Tuesday, February 17, 2015

Grain As Prehistory

[This is another draft post from the year 2010 that is resurrected without major further research.  It too was never posted because of its original, overambitious scope that has been abandoned in this post.]
Although corn was domesticated only 8,000 to 10,000 years ago from the grass teosinte, the genetic diversity between any two strains of corn exceeds that found between humans and chimpanzees, species separated by millions of years of evolution. For instance, the strain B73, the agriculturally important and commonly studied variety decoded by the maize genome project, contains 2.3 billion bases, the chemical units that make up DNA. But the genome of a strain of popcorn decoded by researchers in Mexico is 22 percent smaller than B73’s genome.

“You could fit a whole rice genome in the difference between those two strains of corn,” says Virginia Walbot, a molecular biologist at Stanford University.
From here.

Mostly, this is a sideshow. Different kinds of wheat (which comes in duploid to hexaploid varieties, with the hexaploid monster genome found in the most commonly consumed varieties) for example, also shows great genetic diversity as measured by numbers of bases, but some of this is a product of how you choose to measure genetic diversity, because the variations aren't simply random mutations.

In fact, other recent research on grain genetics shows that just a handful a key, independent, simple mutations account for the traits that led to the practical differences between the wild plants that were the marginally nourishing ancestors of the world's staple grains (e.g., rice, maize a.ka. corn, and wheat) and the contemporary domesticated varieties that feed the world.

Far more interesting to me is the extent to which a genetic analysis of foods and domesticated animals makes it possible to localize the epicenters of the Neolithic revolution and date the times when the domestication occurred. Modern humans (i.e. post-Neanderthals) transitioned from many tens of thousands of years as a hunter-gatherer society to a farming society at something like a dozen independent locations around the world at roughly the same time give or take a few thousand years. Each location domesticated different plants and animals.

Seeds from domesticated plants can often be recognized on sight, survive thousands of years better than dead animal matter, and can be carbon dated.

Comparisons of the genomes of domesticated plants and animals to wild species usually makes it possible to make a very specific identification of a common wild ancestor, often an ancestors with a quite narrow geographic range. For example, genetic evidence provides strong support for the notion that all maize has a single common domesticated ancestor about 9,000 years ago in a fairly specific highlands part of Southern Mexico.

Similarly, the story of wheat is also a high definition bit of genetic history.
Einkorn wheat was one of the earliest cultivated forms of wheat, alongside emmer wheat (T. dicoccon). Grains of wild einkorn have been found in Epi-Paleolithic sites of the Fertile Crescent. It was first domesticated approximately 9000 BP (9000 BP ≈ 7050 BCE), in the Pre-Pottery Neolithic A or B periods. Evidence from DNA finger-printing suggests einkorn was domesticated near Karacadag in southeast Turkey, an area in which a number of PPNB farming villages have been found.
The origins of maize and wheat are known in time to a period of plus or minus a few hundred years, and in place to regions the size of a few contiguous counties. We can trace the origins of the greater Mexican squash-maize-bean triad of staples to the separate but adjacent areas, resolve order they were developed in (squash came first), and determine how quickly this pattern of domestication spread from carbon dated archeological evidence.

No comments: