Encyclopedia of Evolution.pdf - Online Reading Center

0 AIDS, evolution of
to stay in one place at least long enough for one harvest.
Agriculture promoted the rise of civilization. Civilization
rose earlier in Mesopotamia than in other places because
agriculture began earlier there. With civilization came
advanced technology. Because they needed to defend particular
tracts of territory, the armies now had a lot more to
fight about. The cultural groups that developed agriculture
first were the first to be civilized and to have advanced technology,
which allowed them to conquer the cultural groups
in which this process had not progressed as far. Biologist
Jared Diamond explains that this is why Europeans conquered
America, driving natives into reservations, rather
than Native Americans conquering Europe and driving
Europeans into remote corners of the Alps and Pyrenees.
The evolution of agriculture is a perfect illustration that
evolution does not operate for the good of the species (see
group selection). Agriculture did not improve the average
health of human beings. In fact, the average life span in early
agricultural societies was less than that in contemporaneous
hunter-gatherer societies. This occurred for two reasons:
• Diseases spread more rapidly in cities in which people were
trapped with one another’s wastes, garbage, and germs.
• Agriculture actually decreased the quality of human nutrition
by making people dependent upon a few crop plants
rather than a diversity of wild foods. In particular, the
human body evolved under conditions in which ascorbic
acid (vitamin C) was readily available from wild fruits.
When entire populations became dependent upon crops
with little vitamin C, scurvy became a way of life.
Along with agriculture, herding began, starting with the
wild animal species most amenable to survival and breeding
under captivity. Unconscious natural selection, then conscious
artificial selection, resulted in the evolution of livestock species,
such as the cow, which evolved in western Eurasia and
Southeast Asia from two distinct wild species. Once again,
the earlier development of herding in the Middle East than in
other areas occurred because many wild animal species of the
Middle East (such as goats and sheep) were amenable to herding,
while wild animal species such as deer in North America
were not. Livestock animals provided high quality food (meat
and milk), often by consuming wild foods that humans could
not eat. This was especially true of goats. Pigs, on the other
hand, eat many of the same foods as humans. This competition
between pigs and humans for food may be one reason
that pigs were considered undesirable (“unclean”) by some
cultures in the arid regions of the Middle East. As with agriculture,
herding caused a narrowing of the food diversity base
from many wild animal species to a few livestock species.
During the migrations of people during the past few
thousand years, new assortments of genes have occurred, producing
racial diversification. The human species as a whole
has undergone no significant evolutionary changes during
the time since the origin of agriculture. Humans a hundred
thousand years ago were physically indistinguishable (as
far as is known from fossil remains) from modern humans.
Nine-tenths of the history of the human species occurred
under hunter-gatherer conditions, without significant genetic
evolution. Therefore, while the evolution of agriculture is a
prime example of the evolution of mutualism, it is not an
example of coevolution, because the crop species genetically
evolved, and the humans did not.
Further Reading
Diamond, Jared. Guns, Germs, and Steel: The Fates of Human Societies.
New York: Norton, 1997.
Kislev, Mordechai E., Anat Hartmann, and Ofer Bar-Yosef. “Early
domesticated fig in the Jordan Valley.” Science 312 (2006): 1,372–
1,374.
Konishi, Saeko, et al. “An SNP caused loss of seed shattering during
rice domestication.” Science 312 (2006): 1,392–1,396.
Rindos, David. The Origins of Agriculture: An Evolutionary Perspective.
San Diego: Academic Press, 1984.
Tenno, Ken-ichi, and George Willcox. “How fast was wild wheat
domesticated?” Science 311 (2006): 1,886.
AIDS, evolution of HIV, the human immunodeficiency
virus, can ultimately result in Acquired Immune Deficiency
Syndrome (AIDS) in humans. Many scientists and public
health professionals consider AIDS to be the major epidemic of
modern times. Evolutionary science contributes greatly to an
understanding of the origin of and changes within this disease.
The human immunodeficiency virus (HIV) infects some
human white blood cells and eventually causes them to die.
This partial disabling of the human immune system allows
opportunistic infections to occur as normally harmless
microbes become parasitic (see coevolution). Because this
immune deficiency is acquired through infection, and results
eventually in a whole set of symptoms, it is called Acquired
Immune Deficiency Syndrome (AIDS). Most people, when
not infected with HIV, can easily resist microbes such as the
cytomegalovirus, the bacterium Mycobacterium avium, or the
protist Pneumocystis carinii, but these microbes can cause
complications such as fatal pneumonia if HIV partially disables
the immune system. The immune system apparently
also protects the human body from viruses that can induce
certain kinds of cancer, such as Kaposi’s sarcoma, which also
afflict people infected with HIV.
HIV infection spreads slowly within the body of the victim.
After an initial acute phase of infection, during which the
immune system launches a partly effective response, a chronic
phase of invisible spread follows. It may be several years after
HIV infection, during which individuals are HIV-positive,
before those individuals exhibit symptoms resulting from the
loss of immune competence, at which time they are said to
have AIDS. There are different strains of HIV, of which HIV-
1 is the most common worldwide.
HIV is a retrovirus, which means that it stores its genetic
information in the form of RNA and is able to transcribe this
information backward into DNA. In contrast, most viruses
and all living cells store genetic information in DNA (see
DNA [raw material of evolution]). HIV carries, inside its
protein coat, two molecules of an enzyme known as reverse
transcriptase, which makes DNA from the genetic information
in its RNA. HIV also carries an integrase enzyme, which