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).

Over time, selective breeding modifies teosinte's few fruitcases (left) into modern corn's rows of exposed kernels (right). (Photo courtesy of John Doebley.). From: Genetically Modified Corn— Environmental Benefits and Risks Gewin V PLoS Biology Vol. 1, No. 1, e8 doi:10.1371/journal.pbio.0000008

Domestication involves changes in the genetic makeup and morphological appearance of plants (and animals) but the wild-relatives of crop continue today to be important sources of genetic diversity and of traits for protection of domesticated crops from stress and disease (see Plant breeding) and to ensure food security. Knowledge of crop origins is thus of considerable practical importance.

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 ):

• 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

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).

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.. 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.

Rice domestication in Asia and Western Africa

See also Rice

Maize and bean domestication in the Americas

See also Maize

New crops by hybridization and polyploidy

See also Plant breeding

References

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.. 

• 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.. 

External links

• Damania, A. B., J. Valkoun G. Willcox C.O. Qualset (1997) The Origins of Agriculture and Crop Domestication, Proceedings of the Harlan Symposium 10-14 May 1997 Aleppo, Syria

• Lev-Yadun, Simcha , Gopher, Avi, Abbo, Shahal (2000) The Cradle of Agriculture. Science 2 June 2000: Vol. 288. no. 5471, pp. 1602 - 1603 DOI: 10.1126/science.288.5471.
• PLB143: Evolution of Crop Plants The origins of agriculture and the domestication of plants Spring Quarter, Paul Gepts, Department of Agronomy and Range Science, University of California, Davis

• Bruce D. Smith (2001) Documenting plant domestication: The consilience of biological and archaeological approaches PNAS | February 13, 2001 | vol. 98 | no. 4 | 1324-1326