Mass extinctions past and present: a unifying hypothesis

Abstract. Enzymes are often referred to as the "agents of life" – a very apt term, since essentially all life processes are controlled by them. Typically, these enzymes only function across a narrow band of environmental conditions, particularly temperature and pH. Ambient conditions that challenge these operating conspecifics trigger enzyme dysfunction. Here, it is proposed that the pH-dependent inactivation of a single enzyme, urease, provides a unifying kill-mechanism for at least four of the "big five" mass extinctions of the past 560 million years. The triggering of this kill-mechanism is suggested to be sensitive to both gradualistic and catastrophic environmental disturbances that cause the operating pH of urease-dependent organisms to cross enzymatic "dead zones", one of which is suggested to exist at ~pH 7.9. For a wide range of oceanic and terrestrial ecosystems, this pH threshold coincides with an atmospheric CO₂ partial pressure (pCO₂) of ~560 ppmv – a level that at current CO₂ emission trajectories may be exceeded as early as 2050. The urease hypothesis thus predicts an impending Anthropocene extinction event of equivalence to the "big five" unless future atmospheric pCO₂ levels can be stabilised well below 560 ppmv. Immediate scientific discussion and testing is required to confirm the validity of the urease hypothesis.