At first glance you may think the question seems, well ridiculous; I mean what does a chicken have to do with a soybean anyway... more than you think actually! And if you suffer from a soy allergy and or are one of those that haven't been brain washed by those who tout the soybean as a "super-food", you may want to choose your poultry and egg source wisely.
To answer the question though, you're right it really doesn't matter which came first. It does however warrant mentioning the connection between the two. Although examining the content of soy protein present in poultry and eggs is not a hot topic for researchers, one such study was performed by Professor M. Monica Giust and the results concluded the following
"The transfer of Isoflavones, a potent phytoestrogens found in soybeans, can indeed be manipulated by altering the soy content in commercial chicken feed."
Unfortunately soybean meal is the main ingredient of poultry feed (for both non-organic and organic varieties) used to alter the protein content in the feed itself. Having a handful of free-ranging chickens myself, I came across this unpleasant discovery the hard way and I found it very difficult to find an organic feed sold locally that did not contain soy. This of course, along with what I know and in turn despise about processed foods, led me to mix my own chicken feed made from organic whole oats, whole wheat berries, and whole corn... leaving the protein part of the equation to all of the insects (flies, worms, grubs, etc.) that my free-range birds find on my property.
So what can you do to insure the chicken and eggs that you eat are not packed with these potent phytoestrogens (soy)? How about make sure all of the meats, eggs, and dairy products you consume come from local organic free-range/grass fed sustainable farms.