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June 27, 2008
Paleobotanical evidence pushes
back the time of domestication
Source: American
Society of Plant Biologists (ASPB)
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, 8:30
AM).
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. |
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