How do naturalists — geneticists, in fact — establish where a given species comes from? This question is closely linked to those of domestication and hybridization of a species, hence also to the geographic origins of agriculture.
The towering pioneer was Nikolai I. Vavilov (1887-1943). During the 1920s, Vavilov, who set up and took part in over 100 collecting missions all over the world, established key principles for locating origins of cultivated plants, viz. the centers of origins are areas of greatest diversity and thus the sources of dominant genes.
More recently, analysis of the genomes of plants has become the main tool for ascertaining their geographical origins. Molecular biologists can trace the genealogies of species by comparing populations. Genetic similarity is gauged from PCR-amplified fragment length polymorphisms at hundreds of chromosomal loci, i.e., comparisons of chromosomal maps. Archeological finds are also very helpful. In agricultural archeology, cultural factors can play key roles.
Take glutinous rice. This variety appeals to many Asian consumers with its more cohesive rice grains (‟sticky rice‟). It lacks the sugar molecule amylose, due to a mutation — a guanosine replaced by a thymine — that causes the absence of spliced mRNA, known technically as the Wx splice donor mutation.
When genetic variation is monitored among many different rice plants grown in Asia, from Sri Lanka to Taïwan, including India, China and the whole of Southeast Asia, these comparisons show that glutinous rice appeared in an upland context among the Tai-speaking inhabitants, who migrated to Southeast Asia between 1100 and 1500 years ago.
This single origin did not prevent subsequent spread of glutinous varieties across Asia.
In Southeast Asia, glutinous rice became a staple crop. Indeed, about 200,000 square miles of mainland Southeast Asia (encompassing portions of Laos, Myanmar (formerly Burma), Thailand, Cambodia, and Vietnam) are referred to as the “glutinous rice zone.“
In other words, due to selection of cultivated varieties appealing most to their consumers, a random genetic variation is durably established. Wheat begs to be mentioned in this connection, if only because of its importance (bread) to the diet of Westerners. The plant also displays rich genetics. The original wild wheat with 2n (14) chromosomes gave rise to species having more chromosomes. They are known as polyploid. This is a consequence of gene duplication, an occasional accident during cell division. In the case of wheat, such accidents became crucial to its nutritional value and to its cultivation.
There are wheat plants with up to 6n (42) chromosomes. Mankind selected these polyploid wheats, for their better adaptation to cultivation in warmer climates. Their harvest is also made easier by naked seeds and soft envelopes of the grain — or glumes.
Already in the Bronze Age mankind abandoned cultivation of the original 2n species, einkorn. The ancestor to einkorn, Triticum boeoticum, occurs in the central and eastern regions of the Fertile Crescent.
This expression designates a part of the Middle East comprising present-day Israel, Lebanon and the two valleys of the Tigris and the Euphrates, including some of western Iran and southeastern Turkey.
Archeologists have found kernels of T. boeoticum in sites of the early Neolithic (12,500 BCE) in that region. Moreover, geneticists were able to trace einkorn populations to the Karacadag mountains of southeast Turkey. Wild populations of T. boeoticum from that region are more similar to einkorn than any other, which points to it having been the site of its domestication.
From einkorn, early agriculturalists turned to emmer. This tetraploid (4n) wheat, Triticum diccocoides, likewise arose from a wild progenitor, that our ancestors had both improved and selected. The populations in the wild that are most closely related to emmer, from genomic evidence, are found in southeast Turkey. Until the early Bronze Age, emmer was the dominant crop in the Fertile Crescent. Archeologists found domesticated forms in several Neolithic sites, such as Tell Aswad, in Syria next to the Lebanon border, which dates to about 10,400 BCE.
Other wheat varieties are 4n Triticum durum (hard wheat) and 6n Triticum vulgare (bread wheat). These free-threshing wheats are present as seeds in Damascus basin archeological sites from about 10,000 BCE. They followed subsequently a Mediterranean route into Europe, around 8,000 BCE were found in Mediterranean locations and about 6,000 BCE in northern foothills of the Alps.
Wild rye also originated in the same Karacadag region within the Fertile Crescent, while barley was domesticated in the Israel-Jordan area. At this point, one needs to distinguish domestication and cultivation. The latter consists of planting and harvesting seeds of a plant. The former is the alteration of its phenotype, i.e., selection of culturally more desirable traits of a plant. Only after the domestication process was mastered, could cultivation be performed efficiently. Archeological evidence points to protracted domestication before the onset of cultivation. It occurred in the Karacadag region around 12,000 BCE. The spread of agriculture, into Western Europe for instance, showed an average speed of one kilometer per year.
In the New World, corn was domesticated in the Tehuacan Valley of Mexico, as early as 10,000 years ago. The earliest undisputed domesticated maize cobs are from the Guila Naquitz cave in Guerrero, Mexico, dated about 4280-4210 BCE. However, the wild plant (Zea mays spp. parviglumis) that gave rise to modern maize has a very different morphology. The seeds of wild teosinte are encased in hard shells and arranged on a spike with five to seven rows that shatter when ripe to disperse its seed. Maize became used in the southwestern United States by about 3200 years ago, and in eastern United States beginning about 2100 years ago.
Tomato, cocoa and cotton also originated in Central America. Potato, sweet potato, peanut and tobacco originated in South America.