Pull a product from the shelf of your local grocery store and you may find a label proudly proclaiming it "GMO-free." Those who support the labeling of products containing GMOs, short for genetically modified organisms, argue that people deserve to know what they're putting in their bodies. But opponents argue that these labels are essentially meaningless – the latest episode in the great American tradition of health food hoaxes.

But the question of what is natural and what is genetically modified is more complicated than it seems. In fact, GMOs may be lurking far closer to home than the grocery store. It can be argued that the world's first GMO was … man's best friend.

Dogs are the oldest domesticated species, predating even the dawn of agriculture. There's even some evidence that during the Ice Age, when humans first crossed the Bering Strait, they brought their dogs with them. Given their astounding diversity in size and shape, early naturalists like Charles Darwin theorized that dogs were a mix of several different species. But actually, every single breed of dog is descended from just one canine: the gray wolf.

Even more surprisingly, most modern breeds of dog were created in just the past 200 years, the blink of an eye in evolutionary time. Although we didn't always know it, we did this by messing with their genes. 

To create new breeds, we selectively bred dogs with the most extreme traits. Think of the Komondor, a huge, ferocious guard dog that also happens to resemble a giant mop. Or the Norwegian Lundehund, which has six toes on each foot, is flexible enough to touch the top of its head to its back, and can close its ear canals at will. By selectively breeding dogs with the most bizarre traits, we were also picking out rare genes, ensuring that only those genes were passed down to the next generation.

When modern geneticists started sequencing dog genomes, they found that we had created a species with a unique genetic quirk: The network of genes that controls body size and shape is remarkably simple.

In humans, complex traits are the combined work of many hundreds of genes, each of which has a small effect on its own. But for dogs, over 80 percent of variation in body size – the difference between a five-pound chihuahua and a 200-pound mastiff – comes down to differences in just six genetic regions. What type of ear a breed has is determined by just one genetic region. 

Of course, we weren't content to just mess with body size and ear shape. Researchers in the Soviet Union, led by the geneticist Dmitry Belyayev, carried out a 40-year experiment to find what genetic changes could have turned wild animals into mankind's closest companions. They found that the genes controlling certain physical traits, like floppy ears, white spots, and curly tails, might actually be linked to the genes controlling tameness.

The Russian researchers took silver foxes and selectively bred those that were the most tolerant of human contact. Amazingly, it took only two or three generations to create a population that did not respond to humans with fear or aggression. By the fourth generation, the foxes started wagging their tails. And by the sixth, they actually sought out human contact.

What's more, just by selecting for tameness, foxes with dog-like physical attributes like floppy ears started popping up much more frequently than they normally do.

The connection between floppy ears and friendliness is a phenomenon called "neoteny," which is thought to be one of the ways that new species evolve. Neoteny is a change in developmental timing such that juvenile traits (both physical and behavioral) are retained into adulthood – essentially an extended childhood. And dogs aren't the only species that may have arisen due to neoteny. Humans have many physical characteristics of juvenile apes, like small jaws and flat faces. It's possible that the same extension of childhood that left Belyayev's foxes more willing to interact with humans may have also caused mankind's intelligence, by extending the period of time when we're primed to absorb new information

More recently, geneticists have pinpointed two gene variants that contribute to hyper-sociability in dogs. Strikingly, deletion of the human versions of those genes is known to cause Williams syndrome, a developmental disorder that, among other things, results in extreme extroversion. In other words, we're learning more and more about the precise genetic mechanisms that humans have been manipulating for millennia.

Consider for a moment the frantic news coverage if it were announced that scientists had engineered an entirely new species of animal, tweaking every single aspect of its physiology, freezing it in a permanent state of childhood to make it more subservient. History is rife with other examples of selective breeding, too: everything from corn and peaches to cattle and cats (sort of). Why do we consider extreme genetic modification through breeding to be harmless, but think that minor genetic modification using CRISPR is an affront to nature?

 In fact, at a very basic level, there is nothing more natural than one species changing the genetics of another. Our domesticated animals have done it to us. Humans did not originally make lactase, the enzyme that helps to digest milk, after weaning. But populations that eat dairy have rapidly evolved to keep making lactase into adulthood, thanks to mutations that affect the expression of the lactase gene. There's even a hypothesis that humans may have domesticated each other, helping those who could get along with the group while leaving the most aggressive out in the cold. Hence the neoteny. 

Modern biotechnology has made it possible to make bigger genetic changes more quickly than ever before. As this technology comes of age, we must continue asking tough questions about its moral, social, and environmental implications. 

But if what troubles us is the idea of genetic modification in and of itself, we should remember that mankind has been messing with genetics in pretty extreme ways since before we invented the plow. When it comes to genetic modification, the big difference between a GM papaya and your family's beloved Shetland sheepdog is not that one is modified in a more extreme way than the other. It's that when we created that papaya, we actually understood what we were doing.