Civilization and Plant Domestication

It has been a long held belief that the cultivation of edible plants was a major step toward the establishment of human culture. The most widely held belief was that the domestication of crops took place in two major areas: the valley of the Fertile Crescent between the Tigress and Euphrates Rivers and in Central and South America. It was believed that these two areas provided the spark for the domestic cultivation of crops and then gradually spread out as human agricultural culture spread and incorporated various varieties of edible crops as culture spread. Recent scientific findings have pointed out that agriculture actually developed throughout regions populated by humans in particular "a series of new studies indicate that Chinese river valleys represent a second spot for the emergence of agriculture." A recent issue of Science takes a "look at different aspects of the process, along with a perspective that presents the big picture."

Researchers have found that "material isolated from sites on the west slope of the Peruvian Andes, focusing on material that appears to come from domesticated plants. The big surprises are the dates obtained: squash appeared around 8-9,000 years ago, peanuts just shy of 8,000 years ago, and quinoa at roughly the same time. With agriculture in place, production moved to cotton by about 5,500 years ago. None of these items are likely to have originally been domesticated in this region, suggesting that seeds and techniques arose earlier and elsewhere."

"Genetic studies, using DNA from charred seeds gathered at the world’s first farms, are slowly rewriting the long-told story of how “civilization” began. In an essay in Science this week, Cambridge archaeologists Martin Jones and Xinyi Liu argue that millet spread west long before the Middle Eastern crops (wheat and barley) spread east."

More generally, they say that the Agricultural Revolution took place so slowly that it was probably imperceptible to those humans experiencing the transition. Early farmers continued to harvest wild rice varieties and the percentage of domesticated rice species that they the percentage of domesticated rice harvested versus wild rice increased just a few percent in a human lifespan.

“Rather than a revolutionary shift from hunter-gatherers to farmers in a few human generations, the evidence now suggests that many generations of ‘affluent foragers’ combined the gathering of wild fruits and nuts with the gathering of cultivated cereals,” write Martin Jones and Xinyi Liu, the article's co-authors .

Many researchers estimate that it was 10,000 years ago when agriculture first appeared, which sets up an interesting conundrum; he best estimate of humans populating the Americas was about 13,000 years ago. It has thus been reasoned that "limited local cultivation acted as a supplement to a hunting-gathering society for a few thousand years and only gradually fostered the transition to an agricultural society."

Scientists have speculated on: "what's gone on in the genome of wheat since we domesticated it at about the same time as squash."

"Compared to its wild relatives, domesticated wheat is tetraploid: it has four copies of every chromosome, instead of two. At least some cultivars are hexaploid, with a further set of chromosomes derived from a different species of grain by hybridization.

Another point the researchers have suggested is that: "all this extra genetic material has proven useful to the species in two ways. The domestication process normally creates a genetic bottleneck, in that relatively few ancestral plants wind up being expanded into a large population with little genetic diversity. Normally, low genetic diversity is harmful, but the extra chromosomes (especially those from a different species) have helped wheat remain diverse. The second is that it allows for an extremely flexible genome, one that has produced many variations for humans to select traits from. The extra copies of genes allow deletions, mutations, and even larger rearrangements to tweak gene expression and function in many ways, some of which would probably be fatal in an an organism with only two copies of a chromosome.

Scientists note: "that the South American model—one in which domesticated plants spread rapidly, but only gradually changed people to farmers—appears to apply in most other cases of domestication. It also discusses how slowly the genetic diversity that helped produce modern wheat allowed selection of a domestic species. It turns out that seeds from wild and domestic species are intermixed at agricultural sites for centuries after the tetraploid wheat appeared, and that the percentage of domestic wheat rose only slowly during that period.

The authors of the study conclude that: "It ends up with the big mystery: why did people suddenly think domesticating plants was a good idea? The 10,000 year figure is suspiciously close to the rapid climactic changes that accompanied the end of the last ice age, suggesting that changing temperatures might have disrupted food supplies enough to make domestic plants essential. The end of the ice age also pushed atmospheric carbon levels up by nearly 50 percent, which may have triggered favorable changes in the plants themselves. But it ends on an enigmatic note: maybe there was something cultural going on that we may never be able to reconstruct."