Crop origins and evolution
The origins of agriculture and domesticated crops are intertwined, and the change from a hunter-gatherer mode to tillage, sowing and harvesting was one of the major technologcal innovations of humankind. This occured some 10,000 years ago in several different locations, and involved the domestication of wild-relatives of the major crops (see History of Agriculture).
Domestication involves changes in the genetic makeup and morphological appearance of plants (and animals). These changes occur because people select the variations of a wild plant that best suit their needs. If the desired features can be passed to offspring, over generations of planting and harvesting, the strains of plants grown change. Despite the fact that these domesticated varieties of plants are preferred over their original forebears, the wild-relatives of crop plants continue to be an important resource. Reserves of wild plants offer a pool of genetic diversity. Traits of plants that might have been lost in domestication, sometimes become crucial for protection of domesticated crops from stress and disease (see Plant breeding). Maintaining wild strains for their gene pool helps ensure food security. Knowledge of crop origins is thus of considerable practical importance, even when these original strains are no longer harvested as crops.
Development today of new crops (such as perennial alternatives of currently used annual staples) has potential value is helping meet serious current agricultural challenges such as the need for water use efficiency, better management of land salinization, and soil conservation.
Tracing the ancestors of crops
A Swiss botanist, Alphonse de Candolle started studies of the origins of crops in 1885, and propsed two approaches to anserwing these questions. The first is identification of the geographical distribution wild-relatives of modern crops, based on careful botanical desriptions and tests for cross-pollination between candidate ancestors and the crop in question. Second, archaeological studies of provide clues on how and when a transition from hunter-gatherer existence to agriculture occured.
| Age of earliest C-14 dated crop remains | ||
| Location | Crop | Age |
| (years before present) | ||
| Mesoamerica | Squash | 10,000 |
| Mesoamerica | Maize | 6,300 |
| Central America | Cassava, Dioscorea yam, | 7,000 to 5,000 |
| arrowroot, maize | ||
| Fertile Crescent | Einkorn wheat | 9,400 to 9,000 |
| Fertile Crescent | Lentil | 9,500 to 9,000 |
| Fertile Crescent | Flax | 9,200 to 8,500 |
| China | Rice | 9,000 to 8,000 |
(After Paul Gepts, 2003, in Chapter 13 of Plants, Genes and Crop Biotechnology, Chrispeels and Sadava.)
The earliest origins of major crops based on carbon-14 dating, date back around 10,000 years, just after the end of the last ice age. This allows the geographical regions in which crop domestication took place to be identified.
Six independent centers of crop origin can be nominated [1]):
- Mesoamerica (Southern Mexico and Northern Central America): Maize, Phaseolus beans, Sweet potato, tomato
- The Andes of South America: Potato, cassava (manioc), pineapple
- Southwest Asia (including the "Fertile Crescent": Wheat, barley, pea, lentil
- The Sahel region and Ethiopian highlands of Africa: Sorghum, coffee, melon, watermelon
- China: Asian rice, soybean, adzuki bean, orange, apricot, peach, tea
- Southeast Asia: Cucumber, banana, plantain
Wheat domestication in the Middle East
- See also Wheat
Wheat is a term for a number of related cereals in the genus Triticum [2]
The first domesticated wheat was einkorn wheat, which is cultivated today only as an animal feed in mountainous regions of Spain and Turkey. Einkorn wheat (T. monococcum) is diploid (2 chromosomes).[3]
There is a second lineage of wheats derived from wild emmer, T. dicoccoides which includes tetraploid and hexaploid wheats. Wild emmer is tetraploid (AABB) result of a hybridization between two diploid wild grasses, T. urartu(AA) and a wild goatgrass Ae. speltoides(BB). The hybridization that formed wild emmer occurred in the wild, long before domestication.[4]. From emmer were derived tetraploid domesticated emmer (AABB), and the tetraploid durum (AABB) wheats.
Hexaploid (AABBDD) wheats evolved in farmers' fields when wheat cultivation moved to regions south of the Caspian sea. Either emmer or durum wheat hybridized with yet another wild diploid grass in that region (Aegilops tauschii (DD)) to make the hexaploid (6 chromosomes, AABBDD) wheats, spelt wheat and bread wheat.[4]
Rice domestication in Asia and Western Africa
- See also Rice
Maize and bean domestication in the Americas
- See also Maize
Genetic events during domestication
New crops by hybridization and polyploidy
- See also Plant breeding
References
Citations
- ↑ Gepts, P. (2001) Origins of plant agriculture and major crop plants In M. K. Tolba, Ed., Our Fragile World:Challenges and Opportunities for Sustainable Development, EOLSS Publishers, UK, pages 629-637.
- ↑ Hancock, James F. (2004) Plant Evolution and the Origin of Crop Species. CABI Publishing. ISBN 0-85199-685-X.
- ↑ Cite error: Invalid
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Further reading
- Damania, A.,J., Valkoun, G. Willcox, and C. Qualset, eds. (1998). The Origins of Agriculture and Crop Domestication, 1st ed.. International Center for Agricultural Research in the Dry Areas, Aleppo, Syria.
- de Candolle, A. (1992). Origin of Cultivated Plants, 1sh ed.. Cambridge, U.K..
- Diamond, Jared (1997). Guns, Germs and Steel: A Short History of Everybody for the Last 13,000 Years, 1st ed.. Vintage. ISBN 0-09-939278-0.
- Frankel, O. H., A. H. D. Brown, and J.J. Burdon. (1995). The Conservation of Plant Biodiversity, 1st ed.. Cambridge University Press, Cambridge, U.K..
- Gepts, Paul. Chapter 13. Ten thousand years of crop evolution. In Chrispeels, Maarten J.; Sadava, David E. (editors) (2003). Plants, Genes and Crop Biotechnology, 2th ed.. Jones and Bartlett. ISBN 0-7637-1586-7.
- Hancock, J.F. (2004). Plant Evolution and the Origin of Crop Species, 2th ed.. CAB International, Wallingford, UK.. ISBN 0-85199-685-X.
- Harlan J. R. (1992). Crops and Man, 2nd ed.. American Society of Agronomy, Madison, WI.
- Sun, C. et al. (1998) From indica and japonica splitting in common wild rice DNA to the origin and evolution of Asian cultivated rice. Agricultural Archaeology 1998:21-29
- Vavilov, N. I. (1997). Five Continents.. Rome: International Plant Genetic Resources Institute; St. Petersburg: N. I.Vavilov All-Russian Institute of Plant Industry..