Theoretical feedbacks between neoproterozoic glaciations and eukaryotic evolution
Boyle, Richard A.
PhD Thesis, University of East Anglia
Thesis or dissertation
Physiological consequences of transitions in the level of selection explain the existence of Neoproterozoic lichen symbioses. Between-individual differentiation within groups causes physiological buffering, giving altruists higher fitness than cheaters in variable environments. Altruists increase from rarity. Reciprocal feedback between a genotype frequency and an abiotic state may be equivalent to kin selection. At contemporary degassing rate, biotic silicate weathering enhancement 10 – 20 times greater than present would have been required for a biologically-triggered snowball Earth. However, biotic enhancement could have compensated by declining abiotic silicate weathering rate at low temperatures – sufficiently to accentuate a tectonically-driven glaciation. Marine carbonate speciation causes a slushball ocean to become a net CO2 source at large reservoir sizes, making glacial duration shorter than a hard snowball by a factor of the atmospheric CO2 degassing fraction. Equilibrated oceans require a carbonate weathering:burial ratio Wcarb : Bcarb > 1 for deglaciation by CO2(g) increase. If adequate biotic silicate weathering enhancement rapidly occurs after deglaciation, subsequent reglaciation will occur within 3 – 5Myrs. The contrast with the ~50Ma separating Neoproterozoic events invokes the elapse of a coevolutionary “succession time”, before which biotically-triggered reglaciation is improbable. Low post-glacial silicate weathering is consistent with Si-depleted cap carbonates. Phanerozoic reduction in glacial susceptibility resulted from physiological evolution of biotic weathering and/or calcification. Emergence of terminal cellular differentiation in Ediacara/Metazoa required an environmental context E, provided by Neoproterozoic glaciations, in which the high fitness cost was adaptive. Restriction of the biosphere to refugia experiencing extreme, repeated founder effects raised kin selection for altruism. Between-group isolation limited cheat migration, accentuating group viability selection for altruism. Both processes occurred globally, over multi-million year timescales; explaining subsequent proliferation of Ediacaran macrobiota.