Humans as Consumers of Plants

Given that most wild plants contain chemicals that make nutrients unavailable to herbivores, make the plant taste nasty and/or make an animal sick, how did humans learn how to use plant parts, other than ripe fruits and nuts, as food? Because primates also do this, geophagy (eating dirt that is rich in clay, which binds with toxins) is probably the first detoxification method used by early humans to exploit plant parts such as leaves and roots. Geophagy is a method still used today (e.g., clay is the active ingredient

Table 1 Plant defense hypotheses

Major hypotheses about plant defense Application or viewpoint Major theme

Table 1 Plant defense hypotheses

Optimal defense

Evolutionary (genotypic), but also

Cost of defense, for whole plant or parts, drives evolution of

ecological (phenotypic)


Growth rate

Evolutionary (genotypic)

Resources shape evolved growth rate, which determines defensive profile of plant

Carbon:nutrient balance

Ecological (phenotypic)

Nitrogen availability relative to carbon accumulation determines defensive profile of plant


Ecological (phenotypic), but also

When growth moderately inhibited, allocation to defense is


evolutionary (genotypic)

greatest, due to surplus carbon

in the antidiarrheal Kaopectate). Cooking is another major detoxification method, which probably was not common before about 40 000 years ago. Plant breeding is the third major detoxification method, which developed gradually with the invention of agriculture beginning about 11 000 years ago. However, such domestication of plants has made them more vulnerable to agricultural pests.

To increase plant resistance to pests, now genetically engineered crops are being developed. However, such a cultivar may only have resistance for a few years because that plant genotype acts as a selective pressure on pests and eventually a resistant pest will occur. Creating fields and forests of mixtures of different pest-resistant genotypes may be one solution, but developing enough different types that are resistant without loss in yield (i.e., defense can have a cost to the plant) is a challenge.

Despite how much has been discovered about plant defense in the last 50 years, the concept of plant defense is counterintuitive for modern humans. As people increasingly consume domesticated plants rather than wild plants and depend on medicine dispensed as pills, they have less experience with and so less understanding of the natural defenses of plants. Today most humans eat domesticated plants that have been bred for palatability and yield, and so are much less defended than the wild plant relatives. Thus, the idea that plants have defenses against herbivores is often new (and surprising) to people. For instance, a survey of college students indicated that only about a third realized that caffeine in coffee is a poison that protects coffee plants from some herbivores; others thought caffeine was needed for plant metabolism, was a waste product of plant metabolism, or due to plant breeding by humans. The use of herbal supplements, especially in developed nations, is on the rise. The active ingredients of these supplements are plant-defensive chemicals. Herbal supplements used in large amounts or in combination with other chemicals (including medications) can be harmful. For example, ephedra (containing ephedrine alkaloids) is used for weight loss and enhancement of sports performance. In combination with stimulants such as caffeine, these substances result in various health problems, due to magnified effects on the circulatory and respiratory systems.

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