Studying Human Biological Diversity 291consume milk or milk products. Only 10 to 30 percent of
Americans of African descent and 0 to 30 percent of adult
Asians are lactose tolerant.^15 By contrast, lactase retention
and lactose tolerance are normal for over 80 percent of
adults of northern European descent. Eastern Europeans,
Arabs, and some East Africans are closer to northern
Europeans in lactase retention than they are to Asians and
other Africans.
Generally speaking, a high retention of lactase is found
in populations with a long tradition of dairying. For them,
fresh milk is an important dietary item. In such popula-
tions, selection in the past favored those individuals with
the allele that confers the ability to assimilate lactose, se-
lecting out those without this allele.
Because milk is associated with health in North Amer-
ican and European countries, powdered milk has long
been a staple of economic aid to other countries. In fact,
such practices work against the members of populations
in which lactase is not commonly retained into adulthood.
Those individuals who are not lactose tolerant are unable
to utilize the many nutrients in milk. Frequently they also
suffer diarrhea, abdominal cramping, and even bone de-
generation, with serious results. In fact, the shipping of
powdered milk to victims of South American earthquakes
in the 1960s caused many deaths.
Among Europeans, lactose tolerance is linked with
the evolution of a non-thrifty genotype as opposed to
the thrifty genotype that characterized humans until
about 6,000 years ago.^16 The thrifty genotype permits
efficient storage of fat to draw on in times of food short-
age. In times of scarcity, individuals with the thrifty
genotype conserve glucose (a simple sugar) for use in
brain and red blood cells (as opposed to other tissues
such as muscle), as well as nitrogen (vital for growth
and health).
Regular access to glucose through the lactose in milk
led to selection for the non-thrifty genotype as protec-
tion against adult-onset diabetes, or at least its onset rela-
tively late in life (at a nonreproductive age). Populations
that are lactose intolerant retain the thrifty genotype. As acultures throughout the world adopt a Western high-sugar
diet and low activity pattern, their incidence rates will rise
for diabetes and obesity as well.
Another example of culture acting as an agent of bio-
logical selection is lactose tolerance: the ability to digest
lactose, the primary constituent of fresh milk. To digest
milk, the body has to make a particular enzyme—lactase.
Most mammals as well as most human populations—
especially Asian, Native Australian, Native American, and
many (but not all) African populations—do not continue
to produce lactase into adulthood. Adults with lactose in-
tolerance suffer from gas pains and diarrhea when they(^15) Harrison, G. G. (1975). Primary adult lactase deficiency: A problem in
anthropological genetics. American Anthropologist 77, 815–819.
(^16) Allen, J. S., & Cheer, S. M. (1996). The non-thrifty genotype. Current
Anthropology 37, 831–842.
Loss of traditional cultural practices brought about by forced reserva-
tion life has resulted in high rates of diabetes among American Indi-
ans. The Pima Indians of Arizona have the highest rates of diabetes
in the world today. Diabetes was not a problem for the Pima before
the plentiful high-carbohydrate diet and low activity patterns typical
of U.S. culture replaced their traditional lifeways. Despite the socio-
political roots of this disease in their community, the Pima have par-
ticipated in government-funded research aimed at both understanding
the genetic origins of diabetes and finding effective treatment for it.
Here a Pima woman prepares to give herself an insulin injection.
© Terrol Dew Johnson©^
Terrol
Dew
Johnson
lactose A sugar that is the primary constituent of fresh milk.
lactase An enzyme in the small intestine that enables humans
to assimilate lactose.
thrifty genotype Human genotype that permits efficient
storage of fat to draw on in times of food shortage and conser-
vation of glucose and nitrogen.