Researchers unveiled a 10-million-year-old jaw bone on Tuesday they believe belonged to a new species of great ape that could be the last common ancestor of gorillas, chimpanzees and humans.

The Kenyan and Japanese team found the fragment, dating back to between 9.8 and 9.88 million years, in 2005 along with 11 teeth. The fossils were unearthed in volcanic mud flow deposits in the northern Nakali region of Kenya.

The species -- somewhere between the size of a female gorilla and a female orangutan -- may prove to be the "missing link," the key step that split the evolutionary chains of humans and other primates, Kenyan scientists said.