Lend Me Your Ears: A Tale of Evolution From a 5,310-Year-Old Corn Cob
Corn has come a long way since its grassy beginnings
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Corn is one of the modern world’s most common crops, covering countless acres of fields and popping up in everything from cereal boxes to fuel. But thousands of years ago, when people living in central Mexico first began cultivating its wild ancestors, corn vastly differed from the sweet, crunchy kernels that we know today. The tiny cobs held just a few seeds sealed away in hard shells.
“Maize as we know it looks so different from its wild ancestor that a couple of decades ago scientists had not reached a consensus regarding the true ancestor of maize,” Nathan Wales, a geneticist at the Natural History Museum of Denmark, says in a statement. But a new study, published in the journal Current Biology, details the genetic analysis of a 5,310-year-old corn cob and is helping to fill in the blanks of how corn became a crop.
This ancient corn cob was excavated in the 1960s from a cave in the Tehuacán Valley of Mexico and has since resided in the Robert S. Peabody Museum in Andover, Massachusetts. Wales, an author on the study, and his colleagues got their hands this ancient cob to try to figure out where it fell in the genetic timeline of corn’s transformation into the ubiquitous crop it is today.
The team was able to recover a remarkable 70 percent of the ancient cob’s DNA. Other samples of similar age often only contain around 10 percent of the plants original DNA, according to the release. But surprisingly, the DNA of the ancient cob aligned much more closely to modern corn than it did with its maize ancestors, commonly known as teosintes.
Studying such ancient DNA, however, is no simple matter, Robert Fulton, a geneticist at Washington University in St. Louis’ McDonnell Genome Institute, tells Smithsonian.com. Fulton, who was not involved in the study, was part of the team that first mapped the corn genome in 2009, which allowed researchers to confirm that the modern crops descended from the teosinte plant.
“Typically, strands of DNA can be millions of bases long,” he says. “It’s basically like a large jigsaw puzzle, and for good, high-quality DNA, the puzzle pieces are very large. For ancient DNA, the puzzle pieces are tiny.”
Though the 70 percent recovery of the corn's original genetic material may seem like a windfall, the greater number of pieces also makes the genetic puzzle trickier to assemble and the results more difficult to interpret. And because the genome of the maize family is incredibly complex, Fulton says it can be tricky to draw conclusions from DNA that has degraded over thousands of years.
“The maize genome is very repetitive...there are many sequences that are repeated many times,” Fulton says. “So if you have very small fragments, then it’s challenging to uniquely map those onto the reference sequence because they’ll land perfectly in multiple different places.”
For most sequencing work on such ancient samples, Fulton explains, researchers chop up the DNA many different ways to compare it against a reference genome. But the latest study only did this a few different ways. So although this is a good start to studying the 5,310-year-old cob, much more analysis is needed to confirm their findings.
By studying the ancient corn's genetics, researchers could uncover more about how modern corn was cultivated, says Fulton. The research could also help tease out the specific traits that may have been bred out of the plant over time, which allowed the crop to thrive in many countries around the world.
So before you chow down on your corn this Thanksgiving, take a moment to appreciate the plant's long journey to become the sweet and juicy treat it is today.