Constraints on the chemical composition of the ancient oceans

Addresses the problem of reconciling a Proterozoic “Canfield ocean” scenario (with globally significant euxinia), with the relatively rarity of pyrite-enriched reactive iron during this interval. Uses a simple box model to illustrate how the “electron tower principle” implies that globally significant sulphate reduction necessitates equivalently globally significant nitrate depletion (because denitrifiers will outcompete sulphate reducers). Proposes two distinct geochemical regimes for the low oxygen Proterozoic ocean (nitrate rich/ferruginous and nitrate depleted/euxinic), the latter of which may be partially reverted to in nitrate/oxygen depleted modern day upwelling zones. Suggests that a future empirical cross referencing exercise should find the iron speciation evidence for euxinia to be out of phase in time, with nitrogen isotope evidence for denitrification.





The evolution of recycling (even when its costly)

Addresses the question of why a circular nutrient recycling loop might be produced in a natural environment by two different physiologies, given that it seems just as likely that biology might evolve to degrade a nutrient into a unusable form that leaks out of the system. We applied principles from theoretical biology, concerning the evolution of cooperation (which is promoted by intermediate population mixing and patchy environments), in a simulated agent-based system. We found that in certain spatially structured environments a recycling loop can spread, even when the organisms that produce it are competitively inferior to organisms that “break” the loop by degrading a nutrient into an unusable form that leaks out of the system.


Does symbiosis bias towards climatic regulation?

Addresses the question of why a system conducive to homeostasis (two opposing feedback influences with the same intermediate optimum conditions for growth/expansion, referred to as “integral rein control”) should arise, and whether there is anything about life that biases towards the production of such a system? The hypothesis expressed in this paper is that symbiosis, on average, tends to bring together physiologies with distinct impacts on the natural environment but shared optimal conditions for growth. This tends to bias towards the feedback structure shown to be conducive to homeostasis by the original daisyworld model. This is expressed by introducing a symbiosis between a black and white daisy into the model – with the result of an extension of the habitable range within the system, and succession type dynamics in which the symbiosis colonises the system first but is then displaced due to the evolutionary cost of co-operation.


PhD Thesis: Theoretical feedbacks between Neoproterozoic glaciations and eukaryotic evolution

Incorporating the two multi-level selection/altruism papers below, as well as (i) carbon cycle modelling of ocean acidification in snowball earth conditions (importance of atmosphere-ocean chemical continuity in duration of glacial interval), (ii) quantitative assessment of the position (in temperature “space”) of the unstable region of the ice albedo feedback with respect to glacial entry and duration, (iii)

temperature sensitivity of terrestrial silicate weathering flux (in relation to susceptibility to snowball earth glaciation).


How global scale glaciations might have triggered the evolution of animal form

Geobiological hypothesis that the origin of multicellular animals (anything with form more complex than a sponge) was directly triggered by the Neoproterozoic global scale glaciation events. Basic argument is that complex multicellular animal form requires terminal (i.e. irreversible) cellular differentiation, which imposes a high fitness penalty on individual cells, because mutant cells that do not differentiate (e.g. tumours) leave more descendants than their neighbours. This is more of a problem in animal form than in plants or fungi, because of the more strict controls on cell fate (which is why, for example, animals have more cell types). This sort of cellular “altruism” is promoting by genetic relatedness between the co-operating entities (Kin selection). The hypothesis is that snowball earth glaciations, by creating population bottlenecks (via temperature, water potential, nutrient availability extremes etc), the survivors of which were more closely related than average individuals in a pre-bottleneck population (the “founder effect”), raised average genetic relatedness between individual cells in colonies of sponge-like early animals. This promoted the evolution of terminal cellular differentiation, making organ formation, obligate sexual reproduction, apoptosis etc, adaptive in terms of individual fitness for the first time in Earth history.


The surprising importance of the temperature preferences of daisies

Popular press article referring to how the parable of daisyworld would not work if the combination of the physiological optimum of a species, and the impact of that species on the environment, leads to a destabilizing positive feedback. (e.g. in the parable of daisyworld, “white daisies” cool the planet but self-limit because their optimum temperature for growth is the same as the “black daisies” that warm the planet). The point being that there is no logical reason to expect biological evolution to produce this combination of properties.


Fluctuation in the physical environment as a mechanism for reinforcing evolutionary transitions

Theoretical biology paper expressing the hypothesis that evolutionary co-operation (symbiosis, cellular “altruism” in complex differentiated eukaryotes, social group formation etc) goes hand in hand with physiological robustness, therefore an ability to tolerate variable physical environments. Physiological robustness is proposed to result from a novel capacity to differentiate within co-operative groups, e.g. the formation of organs from co-operative cells. This idea arguably explains why lichens (a co-operative symbiosis) were likely the first organisms to colonise the land surface.