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tains 21% O2, there is widespread agreement that prior to the emergence of oxygenic cyanobacteria, PO2 must have been extremely low. Recent models by Canuto et al. (1983) and Kasting (1987) suggest that the prebiotic atmosphere contained no more than about 10-10 bar of molecular oxygen—enough to make hematite stable, but far too little to provide an effective ozone screen or to support aerobic metabolism. Indeed, a standard tenet of chemical evolution is that prebiotic chemistry could not have proceeded in environments containing significant amounts of O2. Increasingly well resolved phylogenetic trees (e.g., Woese, 1987) complement this perspective. These trees indicate that anaerobic organisms diverged earlier than aerobes, and that aerobes requiring high PO2(i.e., large animals) appeared later than aerobes able to function in less oxic environments. It is, therefore, attractive to link biological to environmental history; however, the entire pattern of biological evolution can potentially be explained quite differently. The facts noted in the previous paragraph really only require a specified initial condition: that is, that the Earth's prebiotic atmosphere was essentially anoxic and that the first organisms were, therefore, anaerobic. There is no a priori reason why an early radiation of cyanobacteria could not have engendered an early and rapid increase in PO2 approximating or even exceeding today's levels. Very different controls would then have to be sought for the observed evolutionary patterns.

Acceptance of what might fairly be called the Cloud model requires that three criteria be satisfied:

  1. geochemical documentation of environmental change;

  2. independent paleontological evidence for coeval evolutionary innovation; and

  3. physiological, phylogenetic, and ecological reasons for linking criteria 1 and 2.

In the following pages, we evaluate the rapidly accumulating data on oxygen and biological evolution during two intervals often inferred to have been critical junctures in the history of life: (1) early in the Proterozoic Eon (ca. 2000 Ma), when increases in PO2 above the Pasteur point are thought to have made possible the evolution of aerobic prokaryotes and mitochondria-bearing protists; and (2) the latest Proterozoic (ca. 600 Ma), when another substantial increase in PO2 may have made possible the initial evolution of macroscopic animals.

THE EARLY PROTEROZOIC EON

Geochemical Evidence for Atmospheric Change

The Paleoproterozoic Era (2500 to 1600 Ma) was a time of profound environmental change (Cloud, 1968a, 1972: Holland, 1984). Two independent sedimentological observations have long been cited in support of the hypothesis that the atmosphere first accumulated significant amounts of oxygen during this interval. Banded iron formations (BIF), quintessentially Precambrian sediments composed of iron-bearing minerals and silica, are abundant in successions older than ca. 1900 Ma, but are rare in younger sequences (Figure 1.1). Continental red beds display an inverse distribution. The origin of marine iron

FIGURE 1.1 A summary of geochemical and paleobiological data relevant to considerations  of Paleoproterozoic evolution and environmental change (PAL = present atmospheric level).



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