Woody cover and hominin environments in the past 6 million years

The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). (13)C/(12)C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than ～40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years.     

Optimal nitrogen-to-phosphorus stoichiometry of phytoplankton

Redfield noted the similarity between the average nitrogen-to-phosphorus ratio in plankton (N:P = 16 by atoms) and in deep oceanic waters (N:P = 15; refs 1, 2). He argued that this was neither a coincidence, nor the result of the plankton adapting to the oceanic stoichiometry, but rather that phytoplankton adjust the N:P stoichiometry of the ocean to meet their requirements through nitrogen fixation, an idea supported by recent modelling studies. But what determines the N:P requirements of phytoplankton? Here we use a stoichiometrically explicit model of phytoplankton physiology and resource competition to derive from first principles the optimal phytoplankton stoichiometry under diverse ecological scenarios. Competitive equilibrium favours greater allocation to P-poor resource-acquisition machinery and therefore a higher N:P ratio; exponential growth favours greater allocation to P-rich assembly machinery and therefore a lower N:P ratio. P-limited environments favour slightly less allocation to assembly than N-limited or light-limited environments. The model predicts that optimal N:P ratios will vary from 8.2 to 45.0, depending on the ecological conditions. Our results show that the canonical Redfield N:P ratio of 16 is not a universal biochemical optimum, but instead represents an average of species-specific N:P ratios.     

Biological and biomedical implications of the co-evolution of pathogens and their hosts

Co-evolution between host and pathogen is, in principle, a powerful determinant of the biology and genetics of infection and disease. Yet co-evolution has proven difficult to demonstrate rigorously in practice, and co-evolutionary thinking is only just beginning to inform medical or veterinary research in any meaningful way, even though it can have a major influence on how genetic variation in biomedically important traits is interpreted. Improving our understanding of the biomedical significance of co-evolution will require changing the way in which we look for it, complementing the phenomenological approach traditionally favored by evolutionary biologists with the exploitation of the extensive data becoming available on the molecular biology and molecular genetics of host-pathogen interactions.     

Action research and critical systems thinking: Two icons carved out of the same log?

My intention in this paper is to make sense of the relationship between Action Research (AR) and Critical Systems Thinking (CST). On the surface, the two strands of thinking do not seem to have much in common. There is hardly any common reference in texts written within each tradition. Deeper down, however, there may be common concerns. The point of departure is to focus on how professionals interact with real-world problems and to clarify the underlying values for professional practice. To facilitate a discussion on the relationship between AR and CST, a frame of reference is developed based on analyzing the meaning construction process. Based on this platform, the relationship among theory, practice, and people shapes the ground for the discussion. Within this discussion three questions are raised: (1) Is the theory understood by the people and based on their interests? (2) Are the research questions relevant for the people? and (3) Are people emancipated to act in their own interests? The relationship between AR and CST can be examined based on these three questions. In concluding, I argue for the potential of a mutual and fruitful dialogue between AR and CST.     

Intestinal monosaccharide transport in experimental protein deprivation

Zusammenfassung An jungen Ratten wurde eine bedeutende Steigerung der Absorption vond-Glukose beobachtet, jedoch keine solche derd-Fruktose. Offenbar ist der Effekt and-Glukose ein adaptives Phänomen, verbunden mit einem veränderten Zellmetabolismus im Dünndarm.