Crop origins and evolution

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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. There is good evidence that this occurred 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). 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 in 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 proposed two approaches to answering these questions. The first is identification of the geographical distribution of wild-relatives of modern crops, based on careful botanical descriptions and tests for cross-pollination between candidate ancestors and the crop in question.

Second, archaeological studies often provide clues on how and when a transition from hunter-gatherer existence to agriculture occurred.

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]):

Wheat domestication in the Middle East

See also Wheat

Wheat is a term for a number of related cultivated cereals in the genus Triticum [2] that originated from wild-grasses some 10,000 years ago in the general region of current day Iraq.

The earliest domesticated wheats were einkorn wheat (T. monococcum) which is diploid (2 sets of chromosomes) and emmer (a tetraploid) .

Recent findings narrow the first domestication of wheat down to a small region of southeastern Turkey.[3]

Genetic analysis of wild einkorn wheat indicates that this wheat was first domesticated in the Karacadag Mountains in southeastern Turkey. Dated archeological remains of einkorn wheat in settlement sites near this region, including those at Abu Hureyra provides confirmation for the domestication of einkorn near the Karacadag Mountain Range. The earliest carbon-14 date for the einkorn wheat remains at Abu Hureyra is 7800 to 7500 years BCE.[4] Recent genetic and archeological discoveries indicate that emmer wheat and durum (hard pasta wheat) originated from this same Karacadag region of southeastern Turkey. Remains of harvested emmer from several sites near the Karacadag Range have been dated to between 8,800 BCE and 8,400 BCE, that is, in the Neolithic period.[5]

Cultivation and repeated harvesting and sowing of the grains of wild grasses led to the creation of domestic strains, as mutant forms ('sports') of wheat were preferentially chosen by farmers. In domesticated wheat, grains are larger, and the seeds (spikelets) remain attached to the ear by a toughened rachis during harvesting. In wild strains, a more fragile rachis allows the ear to easily shatter and disperse the spikelets.[6] Selection for these traits by farmers might not have been deliberately intended, but might simply have been because these traits made gathering the seeds easier; nevertheless such 'incidental' selection was an important part of crop domestication. As the traits that improve wheat as a food source also involve the loss of the plant's natural seed dispersal mechanisms, highly domesticated strains of wheat cannot survive in the wild.

Cultivation of wheat subsequently to spread beyond the Fertile Crescent after about 8,000 BCE. Jared Diamond traces the spread of cultivated emmer wheat starting in the Fertile Crescent about 8500 BCE, reaching Greece, Cyprus and India by 6500 BCE, Egypt shortly after 6000 BCE, and Germany and Spain by 5000 BCE [7].

Wild emmer T. dicoccoides is a tetraploid (AABB) resulting from 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.[8]. From emmer were derived tetraploid domesticated emmer (AABB), and the tetraploid durum (AABB) wheats.

Hexaploid (AABBDD) wheats first 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 and bread wheat.[8]

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

  1. 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.
  2. Hancock, James F. (2004) Plant Evolution and the Origin of Crop Species. CABI Publishing. ISBN 0-85199-685-X.
  3. Lev-Yadun S et al. (2000) The cradle of agriculture Science 288471:1602-3 PMID 10858140
  4. Heun MR et al (1997) Site of Einkorn Wheat Domestication Identified by DNA Fingerprinting Science 278:1312-4 DOI: 10.1126/science.278.5341.1312
  5. Özkan H et al. (2002) AFLP analysis of a collection of tetraploid wheats indicates the origin of emmer and hard wheat domestication in southeast Turkey Molecular Biology and Evolution19:1797-1801 (2002) PMID 12270906
  6. Tanno, K Willcox, G (2006) How fast was wild wheat domesticated? Science 311 1886 DOI 10.1126/science.1124635 PMID 16574859
  7. Diamond J (1997) Guns, Germs and Steel, A short history of everybody for the last 13,000 years. Viking UK Random House ISBN 0-09-930278-0
  8. Cite error: Invalid <ref> tag; no text was provided for refs named Hancock

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. 

External links