You've heard this one right? Some sources, often low quality kibble brands, boast about dogs being omnivores in their commercials and articles. They explain that dogs have evolved to eat both meats and vegetables while in the care of humans. This is why dog kibble includes corn and even grains.
Let's start with the teeth. Dogs do not have omnivorous teeth. They do not have flat molars (like us) designed to grind and chew. They have sharp, scissor like teeth designed to rip and tear. They're teeth are carnivorous just like those of wolves, cyotes, foxes, and so on.
Evolution takes millions of years. Even those sudden, unexplained leaps that a species can make takes significantly longer than the period of time that dogs have been domesticated and selectively bred by humans. Dogs do adapt to eat whatever is fed to them rather than starve to death. Most living things will do that. A horse will eat dry weeds as long as it has to in hopes of survival or at least until the lack of proper nutrients leads to deficiencies significant enough to cause death. Humans will eat "twigs and berries" in third world countries or areas where food is unavailable until their bodies give up as well. This is probably more along the lines of what we see when dogs eat kibble that contains more grains and vegetables than meat. Those dogs are seldom particularly healthy. This is often the basis for the argument to feed either a grain free kibble or a raw or natural diet.
Dogs may be carnivorous but they are also very opportunistic and have managed to survive on carbohydrate heavy diets. Whether or not that is actually good or healthy for them is another consideration altogether.
Dogs have a very short gut or digestive system which is a carnivorous trait. Note: cats have an even shorter digestive tract and as such are even less equipped to tolerate carbohydrates than are dogs. Dogs can take literally half the time to digest their food than we do (with a whole, healthy meal fed to both human and dog). Because of this they simply do not have the digestive system to process grains or carbohydrates which take far more time than meat or vegetables to completely digest. Our bodies produce a number of enzymes and bacteria to help us break down our carbohydrates. Dogs don't have those same tools to ferment and break down sugars for energy. As such grains are often a bad idea and a source of allergies or allergy like symptoms in dogs. Corn is especially bad because of the high sugar content. It's arguable that dogs can and do benefit from small amounts of vegetables and fruits in their diets for fiber, vitamins, minerals, etc that are not found in meat. With this argument we consider the foraging nature of our dogs and their desire to often eat feces from herbivorous animals.
Lastly, we really want to consider the source of this assumption that dogs are not carnivorous. You've all seen those commercials with the falling bits of grains, corn, and veggies with some meat thrown in? Or the "Indoor" pet formulas with their greens, grains and veggies? These kibbles are often more than 50% grain. Of course, if we as the audience believe our dogs are built to eat these food stuffs than we are much more likely to feed them to our dogs. If a dog food company tells you that their food is what dogs are meant to eat...but it perhaps sounds questionable...consider searching for objective sources of information that do not profit from your interpretation of their theories.
Careful research with consideration to feeding dogs as if they were omnivorous has been performed and shows that dogs on these types of kibble diets often have kidney and liver damage, degenerative joint conditions, rotted teeth, diabetes and other debilitating conditions as a result of excessive effort on behalf of their digestive systems to survive on food they are not designed to eat. Note this does not particularly include dogs on grain free packaged diets or raw or natural diets. We'll save the argument about what issues these diets can cause for a later article!