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Earth's Oxygen Cycle and the Evolution of Animal Life

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Earth's Oxygen Cycle and the Evolution of Animal Life Earth’s oxygen cycle and the evolution of animal life Christopher T. Reinharda,1, Noah J. Planavskyb, Stephanie L. Olsonc, Timothy W. Lyonsc, and Douglas H. Erwind aSchool of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332; bDepartment of Geology & Geophysics, Yale University, New Haven, CT 06511; cDepartment of Earth Sciences, University of California, Riverside, CA 92521; and dDepartment of Paleobiology, National Museum of Natural History, Washington, DC 20560 Edited by Andrew H. Knoll, Harvard University, Cambridge, MA, and approved June 17, 2016 (received for review October 31, 2015) The emergence and expansion of complex eukaryotic life on Earth is The requisite environmental O2 levels for each of these biotic linked at a basic level to the secular evolution of surface oxygen milestones may vary substantially, and some may depend strongly levels. However, the role that planetary redox evolution has played on environmental variability, whereas others may be more directly in controlling the timing of metazoan (animal) emergence and linked to the environment achieving a discrete “threshold” or diversification, if any, has been intensely debated. Discussion has range of oxygen levels. Our focus here is on reconstructing an gravitated toward threshold levels of environmental free oxygen ecological context for the emergence and expansion of early animal (O2) necessary for early evolving animals to survive under controlled life during the Middle to Late Proterozoic [∼1.8–0.6 billion years conditions. However, defining such thresholds in practice is not ago (Ga)] in the context of evolving Earth surface O2 levels. straightforward, and environmental O2 levels can potentially con- Most work on the coevolution of metazoan life and surface oxy- strain animal life in ways distinct from threshold O2 tolerance. Herein, gen levels can be characterized as either biological (e.g., attempting we quantitatively explore one aspect of the evolutionary coupling to constrain threshold environmental O2 levels for different organ- between animal life and Earth’s oxygen cycle—the influence of spa- isms), or geochemical (e.g., attempting to constrain environmental tial and temporal variability in surface ocean O2 levels on the ecology O2 levels before, during, and after the emergence of metazoan life). of early metazoan organisms. Through the application of a series of This discussion is typically framed in terms of a relatively sharp quantitative biogeochemical models, we find that large spatiotempo- dichotomy—e.g., either environmental oxygen levels have had ral variations in surface ocean O2 levels and pervasive benthic anoxia little to do with the timing of the rise of animal life (e.g., refs. 9 p areexpectedinaworldwithmuchloweratmospheric O2 than at and 10) or oxygenation of Earth’s surface has been of primary present, resulting in severe ecological constraints and a challenging importance (e.g., ref. 11). Both approaches have emphasized the evolutionary landscape for early metazoan life. We argue that these relationship (or lack thereof) between metazoan evolution and the EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES effects, when considered in the light of synergistic interactions with secular oxygenation of Earth’socean−atmosphere system, and other environmental parameters and variable O2 demand through- both provide a critical backdrop for efforts to understand the out an organism’s life history, would have resulted in long-term evo- importance of environmental factors in early animal evolution. lutionary and ecological inhibition of animal life on Earth for much of Experimental work with the modern demosponge Halichondria Middle Proterozoic time (∼1.8–0.8 billion years ago). panacea suggests an empirical O2 threshold of ∼0.5–4.0% of the present atmospheric level (PAL; corresponding to an equilibrium − oxygen | animals | evolution | Proterozoic concentration of ∼1–10 μmol·kg 1 for air-saturated water at 25 °C and a salinity of 35‰) for basal metazoan life (12) and perhaps long-standing and pervasive view is that there have been in- lower for smaller sponge species. Theoretical calculations for the Atimate mechanistic links between the evolution of complex life LCA of bilaterian life indicate an even lower threshold range of on Earth—in other words, the emergence and ecological expansion ∼0.14–0.36% PAL (ref. 13, Fig. 1). In addition, all major eukaryotic of eukaryotic cells and their aggregation into multicellular organ- lineages contain genes and enzymes allowing for facultatively isms—and the secular evolution of ocean−atmosphere oxygen anaerobic energy metabolism (14), which indicates that either levels (1). Molecular oxygen (O2) is by far the most energetic of the (i) anaerobic pathways of ATP synthesis in primitively aerobic abundant terminal oxidants used in biological metabolism (e.g., ref. eukaryotes were obtained through interdomain and intradomain 2). When this energetic capacity is harnessed by mitochondria in eukaryotic cells, the energy flux supported by a given genome size Significance increases by a factor of ∼8,000 (3), potentially paving the way for increased complexity at the cellular level (but see ref. 4). Oxygen is Earth is currently the only planet known to harbor complex life. also a crucial component of enzymatic pathways leading to the Understanding whether terrestrial biotic complexity is a unique synthesis of regulatory membrane lipids (5) and structural proteins phenomenon or can be expected to be widespread in the universe (6) in eukaryotic organisms and provides a powerful shield against ’ depends on a mechanistic understanding of the factors that led to solar UV radiation at Earth s surface through stratospheric ozone the emergence of complex life on Earth. Here, we use geochemical production. Furthermore, O2 is the only respiratory electron ac- constraints and quantitative models to suggest that marine envi- ceptor that can meet the metabolic demands required for attaining ronments may have been unfavorable for the emergence and the large sizes and active lifestyles characteristic of metazoan life ’ large-scale proliferation of motile multicellular life for most of (e.g., ref. 7). There is thus ample support for the view that Earth s Earth’s history. Further, we argue that a holistic evaluation of oxygen cycle provided a crucial evolutionary and ecological con- environmental variability, organismal life history, and spatial eco- straint on the road to increased biotic complexity, both at the logical dynamics is essential for a full accounting of the factors that cellular level and on the road to motile multicellular life. have allowed for the emergence of biological complexity on Earth. However, it is important to distinguish between the O2 levels required for: (i) the emergence and ecological expansion of com- Author contributions: C.T.R., N.J.P., and D.H.E. designed research; C.T.R. and S.L.O. per- plex (multicellular) eukaryotes, including red/brown algae, land formed research; C.T.R., N.J.P., S.L.O., and D.H.E. analyzed data; and C.T.R., N.J.P., S.L.O., plants, fungi, and animals; (ii) the initial origin of the metazoan T.W.L., and D.H.E. wrote the paper. (animal) last common ancestor (LCA); (iii) the diversification of The authors declare no conflict of interest. basal (e.g., Ctenophora, Cnidaria, Porifera) and more derived (e.g., This article is a PNAS Direct Submission. Bilateria) metazoan clades; and (iv) emergence of the larger body 1To whom correspondence should be addressed. Email: [email protected]. sizes and more complex ecological interactions (such as predation This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and burrowing) likely to leave robust signals in the fossil record (8). 1073/pnas.1521544113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1521544113 PNAS Early Edition | 1of6 Phanerozoic Proterozoic Archean charcoal/ 100 -1 p combustion sponge O (atm) paleosols oxygen -2 2 2 oases 10 -3 (PAL) O p LCA Cr/Mn -4 -5 10 log NMD-S Fig. 1. Biological thresholds and geochemical constraints on Earth surface O2 levels. (Left) Estimated minimum metabolic O2 requirements of sponges (gray) (12) and the LCA of bilaterian metazoans (brown) (13). (Right) Estimated environmental O2 levels based on the plant fossil and charcoal record during the Phanerozoic (37); fossilized soil profiles during the Proterozoic (blue), recalculated following ref. 38 according to pCO2 values in ref. 39; chromium (Cr) and manganese (Mn) systematics of marine chemical sediments and ancient soil profiles (red) (11); non-mass-dependent sulfur (S) isotope fractionation in sedimentary rocks (orange) (40); and 0D and 3D Earth system models for Archean oxygen oasis systems (green) (20−22). Downward arrows represent upper bounds on atmospheric O2 levels, and the range for oxygen oases refers to the estimated atmospheric pO2 value at gas exchange equilibrium. lateral gene transfer (15, 16), or (ii) the earliest eukaryotic organ- ocean may have allowed for O2 levels above the respiratory re- isms contained mitochondria that were facultatively anaerobic (17). quirements of basal metazoan organisms regardless of background In any case, it is possible that early animals were able to respire at atmospheric pO2 (10). very low environmental O2 levels and possibly even survive periodic In principle, one could compare a geochemical estimate of at- episodes of local anoxia. mospheric pO2 to an estimate of the dissolved O2 level required Geochemical
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