Wheat is a grain crop with some 25,000 diffrent cultivars in the world today: most of these forms are varieties of two groups: common wheat and durum wheat. Common or bread wheat Triticum aestivum accounts for some 95% of all the consumed wheat in the world today; the other five percent is durum or hard wheat T. turgidum ssp. durum, used in pasta and semolina products. Bread and durum wheat are both domesticated forms of wild emmer wheat (reported variously as T. araraticum, T. turgidum ssp. dicoccoides, or T. dicocoides). Another early form of wheat called einkorn (T. monococcum), was domesticated at about the same time, but has limited distribution today.
Origins of Wheat
The origins of our modern wheat, according to genetics and archaeological studies, are found in the Karacadag mountain region of what is today southeastern Turkey. There, some 12,000 years ago or so, both einkorn and emmer wheats were domesticated. The earliest collected wheat was wild emmer, at the Ohalo II site, about 23,000 years ago. Emmer was first cultivated in the southern Levant (Netiv Hugdud, Tell Aswad, other PPNA sites); while einkorn is found in the north PPNA sites (Abu Hureyra, Mureybet, Jerf el Ahmar, Göbekli Tepe, other southern PPNA). Spelt (T. spelta) and Timopheev's wheat (T. timopheevii) were ancient forms of emmer wheat developed by the late Neolithic, but neither has much of a market today.
The main differences between the wild forms of wheat and domesticated wheat are that domesticated forms have larger seeds with hulls and a non-shattering rachis. When wild wheat is ripe, the rachis--the stem that keeps the wheat shafts together--shatters so that the seeds can disperse themselves. But that naturally useful brittleness doesn't suit humans, who prefer to wait until the wheat is ripe to harvest it. Of course, if farmers harvest wheat when they believe it is ready, they only get the wheat that remains on the rachis: that wheat is what the farmers plant and in the process selected wheats with rachis that didn't become brittle at harvest time.
Other traits apparently selected for include spike size, growing season, plant height, and grain size.
How Long Did Domestication Take?
One of the ongoing arguments about wheat is the length of time it took for this process to occur. Some scholars argue for a fairly rapid process, of a few centuries; while others argue that the process from cultivation to domestication took up to 5,000 years. These scholars are debating the date of the earliest domestication: all of them agree, and the evidence is abundant, that by ca 10,400 years ago, domesticated wheat was in widespread use throughout the Levant region.
The earliest evidence for both domesticated einkorn and emmer wheats found to date was at the Syrian site of Abu Hureyra, in occupation layers dated to the Late Epipaleolithic period, the beginning of the Younger Dryas, ca 13,000-12,000 cal BP; some scholars (Colledge and Conolly 2010) have argued that the evidence does not show cultivation at this time, although it does indicate a broadening of the diet base to include a reliance on wild grains including the wheats.
Another recent study (Haldorsen and colleagues) focusing on one-grained einkorn (Triticum monococcum ssp. monococcum) agrees with this later timeline and suggests that one-grained einkorn was domesticated in southeastern Turkey after the end of the Younger Dryas (~11,600-10,400 cal BP).