It's not a sunflower, so what is it?

When I walked into the veggie garden this morning, I was startled to see that what I had supposed was a volunteer sunflower had somehow changed into some type of alien sunflower-y like plant. hmm. This happened a few years ago, but then what I thought was a sunflower was some type of thick-trunked, spined horrible weed. This one does not have spines, at least (unless I go out there tomorrow morning and it has somehow sprouted some). I don't know what this thing is, but the leaves and form of the plant looked like a sunflower until I noticed the little green spheres instead of a sunflower head. Now that I look at it, the stems are purple-y instead of the green of the other sunflower in the garden. If anybody has any ideas as to what this thing is, please let me know! I am always amazed at the number of similar, yet not the same, types of plants that I see in nature.