Cotton and its dispersal: A prime example of this incomplete state of knowledge on plant origins is cotton (Gossypium spp.), which has both Old and New World species. We do not know when or where prehistoric Mesoamericans first began to appreciate the utility of the cotton plant, even though something is known about the complex distribution of wild populations in the New World. As far as has been determined, wild cotton occurs in distinctive and restricted littoral (shoreline) or related habitats. It must have taken considerable time for the species to have dispersed so widely throughout tropical and subtropical Mesoamerica, and to have developed the long-fibered seeds for which it is so highly valued. The seeds of the littoral wild cottons, perennials which do not propagate vegetatively, have very sparse fibers. The oldest known archaeological samples of Mesoamerican cotton are from the Tehuacán Valley, dated 3500-2300 BC; these represent a fully domesticated form, Gossypium hirsutum (Smith and Stephens 1971).
[Fig.2: Map of New World cotton species (after Fryxell 1979).]
Cotton may possibly have been introduced to Mesoamerica as an already domesticated form from the southern hemisphere where, in coastal Ecuador and Peru, cultivated cotton of another species (G. barbadense) has been documented for much earlier periods. Yet most views favor the independent domestication of G. hirsutum in Mesoamerica (Fryxell 1978; Heiser 1990; Pickersgill 1989). Confirmation of this judgment is needed from the archaeological record
An important fiber crop for several civilizations, cotton was undoubtedly domesticated multiple times. Very little argument on this point can be made since domesticated forms were found on both sides of the Atlantic much earlier than any known trans-Atlantic travel. However, independent domestication has even been suggested in the New World, based on several lines of evidence (Pickersgill, 1977). Altogether, cotton has likely been domesticated four times around the world (Sauer, 1993).
There are four domesticated species of cotton. Prior to the relatively recent expansion of cotton production into temperate zones, all cultivars were perrenial shrubs or trees (Harlan, 1992). Only species possessing the A genome produce filamentous fibers used for textiles.
East of the Atlantic, the diploid (AA) Gossypium herbaceum was most likely domesticated in Ethiopia. The exact date is not known, but Theophrastus recorded seeing a "wool bearing tree", which was probably cotton, in 350 B.C. G. arboreum, another AA diploid, seems to have been domesticated in India, where the current center of diversity is. This cotton was more widely used than G. herbaceum in the Old World prior to trans-Atlantic travel.
The New World has four wild tetraploid species (AADD), two of which have been domesticated. Molecular evidence shows that all of the tetraploid cottons contain the same A genome, suggesting that the A genome was incorporated into cotton a very long time ago, perhaps 1-2 million years ago, with subsequent speciation into four distinct groups (Gepts, 1993). Diploid New World cottons do not contain the A genome (Harlan, 1992). The A genome is present, in more or less the same form, on both sides of the Atlantic (Sauer, 1993). This has caused some consternation about the source of the A genome, and possible transport across the Atlantic. The most plausible explanation is that the A genome is very ancient, and probably occupied a range on the ancient continent of Pangea over much of current South America, Africa and India (Harlan, 1992). The immediate ancestor of either cotton species is not definitively known, and may be extinct (Gepts, 1993)
Evidence of multiple New World domestication is primarily archeaological. G. barbadense was domesticated in South America. Remains of this cotton were found in a site dated to 2500 B.C. G. hirsutem was probably domesticated in Mexico, since definitive remains of domesticated cotton have been found at a site dated to 3500 B.C. Both domestication centers are centers of diversity for their respective species (Gepts, 1993). The early presence of both domesticated species in geographically isolated areas strongly suggests independent domestication. Although the two species are morphologically similar, they are easily distinguished by isozyme alleles (Gepts, 1993).
Given the evidence, it can safely be concluded that cotton was domesticated four times in India, Africa, Mexico and South America.