080627163156.htm
Maize (Corn) May Have Been Domesticated In Mexico As Early As 10,000
Years Ago
ScienceDaily (June 27, 2008) — The ancestors of maize originally grew
wild in Mexico and were radically different from the plant that is now
one of the most important crops in the world. While the evidence is
clear that maize was first domesticated in Mexico, the time and
location of the earliest domestication and dispersal events are still
in dispute.
Now, in addition to more traditional macrobotanical and archeological
remains, scientists are using new genetic and microbotanical
techniques to distinguish domesticated maize from its wild relatives
as well as to identify ancient sites of maize agriculture. These new
analyses suggest that maize may have been domesticated in Mexico as
early as 10,000 years ago.
Dr. John Jones and his colleagues, Mary Pohl, and Kevin Pope, have
evaluated multiple lines of evidence, including paleobotanical remains
such as pollen, phytoliths, and starch grains, as well as genetic
analyses, to reconstruct the early history of maize agriculture. Dr.
Jones, of the Department of Anthropology, Washington State University,
Pullman, will be presenting this work at a symposium on Maize Biology
at the annual meeting of the American Society of Plant Biologists in
Mérida, Mexico (June 28, 2008).
While macrobotanical remains such as maize kernels, cobs, and leaves
have been found in dry mountain caves, such remains are not preserved
in more humid lowland areas, so the conclusions based on such remains
are fragmentary. Much smaller parts of the maize plant, like cellular
silica deposits, called phytoliths, and pollen and starch grains, are
preserved under both humid and dry conditions. These lines of
evidence, along with genetic and archeological data, are being used to
reconstruct the history of agriculture to its origins around the
world.
Maize is wind pollinated and sheds large amounts of pollen, which is
deposited in soil and water sediments. The tough outer wall (exine) of
pollen protects it from deterioration for thousands of years. While it
is possible to distinguish the pollen grains of maize and its close
relatives from other grasses, it is more difficult, except at the
largest sizes, to differentiate the pollen of maize (Zea mays) from
its presumed wild ancestor teosinte (Zea sp). Thus, while pollen can
provide evidence of the presence of domesticated maize, along with
that from other plants indicating agricultural activity, maize pollen
alone is not definitive evidence of domesticated plants.
Phytoliths are another type of plant microfossil that is preserved for
thousands of years and can be used to distinguish domesticated from
wild maize. These microscopic bodies are silica or calcium oxalate
deposits that accumulate in the intercellular spaces of plant stems,
leaves, and roots and have characteristic shapes depending on genus
and species. They are preserved even when the plant is burned or
disintegrated. Scientists have found that it is possible to
distinguish the microliths of teosinte from those of maize and other
grasses, thus allowing them to identify the approximate dates and
locations of early agricultural activity. Phytoliths are also
preserved on ceramic and stone artifacts used to process food.
Jones and his co-workers analyzed the sediments from San Andrés, in
the state of Tabasco on the Mexican Gulf Coast. Analysis of area
sediments revealed phytoliths of domesticated varieties of maize as
well as those of agricultural weeds. These data, along with evidence
of burning, suggested that agriculturalists were active in that part
of the Yucatan Peninsula around 7,000 years ago.
Starch grains are the most recent addition to the archeobotanical
toolbox. Maize and its grass relatives produce large quantities of
starch grains with unique morphological characteristics and, like
phytoliths, are preserved in sediments and on cultural artifacts.
Maize produces more starch than its wild relative teosinte, and the
grains are much larger. The paleobotanist Dolores Piperno and her
colleagues have established a number of criteria for distinguishing
the starch grains of different grasses and found that those of maize
and teosinte could be reliably separated on the basis of size and
other morphological characters.
Maize also has a rich genetic history, which has resulted in thousands
of varieties or landraces adapted to different environmental
conditions. Maize scientists and geneticists have used this
information to track the evolution and dispersal of maize varieties as
well as to reconstruct the history of maize domestication. For
example, the locus teosinte glume architecture 1 (tga1), is important
in determining phytolith formation and morphology and, along with
other "domestication genes" can be used to write the history of maize
domestication and use by humans.
All of these methods are being used by paleobotanists, plant
scientists, and archeologists like Jones and his colleagues, to
reconstruct the rich history of maize domestication and evolution.
Many of the ancient varieties were adaptations to different
environmental conditions such as different soils, temperature,
altitude, and drought. Preservation of these varieties and knowledge
of their genetic and adaptive histories are of paramount importance as
farmers around the world cope with changes in soil, temperature, and
water availability and struggle to maintain a food supply for growing
populations.
the press release
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