Interbreeding between archaic and modern humans
A bone fragment found in Denisova Cave, "Denisova 11", is identified as that of a girl who died 50,000 years ago aged at least 13. Genome-wide signatures tell that she had a Neanderthal mother and a mostly-Denisovan father. This first find of a prehistoric first-generation mixed offspring shows that different archaic human subtypes sometimes migrated, and then mixed more frequently than previously thought.
Sources: Live Science NBC News