Purity

Where does the idea of purity come from?

I watched a video on the future of food, which discussed how future food may come form unconventional sources. (you may watch it here) While I am a little adamant about fly burgers and recycled food coming from our sewage, I can still imagine it being acceptable in the near future. But when the video started to talk about genetic programming, I got a little queasy. Yes, it is already acceptable now, thats how we got our seedless grapes and oranges and fatter melons and leaner meats, but still, the idea of artificial selection and playing god kinda scares me. Especially when we insert genes from another species into an egg. The video described how scientists injected genes from spiders into a sheep's egg, so that the wool generated will have the qualities of spiders's silk. While this may create stronger wool products, and has a huge potential in terms of bullet proof vests, climbing material, etc, I have no idea why it irks me just that bit.

Then it hits me, how did I get the idea that natural is better than artificial? Where did the idea of purity come from? I am not a Christian, so all the business about playing god shouldn't affect me this way, but somehow I still cling on to the idea of natural purity so stubbornly. I really wonder why.