Back in the 1800s the underground railroad brought slaves to chicago, and they brought a primitive-ish form of tap dancing and jazz music with them. Somewhere around the 1910s it was brought to Broadway and refined in the theatre, where it combined with ballet technique and classical jazz (now called musical theatre) was also developed. I believe this was influenced by the Depression. Meanwhile, jazz music was spreading into different styles such as swing, blues, rock, and funk. Funk was embraced on the streets of Brooklyn where djs took their turn-tables and in the late 70s "hip-hop" was born. In terms of contemporary, Balanchine started the NYCB, and, perhaps inspired by the new movement of jazz, became the father of contemporary ballet. Modern stemed from this somewhere around the 50s, and as an "alternative" ballet, was very popular with African-American culture, as it still is today. Modern, ballet, jazz, and street evolved around the 70s to what is currently known as jazz, which recombined with ballet to form lyrical, which recombined with modern in the last 10-15 years to form contemporary, todays "fad" of dance. Dance can definitly not be defined as a line. It cant even have a family tree. Dance is a cult, filled with incest, rape, transformations, eras.....
I hope this helped, this is dance in a nutshell.