In nearly all human populations, a majority of individuals can taste the artificially synthesized chemical phenylthiocarbonide (PTC). However, the percentage varies dramatically—from as low as 60 percent in India to as high as 95 percent in Africa. That this polymorphism is observed in nonhuman primates as well indicates a long evolutionary history which, although obviously not acting on PTC, might reflect evolutionary selection for taste discrimination of other, more significant, bitter substances, such as certain toxic plants.

A somewhat more puzzling human polymorphism is the genetic variability in earwax, or cerumen, which is observed in two varieties. Among European populations, 90 percent of individuals have a sticky yellow variety rather than a dry, gray one, whereas in northern China these numbers are approximately the reverse. Perhaps like PTC variability, cerumen variability is an incidental expression of something more adaptively significant. Indeed, the observed relationship between cerumen and odorous bodily secretions, to which nonhuman primates—and to a lesser extent humans—pay attention suggests that during the course of human evolution genes affecting body secretions, including cerumen, came under selective influence.

1. It can be inferred from the passage that human populations vary considerably in their

(A) sensitivity to certain bodily odors.
(B) capacity for hearing.
(C) ability to assimilate artificial chemicals.
(D) vulnerability to certain toxins found in plants.
(E) ability to discern bitterness in taste.


2. Which of the following provides the most reasonable explanation for the assertion in the first paragraph that evolutionary history “obviously” did not act on PTC?

(A) PTC is not a naturally occurring chemical but rather has been produced only recently by scientists.
(B) Most humans lack sufficient taste sensitivity to discriminate between PTC and bitter chemicals occurring naturally.
(C) Variability among humans respecting PTC discrimination, like variability respecting earwax, cannot be explained in terms of evolutionary adaptivity.
(D) The sense of taste in humans is not as discriminating as that in nonhuman primates.
(E) Unlike nonhuman primates, humans can discriminate intellectually between toxic and nontoxic bitter substances.


3. Which of the following best expresses the main idea of the passage?

(A) Artificially synthesized chemicals might eventually serve to alter the course of evolution by desensitizing humans to certain tastes and odors.
(B) Some human polymorphisms might be explained as vestigial evidence of evolutionary adaptations that still serve vital purposes in other primates.
(C) Sensitivity to taste and to odors have been subject to far greater natural selectivity during the evolution of primates than previously thought.
(D) Polymorphism among human populations varies considerably from region to region throughout the world.
(E) The human senses of taste and smell have evolved considerably over the course of evolutionary history.