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Wherefore wheat?
30 MAR 2006

The invention of agriculture in the Middle East is usually credited as the foundation for the first ancient civilizations. Using the food surpluses that farmers grew, some people had time to specialize in skills like working metal, writing, and acting imperious in early empires.

Ear of yellow wheat stands stately against blue sky and green treesWheat and barley, the major grains of the Middle East, descended from wild grasses that still survive in the region. How long did wild grass take to convert into wheat? The first domesticated wheat appears in Middle-Eastern archeological sites dating to about 9,250 years ago, and researchers had estimated that domestication may have occurred in the 200 years before that.

This wild wheat ear sheds its grains at maturity, while domestic wheat retains grains. After years of farming, evolution causes a genetic change toward domesticated wheat. All photos this page courtesy George Willcox

Now a pair of researchers suggest the process took at least 1,250 years, and was not completed until much later than 9,250 years ago.

George Willcox, who studies ancient botany at the French National Center for Scientific Research and the University of Lyon, says the new estimate is based on a bushel of evidence -- charred remains of wheat from archeological sites in Syria and Turkey -- rather than models based on scanty grains of evidence.

As Willcox explained, the distinction between wild grass and domesticated grain comes down to whether the seeds hang onto the stalk after the plant dies. Wild-grass seeds start to drop almost immediately after death, while domesticated grain seeds hang on much longer.

From this you can glean the kernel of the distinction: Tightly adhered seeds are much easier to harvest.

The danger of dropping out
Scientists say this genetic change emerged from farmers' activities, which would select for any natural mutation that causes seeds to hang on longer. As farmers harvest and replant their crops, those tightly adhered seeds are more likely to be retained and replanted than seeds that quickly fall off. Over several generations, geneticists had assumed, this selection pressure would be great enough to domesticate the plant -- producing varieties where every seed adheres tightly.

But how long is "several generations"?

Most previous estimates, based on mathematical analysis of the genetic impact of those plant-harvest cycles, concluded that domestication took a few hundred years, or even fewer.

But domestication equates to secure seed attachment, and wild seeds leave a smooth scar when they separate, while domesticated seeds leave a rough one. And so Willcox and colleague Ken-ichi Tanno of Japan looked at 9,844 spikelets of wheat from four archeological sites (spikelets are the parts of the plant that contain the wheat grain; only 804 of them were intact enough to be analyzed).

Nine kernels of wheat charred in an ancient fire, leaving only grayed husks
Ancient, charred wheat grains from Kosak Shamali archaeological site in northern Syria. Microscopic examination of remains like these help explain how wheat was domesticated. At this site, only 35 percent of the wheat was wild.

The oldest site, dating to 10,200 years ago, produced no definitely domestic spikelets, while the younger sites had increasing percentages of rough, domesticated scars, indicating a gradual process of domestication. By 6,500 years ago, more than 90 percent of the spikelets looked domestic. "Domestication was a series of events occurring at different places over thousands of years, during which wild wheat persisted in cultivated fields," the authors wrote.

"This is the first time anybody has looked in detail at a fairly wide range of evidence for wheat," Willcox told us. "It shows that the change from wild cultivation to domesticated cereals was much slower than people had previously thought." While other researchers used material from at one or two sites, he says, "We were able to look at a good cross-section. Nobody had really done this."

Researchers dig at sun-baked circle of red earth that once was the base of a house
At the Tell Qaramel site in northern Syria, residents of these small round houses ate wild wheat about 10,200 years ago. Excavated by R. Mazurowski, Warsaw, Poland

A matter of evolution
The results did parallel a 1984 study of barley, the other key ancient grain in the Middle East, which also showed a slow change from wild grass to crop, Willcox said.

The rise of domesticated varieties did grow from farmers activities, but Willcox calls them a result of natural, not artificial, selection. He doubts that farmers chose tightly adhered seeds, since they probably harvested early to catch the quick-separating seeds. "It's generally accepted by the scientific community that at this stage there is no real artificial selection ... it's more natural selection. If you imagine a field with some forms that shatter, and some that don't shatter, when you go out and harvest, the shattering forms will be wasted. But the domesticated variety will stay intact. That will result in selection in favor of these forms, but it is not really artificial selection, because humans would not have been able to spot the rare individual with this advantage."

And if they didn't choose the tightly adhered seeds, it makes sense that the process of evolution took much longer than previously thought -- especially since the evidence says so!

-- David Tenenbaumlittle wheat seeds

Bibliography
How Fast Was Wild Wheat Domesticated? K. Tanno and George Willcox, Science, March 31, 2006.

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