In Neoproterozoic sedimentary sequences evidence for low latitude glaciation crops up at two and probably several other times; so-called ‘Snowball Earth’ events. Opinion is divided on several aspects of these events: whether or not they truly coated the Earth in glacial ice; their influence on biological evolution; the processes that started and terminated them. From a biological standpoint, a completely ice-bound surface – both land and oceans – would have stressed organisms to the extreme. Marine life (all that there was in those times) may only have survived in a few refuges from the ice, perhaps around submarine hydrothermal vents or in ephemeral sea-ice leads and polynya. If that were so, then these frigid episodes would have created important evolutionary ‘bottlenecks’, from which sprang several adaptive radiations: ‘Snowball’ epochs may have determined the forms and genetic diversity of all later life, especially among the Eucarya, of which we are a part. Probable deep-ocean anoxia would have been particularly stressful for organisms that depend on oxygen.
The key to establishing whether or not Neoproterozoic frigid episodes did bring eucaryan life to the verge of extinction lies in the diversity of life during those periods. That is not an easy task as all life until just before the Cambrian Explosion was both soft-bodied and minute. One means of assessing diversity is to study biochemical remnants of cell processes preserved in reduced ocean sediments (Olcott, A.N. et al. 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science, v. 310, p. 471-474). Olcott and colleagues studied black shales from Brazil whose age is within that of a frigid episode (740-700 Ma), and which contain textural evidence for abundant sea ice and low temperatures. Recovered biochemical compounds indicate considerable diversity, with a mixture of photosynthetic blue-green bacteria and eucaryan algae, with anaerobic bacteria of several types. The results indicate open water to allow photosynthesis – although it is possible for light to penetrate several metres of sea ice – together with deeper anoxic waters. Since the samples span a section almost 100 m thick, it seems this diversity persisted for a long period. However, the most that it can establish with certainty is that thin sea ice or open water did persist at the low palaeolatitude of late-Precambrian Brazil. The Neoproterozoic record has abundant, widespread black shales, and quite possibly there are others associated with evidence for glacial events. The importance of the paper lies in showing that biomarkers can be used as effectively in the Precambrian as in the Phanerozoic, and an expansion of this approach can be expected.
Oxygen and mammalian evolution
So much in the geological history of surface processes depends on either the dearth or the superabundance of oxygen. That is no surprise for a host of reasons, one being that it is the most reactive common element when free of bonds, and another is that the most powerful means of releasing oxygen is the capture of energetic solar photons by the pigments residing at the heart of photosynthesis. To grossly paraphrase James Lovelock, the principal reason for not sending people to Mars to search for life is that the planet’s atmosphere tells us that even if was there, it wouldn’t be very exciting. Oxygen gas is at vanishing low levels on the Red Planet, even if there is lots locked up in its iron-oxide rich surface.
The greatest event in the history of terrestrial life, apart from its emergence, was exploitation of the means of breaking hydrogen-oxygen bonding in water, which is what common photosynthesis is all about. It opened the entire planet to life from the restricted, though diverse habitats of most Bacteria and Archaea in the earlier anoxic world. First, oxygen-excreting cyanobacteria were able to colonise the entire ocean surface, depending on available nutrients. In doing so and generating free oxygen they threatened every other organism that used metabolisms based on other kinds of chemistry: oxygen is highly toxic because of it propensity to grab free electrons. Balanced by its oxidation of iron in early oceans, severe oxygen stress did not emerge until halfway through Earth’s history. Once it did become able to accumulate in air and water, all ecosystems faced havoc. Dominant prokaryotes slunk to rare places of refuge, while others seem to have combined in resisting oxidation. Their creation of the Eucarya that depend completely on available oxygen led, through the emergence of algae and then plants, to an accelerated stoking up of oxygen generation.
Once vegetation began to cloak the land, an extra 30% of the planet’s surface opened new vistas for animals and increased oxygen production and complementary burial of carbon. Indeed, explosive growth of atmospheric oxygen during the Carboniferous resulted in animal expansion to the air, through ominously huge insects. The first clearly traced ancestors of mammals seem to have appeared in the Permian, though their descendants only got the chance to dominate once reptiles, especially dinosaurians, lost their grip as a result of the K-T extinction. At the time of a far greater loss of living diversity, at the end of the Permian, it is now clear that in a relatively short time oxygen levels had fallen from their highest to one of the lowest in the Phanerozoic record (see New twist for end-Permian extinctions in the May 2005 issue of EPN).
Anoxic oceans were a regular feature of the Mesozoic and early Cenozoic. It is their preservation of abundant buried carbon that holds a key to, in an anthropocentric sense, the greatest of evolutionary leaps; the rise of large mammals and ourselves. A large team of US scientists has used the now abundant records of carbon isotopes in both buried organic matter and marine carbonates to reconstruct changes in atmospheric oxygen content (Falkowski, P.G. and 8 others 2005. The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science, v. 309, p. 2202-2204). Their modelling suggests that at the start of the Jurassic, atmospheric oxygen stood at around only 10%. Through that period it rose dramatically to 16%, fell equally abruptly and then rose again to about 18%, thereby creating the conditions for some of the largest sources of petroleum. Cretaceous times saw a slow rise, until around the time of the global warming at the Palaeocene-Eocene boundary (55 Ma). The middle of the Cenozoic was a further period of dramatic increase in oxygen levels, to their highest (~23% in the Oligocene) since the peak during the Carboniferous. Latterly atmospheric oxygen has waned to around 21% today.
Falkowski et al. compare their new atmospheric oxygen curve with evolutionary spurts among mammals, of which the simplest to understand is the parallel rise of mammalian average size. The metabolism of all mammals, like birds, has 3 to 6 times the oxygen demand of reptiles. Not only were Mesozoic mammals challenged in stature by the air they breathed, reptiles were easily able to grow to monstrous proportions because of their less demanding physiological processes. The first signs of the placental nurturing of mammalian foetuses, which requires a high oxygen level, coincides roughly with the Mesozoic maximum (100-65 Ma). The end-Cretaceous extinction of the dominant dinosaurian reptiles removed the main competition against the subtle advantages of placental mammals, and was followed by further increase in oxygen. The Cenozoic permitted terrestrial mammals to reach sizes almost comparable with dinosaurs, and to go beyond them among whales. Moreover, it saw explosive diversification, one branch of which, the primates, leads to ourselves.