On the use of models in understanding the rise of complex life

Abstract

Recently, several seemingly irreconcilably different models have been proposed for relationships between Earth system processes and the rise of complex life. These models provide very different scenarios of Proterozoic atmospheric oxygen and ocean nutrient levels, whether they constrained complex life, and of how the rise of complex life affected biogeochemical conditions. For non-modellers, it can be hard to evaluate which—if any—of the models and their results have more credence—hence this article. I briefly review relevant hypotheses, how models are being used to incarnate and sometimes test those hypotheses, and key principles of biogeochemical cycling models should embody. Then I critically review the use of biogeochemical models in: inferring key variables from proxies; reconstructing ancient biogeochemical cycling; and examining how complex life affected biogeochemical cycling. Problems are found in published model results purporting to demonstrate long-term stable states of very low Proterozoic atmospheric p O 2 and ocean P levels. I explain what they stem from and highlight key empirical uncertainties that need to be resolved. Then I suggest how models and data can be better combined to advance our scientific understanding of the relationship between Earth system processes and the rise of complex life.

Eukaryogenesis and oxygen in Earth history

“…420-400 million years ago (Ma), during the Paleozoic Era 129,130 (Figure 3). During the majority of the Proterozoic Eon, pO2 was maintained at intermediate levels, likely ~10% PAL and possibly as low as ~1% PAL [20][21][22]131 , leaving the deep oceans predominantly anoxic 132,133 , with predominantly oxic surface waters except in (anoxic) upwelling regions 21 . Others argue for pO2 <0.1% PAL 134 or <1% PAL 135,136 during the mid-Proterozoic, which would have left a mostly anoxic surface ocean containing 'oxygen oases' near primary producers (just as prior to the GOE) 21,137 .…”

“…Indeed, fossil and molecular clock evidence consistently suggest that the last eukaryote common ancestor (LECA) emerged hundreds of millions of years after Earth's initial oxygenation [16][17][18][19] . Furthermore, recent advances in geochemistry and Earth system modeling suggest that the oceans at the time of eukaryogenesis were predominantly anoxic at depth, with only weakly oxygenated surface waters (likely no more than 1-10% of modern atmospheric saturation) [20][21][22][23] . These revisions have fostered major modifications 7,[24][25][26] to the co-evolutionary narrative first synthesized by Sagan 1967 1 , inviting a reexamination of the redox conditions necessary for eukaryogenesis.…”

The case and context for atmospheric methane as an exoplanet biosignature

“…However, ref. 40 found that such model behavior is dictated by arbitrary forcings and is not compatible with the rock record. In any case, hydrogenotrophic methanogenesis in the Archean water column could maintain substantial CH 4 fluxes regardless of organic burial efficiency in sediments ( 35 , 38 , 39 ).…”

The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing

“…The O 2 requirement of some extant sponges has been established and found to be very low [29,30] (but of course one can question whether the O 2 requirements of extant animals reflect those of their earlier ancestors). Establishing environmental O 2 levels over geological time has triggered much ongoing research and debate-addressed in this theme issue [9,31]. Currently it is unclear whether O 2 levels in the atmosphere and surface/ shallow ocean (equilibrated with the atmosphere) were low enough to prevent sponges existing at any point after Earth's Great Oxidation at approximately 2.3 Ga [29].…”

“…For non-modellers, it can be difficult to assess the limitations of individual models, or the applicability of different modelling approaches to successfully address different problems. In a scholarly review of key existing models and their limitations and future possibilities, Lenton [31] opens a window into the world of modelling for the non-specialist. The approach is firmly centred on implications for reconstructing oxygen and nutrient levels as complex life evolved and diversified, but a particularly valuable aspect comprises an overview of the principles that should underpin all biogeochemical models, combined with a critical evaluation of key existing models.…”