Bacteria created a GMO 8,000 years ago that we still eat

If the thought of eating genetically modified food makes you cringe because it seems unnatural, think again.

Bacteria modified the genes of plants on their own long before humans figured out how to do it, and we're still enjoying the fruits (and vegetables) of their work today.

It turns out that the sweet potato, the beloved orange root vegetable frequently eaten on Thanksgiving, has been harboring DNA inserted into its genome by bacteria long before humans started growing it for food around 8,000 years ago.

A team of scientists analyzed the genomes of hundreds of varieties of domestic sweet potatoes and found they had bits of DNA from a microbe commonly used in plant genetic engineering.

The scientists, lead by Jan Kreuze of the International Potato Center in Peru, published their results in the May 5 edition of the Proceedings of the National Academy of Sciences.

One stretch of DNA - the code of which matched with a species of bacteria called Agrobacterium - was present in all 291 cultivated sweet potato varieties studied, but not in closely related wild plants. That makes the scientists think the bit of DNA contains a gene that gave the sweet potato a trait humans found desirable and selected for when domesticating the plant.

If that's the case, the sweet potato's genetic modification may very well be the reason why we can eat (and love) it. Unlike real potatoes, which are tubers that come from the plant's stem, the sweet potato is actually the root of the plant. These woody and fibrous bit of the plant probably needed some genetic TLC to become an edible and delicious Thanksgiving staple.

"We think the bacteria genes help the plant produce two hormones that change the root and make it something edible," virologist Jan Kreuze told NPR's Goats and Soda blog. "We need to prove that, but right now, we can't find any sweet potatoes without these genes."

Whatever the reason the bacteria genes stuck around, the way they got there in the first place is not too mysterious to plant geneticists.

"I don't think that's all that surprising," Greg Jaffe, the GMO expert at the Center for Science in the Public Interest in Washington, told Goats and Soda. "Anyone who's familiar with genetic engineering wouldn't be surprised that the [bacteria] Agrobacterium inserted some DNA into some crops."

That's because Agrobacterium is a bacteria that infects plants sort of like a virus does. The microbe inserts a bit of its DNA into the genome of a plant, a process known as horizontal gene transfer, the scientists explain in PNAS. The DNA insertion from Agrobacterium causes a plant's roots to grow like crazy or grow into tumors, a condition called crown gall disease.

The scientists think an ancient horizontal gene transfer happened between Agrobacterium and the plant that all today's sweet potatos are descended from.

So even though we haven't known it until now, we've been eating genetically modified foods for thousands of years, and it hasn't killed us yet.