Meanwhile the abundance of aerobically poisonous hydrogen sulfide is the subject of much debate in the field.

We thus have four issues that appear at variance with each other: 1) the different timing of anoxic ocean levels and atmospheric oxygen levels, 2) the assumption from chemical origins theories that life began in water, 3) the timing for prokaryotic emergence followed by the various theories on eukaryotic emergence (e.g. endosymbiosis), and 4) the evidence for when photosynthetic life appeared. These four ideas do not seem to add up because eukaryotes emerged before the emergence of oceanic oxygen levels rose, but origin of life theories assume that life started in the ocean.

The evolutionary story, in other words, is being imposed forcibly on the data. This is a pattern we’ve observed before.