Barley (Hordeum vulgare) - The History of its Domestication

How did our ancestors develop such a genetically diverse crop?

Barley landraces in southeast Turkey
Barley landraces in southeast Turkey. Brian J. Steffenson (Morrell and Clegg)

Barley (Hordeum vulgare ssp. vulgare) was one of the first and earliest crops domesticated by humans. Currently, archaeological and genetic evidence indicates barley is a mosaic crop, developed from several populations in at least five regions: Mesopotamia, the northern and southern Levant, the Syrian desert and, 1,500-3,000 kilometers (900-1,800 miles) to the east, in the vast Tibetan Plateau. The first was long though to be that of southwest Asia during the Pre-Pottery Neolithic A about 10,500 calendar years ago: but the mosaic status of barley has thrown a wrench into our understanding of this process.

In the Fertile Crescent, barley is considered one of the classic eight founder crops.

A Single Wild Progenitor Species

The wild progenitor of all of the barleys is thought to be Hordeum spontaneum (L.), a winter-germinating species which is native to a very wide region of Eurasia, from the Tigris and Euphrates river system in Iraq to the western reaches of the Yangtze River in China. Based on evidence from Upper Paleolithic sites such as Ohalo II in Israel, wild barley was harvested for at least 10,000 years before it was domesticated.

Today, barley is the fourth most important crop in the world after wheat, rice and maize. Barley as a whole is well-adapted to marginal and stress-prone environments, and a more reliable plant than wheat or rice in regions which are colder or higher in altitude.

The Hulled and the Naked

Wild barley has several characteristics useful to a wild plant that aren't so useful to humans.

There is a brittle rachis (the part that holds the seed to the plant) that breaks when the seeds are ripe, scattering them to the winds; and the seeds are arranged on the spike in a sparsely seeded two rows. The wild barley always has a tough hull protecting its seed; the hull-less form (called naked barley) is only found on domestic varieties.

The domestic form has a non-brittle rachis and more seeds, arranged in a six-rowed spike.

Both hulled and naked seed forms are found in domesticated barley: during the Neolithic period, both forms were grown, but in the Near East, naked barley cultivation declined beginning in the Chalcolithic/Bronze Ages about 5000 years ago. Naked barleys, while easier to harvest and process, are more susceptible to insect attack and parasitic disease. Hulled barleys have higher yields; so within the Near East anyway, keeping the hull was a selected-for trait.

Today hulled barleys dominate in the west, and naked barleys in the east. Because of the ease of processing, the naked form is used primarily as a whole-grain human food source. The hulled variety is used mainly for animal feed and the production of malt for brewing. In Europe, the production of barley beer dates at least as long ago as 600 B.C.

Barley and DNA

A recent (Jones and colleagues 2012) phylogeographic analysis of barley in the northern fringes of Europe and in the Alpine region found that cold adaptive gene mutations were identifiable in modern barley landraces. The adaptations included one type that was non-responsive to day length (that is, the flowering was not delayed until the plant got a certain number of hours of sunlight during the day): and that form is found in northeast Europe and high altitude locations.

Alternatively, landraces in the Mediterranean region were predominantly responsive to day length. In central Europe, however, day length is not a trait which (apparently) had been selected for.

Jones and colleagues were unwilling to rule out the actions of possible bottlenecks, but suggested that temporary climate changes might have affected the selection of traits for various regions, delaying the spread of barley or speeding it, depending on the adaptability of the crop to the region.

How Many Domestication Events!?

Evidence exists for at least five different loci of domestication: at least three locations in the Fertile Crescent, one in the Syrian desert and one in the Tibetan Plateau. Jones et al. 2013 report additional evidence that in the region of the Fertile Crescent, there may have been up to four different domestication events of Asian wild barley.

The differences within groups A-D are based on the presence of alleles which are differently adapted to day length; and the adaptive ability of barley to grow in a wide variety of locations. It could be that the combination of barley types from different regions created increased drought resistance and other beneficial attributes.

The DNA analysis reported in 2015 (Poets et al.) identified a genome segment from the Syrian desert variety in Asian and Fertile Crescent barleys; and a segment in northern Mesopotamia in Western and Asian barleys. We do not know, says Allaby in an accompanying essay, how our ancestors produced such genetically diverse crops: but the study should kick off an interesting period towards a better understanding domestication processes in general.

Evidence for barley beer making as early as Yangshao Neolithic (ca 5000 years ago) in China was reported in 2016; it seems most likely to have been from the Tibetan Plateau, but that has yet to be determined. 

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Hirst, K. Kris. "Barley (Hordeum vulgare) - The History of its Domestication." ThoughtCo, Aug. 4, 2017, thoughtco.com/barley-history-of-its-domestication-170641. Hirst, K. Kris. (2017, August 4). Barley (Hordeum vulgare) - The History of its Domestication. Retrieved from https://www.thoughtco.com/barley-history-of-its-domestication-170641 Hirst, K. Kris. "Barley (Hordeum vulgare) - The History of its Domestication." ThoughtCo. https://www.thoughtco.com/barley-history-of-its-domestication-170641 (accessed November 20, 2017).