About the question "how humans come to like spices", I came across the following
text, by Paul Rozin, in "Psychobiological perspectives on food preferences and
avoidances", in "Food and evolution", 1987. I hope you like that text as much as
I did.

Best wishes,

Jean-Louis
[log in to unmask]

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"Chili peppers produce oral pain and, at moderately high levels, induce
defensive reflexes, including salivation, running of nose, and tearing of eyes.
They have been consumed in Meso-America and other parts of the New World for
thousands of years and incorporated into many Old World cuisines following their
discovery in the 16th century. The circumstances under which such an aversive
food was rather readily adopted are mysterious, especially in light of the
reluctance to adopt seemingly more nutritive and palatable products of the
Americas, like corn and tomatoes. Chili peppers are among the most commonly
consumed flavorings in the world and are probably eaten on a daily basis by over
one-quarter of the adults in the world.

My emphasis will be on the mechanisms responsible for reversal of innate
aversions. However, one can hardly refrain from discussing the possible
adaptative value of consuming this popular spice. Chili peppers, per unit
weight, are among the best sources of vitamin A and C in the world. Capsaicin,
the substance that causes the mouth burn, activates the gastrointestinal system,
stimulating salivation, gastric secretion, and gut motility. The role of any of
these features in the adoption of chili pepper into new cultures or the
acquisition of a liking for it is not known (...)

We have studied the liking for chili pepper in a Mexican highland village and in
the University of Pennsylvania community. At this time we do not know how this
aversion reversal occurs, but I will summarize some of its basic features, and
some constraints on theories of acquisition. A basic first point of that people
consume chili pepper because they like it. It is a "good taste", and according
to self-reports, is rarely consumed primarily because of its anticipated
effects. Furthermore, people who like chili come to like the very same sensation
(the mouth burn) that initially puts people off. In a Mexican village, children
in the two- to six-year-old range receive gradually increasing amounts but are
permitted to refuse it when it can be removed (e.g. by omitting the hot sauce on
the tortillas). They are not rewarded in any obvious way for eating it; rather
they observe that it is enjoyed by their elders. By age five to eight, most
children in the village were voluntarily adding piquancy to their foods; they
had come to like the "hot" stuff after months to years of exposure to it in a
natural family setting. (American adults sometimes acquire likings for chili
peppers very rapidly, after a few experiences.) Some of the mechanisms I have
discussed may be at work, with initial exposure "forced" by mild social pressure
to do what other members of the family do and the fact that moderate levels of
chili are cooked into some of the rest of the meal, and with the already good
tastes of the main food staples. Furthermore, the salivation facilitates
mastication of an otherwise rather dry and mealy diet and may enhance the flavor
of the food as well. The saliva and the pepper flavor and burn added to a rather
bland diet seem to improve the taste of the food significantly, explaining why
the most frequent explanation for chili eating offered by Mexicans is that it
adds flavor or zest to food.

Two possible explanations of the acquired liking for chili (and perhaps other
innately unpalatable substances) depend on its initial unpalatability. The mouth
pain of chili may become pleasant as people realize that it is not really
harmful. This puts the pleasure of eating chili in the category of
thrill-seeking, in the same sense that the initial terror of a rollercoaster
ride or parachute jumping is replaced by pleasure. People may come that there
really is no danger produced by chili may cause the brain to attempt to modulate
the pain by secreting endogenous opiates, morphine-like substances produced in
the brain. There is evidence that, like morphine, these brain opiates do reduce
pain. At high levels, they might produce pleasure. Hundreds of experiences of
chili-based mouth pain may cause larger and larger brain opiate responses,
resulting in a net pleasure response after many trials.

(...) There are multiple routes to liking (...) In the case of chili pepper and
most other likings, I am inclined to emphasize the importance of the social
matrix and the valuation of the food by important others. A major role for
social valuation might help to explain why non-human omnivores seem to develop
acquired likings for food rather rarely, and why it is extremely difficult to
get animals to liek chili pepper. A few cases of chil-liking in animals have
been reporteed; however, in all such cases, the animals (chimpanzees, dogs,
macaque monkeys) were adored pets in homes where chili pepper was a frequent
part of the diet. Perhaps these personal domesticates participated in the human
social matrix that may be so important in the reversing of aversions and the
development of likings.

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[a few pages later, about flavors]

(...) Flavor principles impart a characteristic to most of the dishes in a
cuisine, This culinary institution can be somewhat speculatively traced to a bit
of human biology, the omnivore's dilemma. Fear and curiosity about new foods are
represented in individual humans by a desire for familiarity in foods, an
indicator of safety, and an opposing desire for variety and reaction against
monotony in diets. Flavor principles may be a response to the desire for
familiarity, since they give almost all foods a characteristic and familiar
flavor. But what about the other side of the omnivore's dilemma, the desire for
variety? Careful analysis indicates that the flavor of a cuisine is usually a
group of flavorings that vary from dish to dish while maintaining a common
identity or family resemblance. Thus, while chili pepper appears in almost all
Mexican food, a wide variety of peppers with differnt flavors are used, and
there is some systematic variation of these peppers from dish to dish.
Similarly, the curries of any region of India are families of flavors that are
mixed in varying ways in the menu cycle. In short, the practice of flavor
principles can be seen to represent both sides of the omnivore's dilemma:
variety within a general familiar constraint, or culinary themes and variation.
This may be a case where a culinary institution can be accounted for as a
transformation of a basic food selection biology."