Evolution, 4th Edition

566 CHAPTER 21

Why do so many of us eat so much that is so bad? Our appetites, digestion, and

metabolism are geared largely to the diet of our preagriculture ancestors. The arrival

of agriculture opened up a world of almost unlimited calories. Suddenly, the ability

to store energy for lean times was maladaptive. Many diseases soared in frequency

with the abrupt changes in lifestyle caused by agriculture, among them heart

attacks, stroke, stomach ulcers, certain cancers, and tooth decay [41]. Some genes

have evolved that partly compensate. In several populations, mutations have spread

at a gene called TCF7L2 that decreases the risk of developing diabetes by affecting

appetite, fat storage, and metabolism [25]. But many, many more generations will go

by before we exhibit full adaptation to our current diet—adaptation that will result

from natural selection caused by higher death rates. By the time populations have

adapted to high-calorie diets, we probably will be eating very different things. Diet

is not the only aspect of modern life that is mismatched with our evolved physiology

and anatomy [41]. Exercise strengthens not only muscles but also bones. Osteopo-

rosis has increased because of our modern sedentary lifestyle.

Humans did not evolve in a sterile environment. Throughout our evolution-

ary history, we have been exposed to diverse bacteria and other microbes from

birth, and we naturally harbor a microbiome of thousands of species throughout

our body. The hygiene hypothesis ascribes the great recent increase in the incidence

of allergies and other autoimmune maladies to insufficient exposure to diverse

microbes. This exposure is necessary, in the first years of life, for proper devel-

opment of our immune system [18, 51]. Humans who grow up in rural environ-

ments and are exposed to farm animals are less likely than city dwellers to develop

autoimmune diseases, ranging from allergies to asthma and inflammatory bowel

diseases. Even parasitic worms have some beneficial effects, reducing the risk of

developing diabetes [2]. Excessive exposure to antibiotics in infancy has many

harmful effects: development of autoimmune disorders, lowered immunity to

viruses, and increased risk of lifelong obesity [20, 60]. Restoring the normal intes-

tinal microbiome by fecal therapy is sometimes necessary to correct a dangerous

imbalance caused by antibiotics (FIGURE 21.22). Basic hygiene is unquestionably

good for us, but a sterile environment is unquestionably bad.

The Evolution of Culture

Anthropologists have uncovered a revolution in human culture that began more

than 50 Kya (FIGURE 21.23). Sophisticated stonework became increasingly wide-

spread. Sublime cave paintings show an aesthetic sense that Picasso admired.

Abstract artifacts suggest mystic or perhaps religious beliefs. Flutes carved from

Futuyma Kirkpatrick bone document the invention of music.Evolution, 4e

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Evolution4e_21.22.ai Date 02-02-2017

Before antibiotic

treatment, the

bacterial population

is diverse.

Antibiotics kill all

bacteria except

resistant individuals.

Antibiotic-resistant

bacteria increase.

Reintroducing

diverse bacterial

species suppresses

the antibiotic-

resistant species.

FIGURE 21.22 Restoring diverse microbes

is sometimes necessary to eliminate harmful

bacteria that proliferate in the intestine after

antibiotic treatment. Antibiotics reduce the

normal diversity of bacteria, often leav-

ing a few resistant harmful species, such as

Clostridium difficile, to proliferate. Reintro-

ducing the full community of other species

suppresses the harmful species. (From [53].)

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