Several Xbox 360 games have been made backward compatible on Xbox One today, according to Microsoft. The most notable of the additions are truly old-school: Phantasy Star 2 and Sonic & Knuckles, both Xbox Live Arcade games now playable on Xbox One.
Sega originally released both games on Sega Genesis; Phantasy Star 2 came stateside in 1990, while Sonic & Knuckles launched worldwide in 1994, months after Sonic the Hedgehog 3. The original Sonic & Knuckles cartridge could be directly locked onto its predecessor's to access the older game's data.
Other 360 games now playable on Xbox One include Dig Dug, Runner 2 and Samurai Shodown 2. With the exception of Runner 2, today's update is certainly a throwback: The majority of these releases are over 20 years old.