Mass extinctions past and present: a unifying hypothesis

Abstract

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.