Encyclopedia of Evolution.pdf - Online Reading Center

agriculture, evolution of
Physicist William Thomson (see Kelvin, Lord) calculated
that it may have taken about 100 million years for
the Earth to have cooled from a molten state to its present
temperature. He also calculated a similarly brief period during
which the Sun would have been able to burn without
exhausting its supply of fuel. These calculations, which Kelvin
published in 1866, seemed to place an upper limit of 100
million years for the age of the Earth, in only the last part
of which the Earth was cool enough for life to survive and
evolve upon it. To many scholars, this was insufficient time
for evolution to occur by means of natural selection as
proposed by Charles Darwin (see Darwin, Charles). Some,
like the zoologist Huxley (see Huxley, Thomas Henry) simply
accepted the figure and proposed that evolution, occurring
occasionally by leaps rather than by gradual change, had
in fact occurred during just the final portion of that 100 million
year time span. The problems with Kelvin’s calculations
became apparent early in the 20th century. Kelvin did not
know, until possibly at the end of his life, about radioactivity
(from the fission of elements such as uranium) as a source of
heat energy in the Earth. Radioactivity could therefore have
kept the Earth warm far longer than the 100 million years
required for an Earth-sized ball of lava to cool. Further, he
knew only of combustion as a possible source of energy for
the Sun, not realizing that the fusion of hydrogen atoms
would allow the Sun to have existed for several billion years.
Radioactivity provided a source of energy that had kept
the inside of the Earth warm for billions of years. Radioactive
elements degenerated on a precise and calculable schedule of
half-lives. This fact allowed geologists to develop techniques
of radiometric dating. Radiometric dating techniques
contained some sources of error, but geologists continue to
find ways to avoid these errors, and radiometric dating has
become a very precise method of determining the periods of
time in which different fossilized species lived, the times at
which major Earth catastrophes occurred (see Cretaceous
extinction; Permian extinction), and of the age of the
Earth.
The Earth formed about 4.6 billion years ago. Its initial
heat, plus the impacts of extraterrestrial debris (see asteroids
and comets) prevented the formation of oceans until
about 3.8 billion years ago. According to most evolutionary
scientists, life began shortly after the oceans formed (see origin
of life).
Further Reading
Dalrymple, G. Brent. The Age of the Earth. Stanford, Calif.: Stanford
University Press, 1994.
Gould, Stephen Jay. “Hutton’s purpose.” Chap. 6 in Hen’s Teeth and
Horse’s Toes: Further Reflections in Natural History. New York:
Norton, 1983.
———. Questioning the Millennium: A Rationalist’s Guide to a Precisely
Arbitrary Countdown. New York: Harmony Books, 1997.
agriculture, evolution of Agriculture is the process by
which animals cultivate plants (or occasionally fungi or protists)
for food or other resources. More broadly defined, agriculture
also includes the breeding and raising of livestock
animals. In most cases of agriculture, the crop or livestock
species and the animal species that raises it are mutually
dependent upon one another for survival.
Several species of ants carry out activities that bear striking
parallels to human agriculture. For example, leaf-cutter
ants (genus Atta) cultivate gardens of fungus. Massive foraging
parties of leaf-cutter ants gather pieces of leaf from
many species of tropical plants and carry them back to their
nests. They do not eat the leaves, which contain many toxins.
Instead they chew them up into compost, on which a kind
of fungus grows. The fungus grows nowhere else except in
the mounds of leaf-cutter ants; when the ants disperse, they
take fungus tissue with them. The ants eat fungus tissue and
almost nothing else. Beneficial bacteria that grow on the bodies
of the ants produce chemicals that inhibit the growth of
other bacteria in the compost. For this reason, some biologists
consider these and other ants to be a promising source
of new antibiotics. Because the ants deliberately prepare compost
for the fungus, and because of the mutual dependence of
ants and fungus upon one another, the ant-fungus relationship
can be considered an example of agriculture.
Several species of ants in the seasonal tropics of Central
America live on and in trees of the genus Acacia. The ants
do not eat the leaves of the acacia; instead they consume
nectar that is produced by glands on the stems (not in the
flowers) of the trees, and they eat globules of protein and
fat, called Beltian bodies, that grow on the tips of immature
leaves. The ants chew out the insides of the acacias’ unusually
large thorns, and live inside the thorns. In some cases,
experimental manipulation has shown the ants to be dependent
upon the acacias for survival. The ants attack and kill
other insects and drive away larger animals that attempt
to feed on the acacias. When vines or other plants begin
to grow in the immediate vicinity of the acacias, the ants
sting them or chew them down. In most cases, the acacias
are completely dependent upon the ants; when the ants are
experimentally removed, vines overgrow the acacias, and
animals browse the leaves heavily. The acacias remain green
during the dry season, when most of the other trees lose
their leaves; but these green targets go undisturbed by herbivores,
because of their protective ant army. Because the ants
weed out other plants from the vicinity of the acacias, and
defend their crops, and because of the mutual dependence
of ant and acacia, the ant-acacia relationship can be considered
an example of agriculture. When, about 10,000 years
ago, agriculture evolved in the human species, it was not the
first time that agriculture had evolved on this planet. Hereinafter,
“agriculture” refers to human agriculture.
Some scholars used to believe that human agriculture was
invented by a brilliant man in a tribal society of hunter-gatherers.
Other scholars pointed out that, since women gathered
most of the plant materials, agriculture was probably invented
by a woman. Both the brilliant-man theory and the brilliantwoman
theory are incorrect, however, because agriculture
could not have been invented in a single step by anyone. It had
to evolve. Agriculture had to evolve because unmodified wild
plants are unsuitable for agriculture. There are four reasons: