562 CHAPTER 21major genetic change can occur in so short a time only if selection is extremely
strong. But as you will soon see, the change to agricultural societies caused such
a radical transformation of our environment and lifestyle that it left evolutionary
skid marks across our genome.
Agriculture has great benefits: it is the foundation for human civilization. But
agriculture also has great costs, to both the health of humans and the planet on
which we live. The biologist and anthropologist Jared Diamond suggested that
agriculture is “the worst mistake in the history of the human race” [12].
Agriculture required changing from a nomadic life to a sedentary one. The
cultural and ecological consequences were enormous. Humans established per-
manent settlements that increased in size and political complexity. Reproductive
rates skyrocketed, and the human population has been growing exponentially ever
since. Agriculture began the devastation of habitats that today threatens countless
species. It introduced many plants and insects to new regions where they have
become invasive enemies of native species. The dense and sedentary populations
in villages, which grew into towns and then cities, triggered outbreaks of infec-
tious diseases. The most devastating was malaria, which became widespread
about 15 Kya and still kills more people than any other disease. More than 50
other diseases, including influenza, tuberculosis, and diphtheria, were acquired
from domesticated animals.
Agriculture radically changed people’s diet, and not entirely for the better [37].
Populations became dependent on a few foods (such as potatoes, rice, and corn),
and they suffered famine when crops failed. Most of the food was, and still is, low
in vitamins and important nutrients but high in carbohydrates. Access to almost
limitless calories—at least when harvests were good—created conditions for obe-
sity, diabetes, and other diet-related diseases to develop.Natural Selection, Past and Present
Agriculture profoundly changed the environment in which humans were living.
It is no surprise that it caused new types of selection on our ancestors. What is a
surprise, however, is how strong and widespread that selection was across the
genome.
Very recently, human geneticists have developed an evolutionary time machine.
It is now possible to sequence the ancient DNA from skeletons unearthed by arche-
ologists. By comparing them with sequences from people still living in the same
place, we can see for the first time directly how gene frequencies changed in time.
Mathieson and colleagues compared DNA from 230 individuals that lived between
8500 and 2300 years ago in Europe and western Asia with samples from over two
thousand living humans [46]. Twelve genes show large swings in allele frequencies
between the past and present, the “smoking gun” of adaptive evolution (FIGURE
21 .19). Four of the 12 genes are involved in adapting to the new diet that came with
an agricultural lifestyle, which arrived in Europe less than 10 Kya.
The strongest signal of selection is on a mutation responsible for lactase persis-
tence. Lactase is the enzyme that digests lactose in mother’s milk. In our ancestors
and in many populations today, the lactase gene turns off after weaning. But in
populations that domesticated livestock and consumed dairy products, there was
a strong fitness advantage to keeping that gene turned on so that adults could
digest lactose. Mutations with that effect became established independently sev-
eral times in Europe, Africa, and the Middle East [74]. Analysis of the ancient DNA
from Europe showed that the mutation appeared there just 4500 years ago [46]. It
rapidly swept nearly to fixation in northern Europe, where today about 98 percent21_EVOL4E_CH21.indd 562 3/22/17 1:51 PM