The Neanderthal genome reveals a long history of interbreeding among at least four different types of early humans living in Europe and Asia, scientists say. Researchers extracted DNA from a woman's toe bone that dates back 50,000 years and compared it with the genomes of modern humans and a recently recognized group of early humans called Denisovans, the University of California, Berkeley, reported Wednesday. The Denisovans were named for the Denisova Cave in Siberia where remains have been found. Neanderthals and Denisovans were very closely related, the comparison showed, and their common ancestor split off from the ancestors of modern humans about 400,000 years ago. Neanderthals and Denisovans then split about 300,000 years ago, and although both eventually died out they left behind bits of their genetic heritage because they occasionally interbred with modern humans, the researchers said. The genetic studies also show Denisovans interbred with a mysterious fourth group of early humans also living in Eurasia at the time, a group had split from the others more than a million years ago and may have been the group of human ancestors known as Homo erectus, which fossils show was living in Europe and Asia a million or more years ago. "The paper really shows that the history of humans and hominins during this period was very complicated," Berkeley population geneticist Montgomery Slatkin said. "There was lot of interbreeding that we know about and probably other interbreeding we haven't yet discovered."