Justin Timberlake, no stranger to viral videos ("Dick in a Box," "Motherlover," etc.) last night joined Late Night host Jimmy Fallon and house band the Roots for the above clip: "A History of Rap." Timberlake and Fallon turn an innocuous conversation about the Sugar Hill Gang into a full-blown medley touching on the past 30 years of hip-hop, including the Beastie Boys, Digital Underground, The Chronic, and "Empire State of Mind." (The Roots' "The Seed 2.0" makes a brief appearance, too.) Perhaps most impressive: Timberlake and Fallon's abilities at imitating Notorious B.I.G., Eminem and Snoop Dogg. Still, "A History of Rap" has a long way to go before it closes in on the 153 million views garnered by the similar "Evolution of Dance" video on YouTube.