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OXYGEN AND PROTEROZOIC EVOLUTION: AN UPDATE 29 that the fossil record of macroscopic animals begins about 600 Ma. Biological Reasons for Linkage to Environmental Change Early hypotheses linking metazoan evolution to increases in atmospheric oxygen stressed the ozone screen and its consequences for UV absorption (e.g., Nursall, 1959: Berkner and Marshall, 1965; Cloud, 1968b). Such scenarios now seem unlikely in that an essentially complete ozone screen was probably in place by the time PO2 reached 1% PAL (Kasting, 1987), a threshold attained long before 600 Ma (see above). Nonetheless, oxygen is important to metazoan evolution for physiological reasons (Raff and Raff, 1970; Towe, 1970; Runnegar, 1982b). Although tiny animals, perhaps ecologically similar to extant millimeter-scale nematodes and other meiofauna, can live at a PO2 of a few percent PAL, macroscopic animals require higher oxygen concentrations to ensure the oxygenation of multiple cell layers. Large animals also require a higher Poitou support collagen manufacture, exercise metabolism, and organismic function within skeletons. Values of 6 to 10% PAL appear to be minimum values for the support of large, unskeletonized animals that have a circulatory system or, in the case of coelenterates, a system of finely divided mesentery folds (essentially thin, flat animals folded into a three- dimensional architecture; Raff and Raff, 1970; Runnegar, 1982b). Much higher oxygen concentrations, approaching present- day levels, are necessary to support macroscopic animals without circulatory systems (Runnegar, 1982b). Thus, there are two potential ways in which increasing PO2 might correlate with metazoan evolution. If tiny ur- metazoans developed circulatory systems, then a PO2 increase to more than 6 to 10% PAL would remove this environmental barrier against the evolution of macroscopic animals. On the other hand, if macroscopic size preceded the efficient internal circulation of fluids, PO2 increases to nearly modern levels would be necessary for large animal metabolism. Most discussions of oxygen and early animal evolution have tacitly or explicitly assumed the former case (e.g., Cloud, 1968b, 1976; Towe, 1970; Runnegar, 1982a); however, this is by no means demonstrated. It is not at all clear that Ediacaran animals had well-developed circulatory systems or finely divided mesenteries. Although many present-day animals live in relatively oxygen-poor waters (e.g., Thompson et al., 1986), this may tell us little about the ancestral habitat of macroscopic animals. Once circulatory and respiratory systems were invented, large animals would have been able to inhabit oxygen-poor environments previously closed to them. Geochemical Data In the preceding section it is argued that oxygen levels greater than 6 to 10% PAL may have been necessary for the evolution of large animals, but we argue earlier that PO2 probably equaled or exceeded 15% PAL as early as 2100 Ma. We, therefore, have three choices. We can reject the Cloud hypothesis; we can suggest that oxygen levels actually decreased during the Meso- or early Neoproterozoic; or we can suggest that the first macroscopic animals did not possess well developed circulatory systems and, therefore, required oxygen levels substantially greater than 15% PAL. It is certainly premature to choose the first option, and we know little about the history of atmospheric oxygen between 1900 and 600 Ma or the internal architecture of Ediacaran animals. There is no a priori reason to believe that the secular curve for atmospheric oxygen has been monotonic, although it has often been drawn that way. In the absence of convincing data, we can ask whether or not the geological record contains any evidence suggestive of immediately pre-Ediacaran environmental change. The answer is clearly yes. At least four independent lines of evidence indicate that the period immediately preceding the Ediacaran radiation was a highly distinctive epoch in Earth history, certainly involving marked environmental change and plausibly including biologically significant variations in PO2 (Figure 1.6; Derry et al., 1992; Knoll, 1992a). The distinctive nature of ca. 850-600 Ma sedimentary successions is clearly seen in two lithologies. After a hiatus of more than 1000 million years (m.y.), iron formationsâsome of them thick and laterally extensiveâreappear on five continents (Young, 1976). It is difficult to envision their formation in the absence of extensive deep ocean anoxia. In general, Neoproterozoic iron formations are associated with the other distinctive lithologies of this period: tillites and related glaciogenic rocks. At least four ice ages punctuated late Proterozoic history; the Sturtian and Varanger glaciations were arguably the most severe in our planet's history (Hambrey and Harland, 1985; Kirschvink, 1992). Independent evidence of environmental change comes from the isotopic composition of Neoproterozoic sedimentary rocks. Over the past few years, stratigraphic variations in the marine Neoproterozoic carbon, sulfur, and strontium isotopic records have been detailed. Carbon in ca. 850 to 600 Ma carbonates and organic matter is isotopically unusual in two respects âthese materials are often anomalously enriched in 13C (δ13C >+5%o PDB), and within this interval there are several negative δ13C excursions of 6 to 8%o, at least in part associated stratigraphically with glaciogenic rocks (Knoll et al., 1986; Kaufman et al.,
