Not long for the first prediction
I discussed in a couple of blogs that the Cambrian radiation would be the focus of much ongoing research in the coming year. I didn't have to wait very long! In today's issue of the journal science Canfield et al. explore the oxygenation of the deep oceans during the Neoproterozoic and its relationship with the Cambrian Explosion. The scientists looked at stable isotope variations of iron in sediments near the Precambrian/Cambrian marker horizon in NewFoundland. The idea here is to examine the ratios of reactive iron (iron oxides, carbonate and sulfides) with non-reactive iron (the rest). During the Phanerozoic (the last 542 Million years) the ratio of reactive iron/total iron is 0.15 and the modern value is 0.26. In oxygen rich environments, the ratio is driven to the low side and in anoxic environments, the ratio may reach 0.38 or higher. These high ratios were seen in the record prior to the appearance of Ediacaran fauna in Newfoundland and during the Gaskiers glaciation. In contrast, the ratios following the Gaskiers showed a drop to near modern values.
The authors also looked at the amount of organic carbon in the sediments and the ratio of sulfur isotopes. These both confirmed their conclusion that the interval following the Gaskiers glaciation resulted in deep water oxygenation of sufficient magnitude to allow for the rise of organisms needing oxygen for the metabolism of larger eukaryotes.
The question, of course, is whether or not this oxygenation should be viewed as the 'sole' trigger for the Ediacaran and Cambrian explosions or whether it was one of many potential favorable events that lead to the arrival of complex metazoan life.
In a paper that was published yesterday in Nature, Acquista et al. showed that transmembrane proteins originally excluded oxygen in ancient organisms but this exclusion decreased as oxygen levels rose. In addition they found that oxygen affected the timing of evolution of cellular compartmentalization (a necessary requirement for complex life forms.