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Scientists Thursday reported the discovery of a 13 million-year-old piece of jawbone from a previously unknown, apelike forerunner of gorillas, chimpanzees and humans.

The discovery "opens up a whole new geographical area for exploration into pre-human ancestry," said Glenn Conroy, one of the researchers reporting the find in Thursday's issue of the journal Nature.The fossil was found in Namibia, showing that such "Miocene hominoids" ranged much farther south in Africa than previously known from fossil finds in Kenya and Uganda, researchers said.

The creature apparently lived before hominoids evolved into two branches, Conroy said. One branch led to humans and the other to chimpanzees and gorillas. The split is thought to have occurred between 10 million and 5 million years ago.

Conroy is a professor of anatomy and anthropology at the Washington University Medical School in St. Louis. He reported the discovery with Martin Pickford of the Paleontology Institute of the National Museum of Natural History in Paris, who found the fossil, and other scientists.

The fossil is the right side of the lower jaw, including some teeth. Conroy said the size of the teeth suggests the creature was about the size of a small chimpanzee, weighing around 35 pounds.

A wisdom tooth had appeared, showing the creature was an adult, perhaps about 10 years old, when it died, Conroy said in a telephone interview. That age is considered mature because of the creature's rate of development, in much the same way dogs become adults at younger ages than humans do.

The tooth shapes show it ate soft plants, probably including lots of fruit, he said. Its environment must have been more humid and forested than the very dry surroundings that exist there now, Conroy said.

The scientists named the creature Otavipithecus namibiensis, because it was found in the Otavi region of northern Namibia.

Conroy said the finding was a surprise. Researchers had been seeking more recent creatures that appeared on the human-bound branch of the hominoid evolutionary split, such as australopithecines or early members of the Homo group, he said.

Brenda Benefit, an assistant professor of anthropology at Southern Illinois University at Carbondale, said scientists had suspected such creatures lived in southern Africa, partly because australopithecines had been found there.