23.2: The Origins of Crop Plants

Domestication of Plants and Animals

Tremendous natural variations exist among the individuals of any plant species. The traits that define color, shape, flavor, height, yield, and resistance to pests, pathogens, and environmental stresses are not fixed within a species. Individual plants and animals from the same species can be easily distinguished based on these characteristics.

Since the beginning of agriculture, humans have unconsciously been selecting plants and animals with desirable traits, such as large-sized grains, pods, fruits, and vegetables; sweeter and less-seeded fruits; less bitter and nonprickly vegetables; cereals with large panicles and tough rachis; non-seed-shattering plants; and so on. As a consequence of such artificial selections over many generations, unprecedented changes occurred in cultivated plants that set them apart from their ancestors and wild relatives. For example, the relentless efforts of humans led to the development of various crops, such as corn from a wild-grass teosinte; long-spiked, six-row barley from short-spiked, two-row wild barley; large tomatoes from a small berry; and a variety of less-seeded fruits and palatable vegetables from their bitter wild ancestors (see figures 2.1 and 2.2). These plants—enriched in traits that favor higher yields, productive harvest, and increased palatability—would not have come into being without the persistence of humans since the dawn of agriculture.

Artificial selection by humans counteracts the process of natural selection. In nature, small fruits are packed with seeds, thistles, thorns, and prickly leaves; have a bitter taste; ripen asynchronously; and have seeds that spontaneously shatter—all traits that favor the survival of the plants. Thus artificially selected and propagated species of cultivated plants, lacking necessary traits for survival, become more vulnerable to diseases, predators, and environmental stresses.

Fig 2.1 Maize is a product of artificial selection by human. A comparison of maize’s ancestor teosinte (Zea mays ssp. parviglumis) and maize (Zea mays). Photo of maize and teosinte by Hugh Iltis is open source.

Fig 2.2 “Changes in crop plants as a result of artificial selection” by Sushma Naithani and OSU OERU is licensed under CC BY 4.0.

For several millennia, humans have put tremendous effort into providing protection and ensuring the continuous propagation of cultivated plants. We provide fertilizers, pesticides, and water and provide services such as weeding to promote the growth of crop plants. Thus domesticated plants need humans for their survival as much as human survival depends on them. These species cannot survive in nature for a long time by themselves, but with human help, they have spread globally. For some species, this dependency on humans has become total. For example, maize absolutely depends on humans for its survival. If you leave a mature cob in the field, some of its seeds may germinate on the cob, but they will soon die due to the lack of space for emerging seedlings to grow. Furthermore, maize seeds do not fall spontaneously and need human help to be detached from the cob and planted in the soil.

This mutual interdependence between crop plants and humans (and, similarly, between humans and artificially selected and bred animals) was achieved over several millennia, and this is the historical process we call “domestication.” Thus to a great extent, all crop plants and domesticated animals are man-made.

Landraces of Crop Plants

n addition to artificial selection, cultivated plants were continuously subjected to the natural selection imposed by their immediate environment, geographical locality, and agricultural practices. Thus domesticated plants are products of artificial selection operating within environmentally enforced natural selection and the agricultural practices prevalent in a given region. The early domesticated plants flourished in their native environment, but when shifted to new locations, they performed poorly. Over time, the few offspring of the introduced plants acquired characteristics (i.e., via spontaneous mutations or hybridization with related species present in the new locality) that helped them stabilize in their new surroundings. This process has produced diversified varieties of crops known as landraces, each adapted to a specific geography, climate, or environment (seasonal variations in day length, temperature, water availability, soil quality, salinity, etc.) and its associated pests and pathogens.

Fig 2.3 Examples of some of the 59 native Mexican maize landraces. Photo of maize by CIMMYT Maize Germplasm Bank is licensed under CC BY 2.0.

For centuries, farmers have been stocking seeds of different varieties separately to ensure their purity and have been careful about not mixing the seeds from multiple varieties. To avoid cross-contamination, farmers usually save seeds for the next year’s sowing from the plants growing in the middle of their fields. This way, the individual varieties are maintained, but their independent evolution and development are also continued. The landraces of the various crops that have survived today are not 100 percent identical to their ancestors that existed 200 or 1,000 years ago. They are constantly changing, keeping in tune with their growing environment and adapting to the changes in it.

Centers of Crop Domestication

Nikolai Vavilov, a Russian agricultural scientist, was one of the first scientists in the world to infer that the process of domestication—the enrichment of desirable traits by human/artificial selection—also led to the loss of many useful traits (see figure 2.4). He noted that in comparison to their wild relatives, most crops easily succumb to parasites, pests, and pathogens and are less resilient under unfavorable environmental conditions. Vavilov proposed that these lost traits can be traced back to the wild progenitors and related species of crop plants, which are likely to still be present in the regions where crops were first domesticated. He further proposed that useful traits can be reintroduced in crops by employing the kind of systematic plant breeding rooted in the principles of Mendelian genetics.

Fig 2.4 Loss of gene pool during crop domestication due to artificial selection. “Loss of gene pool” by Sushma Naithani and OSU OERU is licensed under CC BY 4.0.

In the 1920s, knowledge about the domestication centers of crop plants was lacking. So Vavilov took this herculean task upon himself and set forth on a mission to collect the germplasm (seeds, tubers, roots) of all domesticated crops and their wild relatives. He led nearly a hundred expeditions to sixty-four countries on five continents over the course of twenty years and built the world’s biggest plant germplasm collection, which included 350,000 accessions of seeds, roots, and tubers representing about 2,500 plants. Based on the study of this comprehensive collection and observations about human cultures and linguistics, Vavilov proposed eight geographic centers as the birthplaces or “centers of origin” of crops—the places where the ancestors, wild relatives, and other related species of crops still live and where mankind first began their cultivation. These eight centers include China, India and the Indo-Malayan region, Central Asia (including Pakistan, Afghanistan, Turkestan, and the northwest Indian provinces of Punjab and Kashmir), the Near East, the Mediterranean, Ethiopia, southern Mexico and Central America, and South America (Ecuador, Peru, Bolivia, Chile, and Brazil-Paraguay; see figure 2.5). Overall, Vavilov associated about 640 crops with their biodiversity centers. Five-sixths of these came from the Old World (Asia, Africa, Europe) and one-sixth from the New World (Australia, North and South America).

Fig 2.5 The centers of origin of cultivated plants identified by Nikolai Vavilov: 1. China; 2. India; 2a. the Indo-Malayan region; 3. Central Asia (including Pakistan, Punjab, Kashmir, Afghanistan and Turkestan); 4. the Near East (Fertile Crescent); 5. the Mediterranean; 6. Ethiopia; 7. Southern Mexico and Central America; 8. South America (small regions of Ecuador, Peru, Bolivia, Chile, and Brazil-Paraguay). “Centers of origin of cultivated plants” by OSU OERU is licensed under CC BY 4.0.

It is eye-opening to see how the various kinds of produce, grains, fruits, vegetables, and spices available in supermarkets today have originated in such distant places. Since human societies have been continuously in motion, crops have moved with them, away from their centers of origin, and have been subjected to additional artificial and natural selection in their new environments. They continued to evolve and diversify as a result of spontaneous mutations, hybridization, and agricultural practices. Thus most crops also acquired genotypic and phenotypic diversity post-domestication. As a result, for many crops, their primary center of origin and the center of diversification (where many varieties of crops evolved) differ.^[2] However, compared to the primary centers of origin, the secondary centers occupy a large area and possess less biodiversity of plant species. The secondary centers are relatively rich in domesticated crop varieties (landraces) but lack the immediate wild progenitor and other related wild species.

Since, typically, the “centers of origin” of crops are rich in their biodiversity,^[3] their exploration helps scientists comprehend the full spectrum of the gene pool available for a given crop, which can then be utilized for breeding experiments. It is important to note that the plant breeders only transfer useful traits from one variety to another by crossing and then selecting the progeny that have desirable combinations of traits. The breeders do not create traits. If a trait is absent in cultivated varieties of a crop species and its ancestors, progenitors, and related species, it cannot be introduced by classical breeding. Biodiversity serves as the resource bank from which scientists can borrow useful traits. It also defines the limits of classical breeding. Therefore, biodiversity centers are the insurance policies for the continuation of today’s crops. Scattered across these centers are the genes/traits that provide resistance to pests and pathogens or the ability to tolerate environmental stresses—the raw materials for breeding advanced varieties of crops.

Table 2.1: Centers of origins (biodiversity) of crops

Center	Remarks
1. China	136 crops were domesticated in this region including rice, sorghum, soybeans, barley, radish, cabbage, mustard, onion, cucumber, pear, apple, apricot, peach, cherry, walnut, litchi, sugarcane, and poppy. Rice was one of the first crop (~ 8,000 years ago) cultivated in the Yangtze River Valley. Pigs, roosters, and dogs were also domesticated here.
2a. Indo-Malay	This region includes parts of India, parts of China and the Malay Archipelago. Here, clove, nutmeg, black pepper, coconut, hemp, banana, grapefruit, reed and velvet beans were domesticated.
2b. Indo Burma	This center includes North-East region of India and present day Myanmar (Burma). Here, ~117 crops including jute, sandalwood, indigo, bamboo, neem, rice, gram, pigeon pea, mung, cowpea, eggplant, cucumber, radish, carrot, mango, orange, lemon, tamarind, coconut, banana, hemp, pepper, cloves, nutmeg, reed, sesame, and cotton were domesticated.
3. Central Asia	This center includes North-Western India (Punjab, Haryana, and Kashmir provinces), Pakistan, Afghanistan, Tajikistan, and Uzbekistan. ~ 43 crops developed in this area, including three varieties of wheat, peas, lentils, horse lentil, gram, mung bean, mustard, linseed, sesame, cotton, hemp, onions, garlic, spinach, carrots, pistachio, almond grapes, pears and apples.
4. Near East (Fertile Crescent)	This center includes present day Turkey, Israel, Syria, Jordan, Lebanon, Iran, Iraq, Turkmenistan and the interiors of Asia Minor. ~ 150 crops including rye, barley, oats, Einkorn wheat, Durum wheat, Persian wheat, Pollard wheat, common Bread Wheat, Oriental wheat, lentils, lupine, peas, gram, pomegranate, mulberry, apple, grapes, pears, cherries, walnut, almonds, pistachios, dates, fennel, cumin, carrots, onions, and garlic were domesticated in this region. ~ 10,000 year old fossils of rye have been found at many archeological sites in this area. Evidence of earliest rye cultivation ~13,000 years ago has been found in Syria and the remains of ~ 9000 year old domesticated sheep, goats and pigs have been found in Turkey.
5. Mediterranean	This center includes regions around the Mediterranean Sea. Here, 84 crops including durum wheat, Emmer wheat, Polish wheat, oats, peas, lupine, clover, black mustard, olives, beets, cabbage, turnip, lettuce, asparagus, rhubarb, mint, hop, sage, celery, etc. were domesticated.
6. Ethiopian Center	This center includes Abyssinia, Eretria, Somaliland, and Ethiopia. 38 important crop plants including Abyssinian and emmer varieties of wheat, millet, sorghum, cowpea, flaxseed, tef, sesame, coffee, okra, indigo, castor, and gum Arabica were domesticated in this region.
7. Southern Mexico and Central America	This center includes Southern Mexico, Guatemala, Honduras and Costa Rica. Here, maize, potato, tomato, pumpkin, capsicum, chili, papaya, guava, cashew, chocolate, cotton, passion flower, tobacco, various beans, sisal, sweet potato, arrowroot etc. were domesticated. in this region, ~7000-year-old remains of maize and 10,000-8,000 years old seeds of squash have been found in the archaeological excavation.
8a. Peru, Ecuador, Bolivia Sub center (South America)	In this region, 62 species of crop plants including potatoes, maize, lima beans, tomatoes, pumpkin, capsicum, cotton, guava, passion flower, tobacco etc. were domesticated.
8b. Chile (South America)	Several varieties of potato and strawberry were domesticated in this center.
8c. Brazilian-Paraguay (South America)	Peanuts, pineapples, cashew nuts, Brazil nuts, rubber, etc. developed here.
9. New guinea* (Far East)	Recent research proves that ~ 7000 years ago, agriculture started independently in the mountainous region of New Guinea. In this area, bananas, knives, reeds etc. developed. *This center was identified after Vavilov death.

Further Readings

Darwin, C. (1859). On the origin of species by means of natural selection, or the preservation of favored races in the struggle for life. John Murray, London.

Harlan, J. R. (1973). On the quality of evidence for origin and dispersal of cultivated plants. Current Anthropology, 14(1–2), 51–62.

Harlan, J. R. (1975). Agriculture origins: Centers and noncenters. Science, 174, 465–74.

Nabhan, G. P. (2009). Where our food comes from: Retracing Nickolay Vavilov’s quest to end famine. Island Press.

Pringle, P. (2009). The murder of Nikolai Vavilov: The story of Stalin’s persecution of one of the great scientists of the twentieth century. JR Books.

Vavilov, N. I. (1931). The problem of the origin of the world’s agriculture in the light of the latest investigation. https://www.marxists.org/subject/science/essays/vavilov.htm.

Vavilov, N. I. (1987). Proiskhozhdenie i geografi i a kul’turnykh rastenii [The origin and geography of cultivated plants]. Nauka.

Attribution

This page was edited by students in the Spring 2024 General Botany (BIO-5) class at Norco College

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1. A clickable world map showing the center of origin of various crops is available at https://www.biodiversidad.gob.mx/v_ingles/genes/centers_origin/centers_plants1.html↵
2. This is also sometimes due to reasons other than human migration. One classical example is pine, which has its center of origin in northwestern China but its center of diversification in Central America (Mexico, Guatemala, and Honduras). At present, 49 out of a total of 111 species of pine are found in Mexico. The difference in the center of origin and center of diversification of pine resulted from a geological event (continental drift) long before the beginning of agriculture, which led to the isolation of various flora and fauna and thus their diversification in a new environment. ↵
3. In simple terms, biodiversity refers to all cultivated varieties of crops along with their wild progenitor(s) and evolutionarily related species. ↵