Why does the sensitivity vary so much?
As seen above, it requires extremely small amounts of HCl and quinine to trigger the detection of sourness and bitterness. This is because HCl and other sour tastants are present in foods that have begun to go off and it was advantageous for our hunter-gatherer precursors to be able to detect that. The ones that could detect the tastants at small amounts would not then suffer the illnesses that came with rotten food and as a result would be able to reproduce, passing down their genes via Natural Selection.
The same goes for bitter foods. The ability to perceive bitter tastants was advantageous as there are numerous natural bitter compounds, of which a large number of which are known to be toxic. The ability to detect bitter-tasting and therefore toxic compounds at extremely low thresholds was considered to provide an important protective function.
The umami taste is one which researchers are still trying to find evolutionary reasoning for. Glutamate is the most abundant amino acid in nature and like other amino acids, is used by the body for growth. By that logic, the ability to taste umami is most likely down to our need for glutamate and other amino acids, the building blocks of proteins in the body.
Sweet tastants are detectable at low values because when one thinks of a hunter-gatherer scenario that was all too common in times gone by, food that contained sufficient calories for the survival of a group was relatively scarce. Therefore, the ability to perceive sweet tastants would be vital as it would draw the hunter-gatherers to foods with high energy content which could be broken down by the body.
Salty foods contain electrolytes, which help your body with homeostasis – the regulation of its internal environment. Electrolytes are important because they are what your cells use to maintain voltages across the cell membranes in order to carry electrical impulses (nerve impulses, muscle contractions) to other cells. When you exercise heavily, you lose electrolytes in your sweat, particularly sodium and potassium.
Since the body is two-thirds water, it is incredibly important that the body have enough electrolytes and this is the reasoning behind the relatively higher threshold value of the salty tastant. Hunter-gatherers would lose large amounts of electrolytes through sweat and diseases (like diarrhoea), so the ability to taste salty foods would be vital for survival.