The distinct neighborhoods that grew up around downtown Dallas in the decades following the turn of the century and helped to create the city’s unique character are the focus this KERA production. “Big D Back When” combines archival film footage, still photos and interviews with a wide range of Dallasites to create a portrait of the past that offers fascinating insights about how Dallas has evolved. Among the neighborhoods featured in “Big D: Back When” are Little Jerusalem — which later became Little Mexico — Deep Ellum, the Thomas and Hall community now known as State-Thomas, Old South Dallas and Oak Cliff — a separate city until it was annexed by Dallas in the early years of the century. The program also highlights La Reunion, a short-lived utopian colony along the Trinity that was settled by European immigrants. For new Dallasites and longtime residents alike, “Big D: Back When” offers an entertaining and informative look at how early neighborhoods grew up and changed in relation to downtown, the river and the railroads. Most of all, however, this is the fascinating story of the people themselves, the pioneers, the visionaries and the families who made something special out of Big D — back when.