The terrestrial biosphere, constituting a less than 20 km-thick zone constrained by the crustal depth at which bacteria occur and the atmospheric level to which birds can fly, has been repeatedly affected by external forcing, including deep Earth-derived volcanic events, asteroid and comet impacts, solar insolation and orbital forcing cycles, and likely supernovae. Intrinsic biological evolution and diversification through natural selection and adaptation has been repeatedly overprinted, and in some instances almost obliterated, by these events. Both, the ‘Medea hypothesis’ and the ‘Cronus hypothesis’, hinging on metaphors focused on biological self-destruction, appear to underestimate the role of externally forced destruction. The search for unifying principles and for a ‘dynamic balance’ in biological evolution must not overlook the unique origin and consequences of each of the mass extinction events.

The discovery of the K-T boundary impact and its coincidence with the end- Cretaceous mass extinction c. 65 Ma-ago (Alvarez et al., 1980; Alvarez, 1986) heralded a major shift in the long-running debate regarding the respective role of uniformitarian principles vs catastrophic events in natural evolution (Stanley, 1987; Raup, 1992; Sepkoski, 1996; Watson, 2008). A large body of stratigraphic and isotopic age evidence points to close temporal overlaps, within age determination error, between extraterrestrial impacts, volcanic events and mass extinction of species in the Phanerozoic (Glikson, 2005, 2008; Keller, 2005) (Table 1; Figure 1). Whereas age coincidence does not prove cause-and-effect relationship, the effects of large impacts and major volcanic eruptions on the atmosphere and oceans inevitably played a role in contemporaneous mass extinction events. That an overall directionality pertains with regard to the complexity, brain development and intelligence of species across extinction boundaries (Watson, 2008), may be attributable to genetic transmission by the surviving species (Convey Morris, 2003).

According to Ward’s (1994, 2009) ‘Medea’ hypothesis, the origin of large mass extinctions of species and loss of biodiversity are in part driven by life itself. Whereas the triggers for mass extinctions could arise from external forcing, in terms of the ‘Medea’ hypothesis the magnitude of a mass extinction events may be greatly enhanced by the response of sediments produced by biological processes, such as carbonaceous shale and limestones. Examples include (1) large-scale release of microbially metabolized methane, identified by its low δ¹³C signature, as recorded at the c.55 Ma-old Paleocene-Eocene Thermal Maximum (PETM) (Zachos et al., 2001, 2008); (2) the Permian-Triassic mass extinction, which in part resulted from oceanic anoxia induced by enrichment of the oceans in methane, hydrogen sulfide and other compounds, associated with large scale basaltic volcanism (Siberian taps) and a large asteroid impact (Araguinha, D ~ 40 km; Table 1); (3) The release of CO2 from carbonate sediments impacted by an asteroid, for example the c.65 Ma-old K-T Chicxulub impact (Alvarez et al., 1980; Alvarez, 1986). On the other hand, the Homo sapiens-driven “Sixth mass extinction” (Crutzen and Stoermer, 2000; Ruddimann, 2005; Steffen et al., 2007) can hardly be regarded as a response to external forcing.

The ‘Cronus’ hypothesis (Bradshaw and Brook, 2009), while acknowledging the effect of external factors on terrestrial evolution, appears to describe the “ebb and flow of life on Earth along a thermodynamic spectrum” in terms which, in the main, appear to be intrinsic to the biosphere. According to these authors ”extinction is an inevitable part of evolution” and “extinction is a part of natural selection”. An implication may follows as if mass extinction of species is not necessarily related to external forcing of the type indicated in Table 1 and Figure 1. The authors proceed with analogies between individual life cycles, speciation and extinction, stating, for example: “the causes of extinction can be thought of as equivalent to the different processes that lead to individual deaths within a population“. However, although natural selection through inter-species competition and abrupt environmental changes may be intertwined, they are far from the same.

Table 1. Phanerozoic stage boundaries, mass extinctions, large asteroid impacts and correlated volcanic and tectonic events. References to isotopic ages in Glikson, 2005. Genera extinction rates after Keller, 2005.

Figure 1. Phanerozoic genera extinction rates, extraterrestrial impact events (circles denote relative magnitude of impacts) and major volcanic events. After Keller, 2005.

The term ‘Gaia’ (Lovelock and Margoulis, 1974) highlights the intertwined nature and the synergy of terrestrial life. On the other hand the metaphors ‘Medea’ (Ward, 2009) and ‘Cronus’ (Bradshaw and Brook, in press), though colorful, appear to be inconsistent with the theories they symbolize. Given that Darwinian evolution includes both elements of cooperation and of competition, within and between species, the specters of infanticide, patricide and self-destructive life are distinct from and hardly take account of the volcanic or impact factors which have triggered the great mass extinction of species.

In advocating their ‘Cronus’ hypothesis, Bradshaw and Brook (2009) claim “Lovelock’s Gaia and Ward’s ‘Medea’ can be best viewed as extremes of a continuum between cooperation and self destruction” and “as such we posit that the background processes of natural history mostly operate closer to the centre of these extreme views”. This misrepresents both the ‘Gaia’ and the ‘Medea’ notions since:

A. The ‘Gaia’ hypothesis (Lovelock and Margoulis, 1974) does not rule out natural selection through competition between species.

B. The ‘Medea’ hypothesis (Ward, 2009) does not regard biological self destruction as the sole agent of mass extinction.

Nor does the notion as if the ‘Cronus’ hypothesis represent a “dynamic balance” between extremes consistent with the terrestrial record. For example, the survival of the dinosaurs for nearly 200 million years from the Triassic to the end- Cretaceous, through a wide range of environments, was terminated due to an extraterrestrial impact and contemporaneous volcanism, rather than intrinsic evolutionary or self-destructive factors (Alvarez et al., 1980; Alvarez, 1986). Further, in terms of its unique origin, the Anthropocene-induced sixth mass extinction (Crutzen and Stormer, 2000; Ruddimann, 2005; Steffen et al., 2006) can hardly be compared with earlier mass extinctions.

The search for common principles in the history of the rise and fall of species should not overlook the temporally and spatially unique character of each of these cataclysms and its distinct effects on biological evolution.

Conclusions

1. Inherent in metaphors such as ‘Medea hypothesis’ or ‘Cronus hypothesis’ is a focus on intra-biosphere self-destructive processes, which overlook the major to critical role of extra-biosphere forcing events, including asteroid/comet impacts and volcanic eruptions.

2. Attempts at arriving at common intrinsic principles with regard to the respective roles of gradual evolutionary processes vis-a-vis mass extinction events must not overlook the unique nature and consequences of each extra-biosphere forcing event and related mass extinction.

3. The anthropogenic “Sixth mass extinction” appears to be unique in terrestrial history. The question remains subject to philosophical notions, for example in terms of the vulnerability of extreme complexity in nature to self-destruction and the “price” in terms of information entropy of the achievement of deep insights into nature. Homo sapiens may never know the answer to the deepest questions.