The complex evolution of humans has allowed for the development of a mammal so advanced that its population has grown beyond exponentially in a relatively short time scale. The complexity of humans is due in part to intelligence in the form of a large, well-developed brain. An organ of this size and importance is what is known as an expensive tissue, meaning the body requires a substantial amount of energy to produce it (Mink et al. 1981). To explain the relationship between energy and brain development, anthropologists Peter Wheeler and Leslie Aiello developed the expensive tissue hypothesis (Aiello et al. 1995). The hypothesis states that the development of a relatively large brain is extremely costly, and must be balanced by either an increase
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Fortunately, the immune system also has evolved to not only contain the microbiota, but preserve the symbiotic relationship between host and microbiota. The homeostasis in mucus layer in the gut is maintained by a system of checks and balances between potentially pro-inflammatory cells, such as Th1 cells that produce interferon-γ, lymphoid cells with cytokine effector features resembling Th2 and Th17 cells, and anti-inflammatory regulatory T cells (Hooper et al 2012). There are different regions of the gastrointestinal tract that differ in function and microbiota (Walter et al 2011 and Nicholson et al 2012). The divided sections allow for distinction between nutrient absorption and digestion to be separated distally from the microbiota in order to reduce conflicts (Walter et al 2011). The chemical communication between the host and microbiota varies based on location and can influence factors by signaling through low molecular weight metabolites, peptides, and proteins (Nicholson et all 2012). PH levels in the stomach, secretion of bile salts, proteins such as immunoglobulin A (igA) all mediate microbial levels in the stomach as well (Walter et al 2011). Chemical methods also combat microbial dangers, for example, the low pH of stomach acid prevents microbial growth. Bile salts are bacteriocidal, and igA also targets bacteria although mechanisms are still not well
Fortunately, the immune system also has evolved to not only contain the microbiota, but preserve the symbiotic relationship between host and microbiota. The homeostasis in mucus layer in the gut is maintained by a system of checks and balances between potentially pro-inflammatory cells, such as Th1 cells that produce interferon-γ, lymphoid cells with cytokine effector features resembling Th2 and Th17 cells, and anti-inflammatory regulatory T cells (Hooper et al 2012). There are different regions of the gastrointestinal tract that differ in function and microbiota (Walter et al 2011 and Nicholson et al 2012). The divided sections allow for distinction between nutrient absorption and digestion to be separated distally from the microbiota in order to reduce conflicts (Walter et al 2011). The chemical communication between the host and microbiota varies based on location and can influence factors by signaling through low molecular weight metabolites, peptides, and proteins (Nicholson et all 2012). PH levels in the stomach, secretion of bile salts, proteins such as immunoglobulin A (igA) all mediate microbial levels in the stomach as well (Walter et al 2011). Chemical methods also combat microbial dangers, for example, the low pH of stomach acid prevents microbial growth. Bile salts are bacteriocidal, and igA also targets bacteria although mechanisms are still not well