The Proterozoic aeon (2,500-540 million years ago) saw episodic increases in atmospheric oxygen content, the evolution of multicellular life and, at its close, an enormous radiation of animal diversity. These profound biological and environmental changes must have been linked, but the underlying mechanisms have been obscure. Here we show that hydrocarbons extracted from Proterozoic sediments in several locations worldwide are derived mainly from bacteria or other heterotrophs rather than from photosynthetic organisms. Biodegradation of algal products in sedimenting matter was therefore unusually complete, indicating that organic material was extensively reworked as it sank slowly through the water column. We propose that a significant proportion of this reworking will have been mediated by sulphate-reducing bacteria, forming sulphide. The production of sulphide and consumption of oxygen near the ocean surface will have inhibited transport of O2 to the deep ocean. We find that preservation of algal-lipid skeletons improves at the beginning of the Cambrian, reflecting the increase in transport by rapidly sinking faecal pellets. We suggest that this rapid removal of organic matter will have increased oxygenation of surface waters, leading to a descent of the O2-sulphide interface to the sea floor and to marked changes in the marine environment, ultimately contributing to the Cambrian radiation.

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Unrestricted Hartree-Fock and density functional theory calculations have been carried out to investigate the detailed kinetics and mechanisms of hydrocarbon thermal cracking. The calculations of the elementary reactions involved in the overall cracking of paraffin molecules agree well with the generally accepted free radical mechanism. The results can be summarized as follows: (1) Initiation cracking, the calculated bond dissociation energy (BDE) for C-C homolytic scission is ∼95 kcal/mol at the MP2/6-31G* level and ∼89 kcal/mol at the B3LYP/6-31G* level. No transition states are found in the reactions. (2) H-transfer reaction, the calculated energy barriers (activation energy) are 15-17 kcal/mol at the MP2/6-31G* level and 10-12 kcal/ mol at the B3LYP level. The reverse reaction has about the same energy barrier. The calculated transition state structures for both reactions lie in the middle between the reactant and product.(3) Radical decomposition-"β scission", the activation energy for the radical decomposition is calculated to be 30-33 kcal/mol. The activation energy for the reverse addition reaction is 5 kcal/mol. The optimized transition state structure is product-like for the radical decomposition reaction and reactant-like for the addition reaction.