Two papers have appeared documenting an association between the end of the late Proterozoic glacial deposits, a rise in oxygen levels in oceanic waters and the appearance of multicellular animals (the Ediacaran fauna). Associations are undoubtedly interesting and they trigger hypotheses about cause and effect. This is apparent in the various commentaries:
There is a tendency to for evolutionists to say that if conditions are right, the evolution of life is inevitable. So, given a prebiotic soup in a reducing chemical environment, single cells will form. (Judging by the latest headlines about water on Mars: given just water, life will form!). Having started with single-celled life in the Precambrian, the suggestion is now being made that as oxygen levels rose, new selection forces were unleashed to evolve multicellular organisms. This hypothesis appears to be thrown out without offering any supporting reasons. This illustrates one of the major problems we have with evolutionary theory: a tendency to talk in terms of physics and chemistry to the neglect of information. Without addressing the origin of complex biological information, a hypothesis like this is an evolutionary veneer on some very interesting observations.
Editor of Nature: â€œan increase in oxygen in the deep ocean [â€¦] appears to have been associated with the evolution of complex animals.â€
Fike et al (in Nature): â€œ [â€¦] indicating that this event may have had a key role in the evolution of eukaryotic organisms.â€
Kerr (in Science): "A Shot of Oxygen to Unleash the Evolution of Animals"
Canfield et al (in Science): â€œThe first known members of the Ediacara biota are found shortly after the Gaskiers glaciation, suggesting a causal link between their evolution and this oxygenation event.â€
Oxidation of the Ediacaran Ocean
D. A. Fike, J. P. Grotzinger, L. M. Pratt and R. E. Summons
Nature 444, 744-747 (7 December 2006) | doi:10.1038/nature05345
Oxygenation of the Earth's surface is increasingly thought to have occurred in two steps. The first step, which occurred ~2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean1, 2. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period3, 4 (~800â€“542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates4, 5, and may have led to oxygenation of the deep ocean6. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (~635 to ~548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period7, 8, 9, 10, 11, 12, 13, 14, 15, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record16 and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean6, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon6, 16, 17 and sulphur cycles3, 18, 19 that occurred during this significant period of Earth's history.
Late-Neoproterozoic Deep-Ocean Oxygenation and the Rise of Animal Life
Don E. Canfield, Simon W. Poulton, and Guy M. Narbonne
Science 315, 5 January 2007: 92-95. doi: 10.1126/science.1135013
Animals have an absolute requirement for oxygen, and an increase in late Neoproterozoic oxygen concentrations has been forwarded as a stimulus for their evolution. The iron content of deep-sea sediments show that the deep ocean was anoxic and ferruginous before and during the Gaskiers glaciation 580 million years ago, becoming oxic afterward. The first known members of the Ediacara biota are found shortly after the Gaskiers glaciation, suggesting a causal link between their evolution and this oxygenation event. A prolonged stable oxic environment may have permitted the emergence of bilateral motile animals some 25 million years later.
See also: Kerr, R.A. A Shot of Oxygen to Unleash the Evolution of Animals, Science 314, 8 December 2006: 1529.
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