Turns out that's a really good question, and the answer seems to be "By tagging along with their human buddies." When you think that your basic model of chicken is a creature native to Southeast Asia, and then drive down the street in urban Portland past somebody's front yard with oh so hip coopful of fowl, you have to ask how that happened.
Luckily for all of us chicken fans, there's "Cultural & Scientific Perceptions of Human-Chicken Interactions." A project including no less than six British universities, scicultchickens.org aims to research and explain what chickens have been to us, and when and how. There's a database of archaeological chicken bone finds, a blog of various activities in chicken-human research (the latest post at time of writing is an examination of chickens in video games), a gallery with an interpretation of imaginary chicken breeds, and much more.
There's a related, more informal website on the topic too - Chicken Coop.