DO MASS EXTINCTIONS IMPART PREDICTABILITY OR CONTINGENCY TO THE HISTORY OF LIFE?

Is the history of life predictable, or is it contingent on unpredictable events? Mass extinctions are generally considered to be agents of contingency that disrupt directional trends, but we argue the opposite: mass extinctions were potentially one of the greatest sources of directionality in the history of life (for marine metazoa, at least).

Our argument rests on several observations. First, high-level taxonomic turnover in marine animals was largely driven by mass extinction, and the dominant organisms today are those whose ancestors survived repeated catastrophic culling. Second, to an extent not appreciated until recently, many mass extinctions had a similar pattern of selectivity: taxa with a high ratio of CaCO₃ shell to biomass suffered preferentially. The end-Permian was the most severe of these events, but others occurred in the early Cambrian, Late Devonian, Guadalupian, Toarcian, latest Triassic, and Aptian. Geological and geochemical evidence suggest that selective extinction, at least in these events, was potentially driven by ocean acidification.

The surface ocean is typically oversaturated with respect to CaCO₃, which is used by most biomineralizing marine organisms. However, the saturation state is occasionally lowered by rapid CO₂ release into the ocean-atmosphere (and, perhaps, other processes), causing the selective extinction of vulnerable taxa. The carbonate chemistry of the oceans—buffered but sporadically disturbed—establishes a conflict between short- and long-term fitness.

Given the observed frequency of these selective extinction events, as well as the nature of perturbations to the carbonate cycle of the oceans, these extinctions impart predictability to the history of life. Organisms that were less susceptible to these catastrophes (e.g., arthropods, mollusks, vertebrates, various worms) dominate the modern ocean, and they probably would again if we “rewound the tape of life” to the early Paleozoic. The timing of these changes might be difficult to predict, because a massive, end-Permian-sized event—while likely given enough time—could have taken place earlier or later. However, a fauna resembling the living marine fauna should eventually dominate.