Technically corn is a grain, so I’m being a bit redundant. Corn however holds a special place in the hearts of Paleo adherents as a grain to be avoided because of how heavily modified corn has gotten in the last century or so. While all plants that we have domesticated have been radically transformed in order to better serve us, corn has dominated our food system more for the last 75 years than it has at any other time in history. We have never consumed more corn per capita than we have today. You don’t recall the last time you ate corn? That doesn’t matter. In fact most of the corn you eat isn’t as corn proper. There will be corn in certain dishes, or cornmeal used in the preparation of some prepared foods. However it is really in the form of corn starches and corn syrups that most of our corn consumptions come from. Corn is the “C” in HFC (high fructose corn syrup). You recognize that evil acronym but won’t be seeing it much longer as the corn lobby has successfully convinced people that “corn sugar” is both more “accurate” and more marketable.
Forget your consumption of corn, the animals we eat are often fed corn. Corn is a primary source of calories (along with soy) for cattle, chickens, pigs, farm raised fish, and pretty much anything else that you can think of that eats something. Whether these animals digestive systems were ever consuming these products before the 20th century or not, this is now the primary food source for most of what we end up consuming. That was the entire plot of the movie King Corn. One way or another most of our calories are coming from corn. That’s just one of the reasons why Paleo people shun this product even more than other grains.
Another reason is the whole GMO aspect of the product. In the Paleo community there is a general distrust of GMO foods of all stripes. The arguments against GMOs range from theoretical ones like lack of proper testing, biased corporate interests blinding regulatory policy, and lack of consideration for biodiversity even more than in conventional agricultural crops. They also point to some studies which claim to show substantially increased rates of cancer and other diseases from rodent studies on GMOs. All of that combined with the overwhelming majority of corn products being of the GMO variety has a skull and cross bones on any corn product that a Paleo person would look at eating.