DNA looks like a twisted ladder, and each rung of the ladder is made up of four types of molecules called nucleotides. They are represented by the letters G, A, T and C, and they code how living things look and function.
The finding, published this week in the journal Genetics, shows just how complex the molecular makeup of life can be, and how a single molecule among millions just like it can change the course of history.
"There were lots of mutations (in corn)," said John Doebley, an author of the study and a professor at UW-Madison. "But this was the big one."
Today, a huge proportion of the world is economically dependent on the crop, with more than 80 million acres in the United States dedicated to it (that's similar to the size of New Mexico). The grain can be found in just about everything Americans eat, especially since sugar cane has mostly been replaced by high fructose corn syrup.
But scientists now show that the agricultural giant started with much more humble beginnings. The weed-looking teosinte, whose name means "grain of the gods," has an ear with only 10 to 12 kernels, each tightly embraced in difficult to penetrate cases. The plant originates in Mexico and is thought to have been domesticated between 10,000 and 9,000 years ago.
The plant started to look like what we now call corn (or maize) due to a single mutation that affected essential architect-like proteins in the plant. The change made the grain disrobe its protective covering — probably a bad thing for the plant, but an absolute boon for humans.
After centuries of artificial selection and more rounds of mutations, the plant grew taller and developed more broad leaves. By 1493, Christopher Columbus was taking a pocket full of the strange — but delicious — plant's seeds back home to Europe.
The scientists were able to identify the single gene responsible for the major change essentially by reenacting the mutation. They mapped out the genomes of generations of teosinte plants and set up molecular markers in the data — sort of like highway signposts — to point out clues to which gene controls what physical trait. Eventually they found the exact DNA nucleotide that, when changed, resulted in the husk disappearing.
"It's the same thing people did to find the breast cancer gene or the cystic fibrosis gene," Doebley said.
In time, the discovery may translate into new ways of genetically modifying corn, as the gene identified in the study also affects the shape of corn kernels.
It also piles onto the already rapidly expanding field of genomics, extending our knowledge of how dramatic genetic shifts can contribute to the vast biological diversity of the world. Doebley compared the teosinte's mutation to a evolutionary step that changed humans from four-legged to upright bipeds.
Most mutations, he said, are neutral and harmless. A lot of genetic information is essentially unimportant — what his research team calls "junk DNA." Every once in a while, though, big changes can be made for the worse or for the better, referred to as "the currency of evolution." Sometimes entire paragraphs are taken out of the code, but in other cases, the change is just one letter.
"This has been something of a controversy, that a single letter could make such a difference," he said. "But that's pretty much been resolved."
Similar single-letter mutations have also been identified in other plants such as wheat, and it's been theorized that something as complicated as human eye color could be the result of a similar very slight DNA change. That's a bit more difficult to confirm, though, because scientists aren't able to perform genetic experiments on people.