A Harvard museum official has discovered a five-million year old bone of a primitive man.

Arnold D. Lewis, head of the preparation laboratory of Harvard's Museum of Comparative Zoology, found the jaw bone and molar during an expedition to Lake Rudolph in Northern Kenya in 1967. The find was not announced until yesterday because extensive research was needed to determine the age of the fossil.

The expedition was one of five led by Bryan Patterson, professor of Vertebrate Paleontology. The expeditions, sponsored by Harvard's Museum of Comparative Zoology with support from the National Science Foundation, searched for fossils in northwestern Kenya.

"The find is important," Lewis said, "insofar as it fills a little gap in the history of man that we are trying desperately to complete. But it is only part of the whole story-the expedition brought back ten tons of fossils. As an individual specimen it could be better. After all, it is only half a jaw and a tooth."

Commenting on the discovery, William W. Howells, professor of Anthropology, said, "Until recently some scientists seemed to think that man kind of jumped down from the trees and said 'Give me a tool!' Now we know that man did not develop so rapidly as people thought."

"The sensation of this particular discovery," Howells continued, "is that it goes back as far as five million years. Ten years ago, scientists thought the Australopithecus, a direct ancestor of Homo sapiens, was only a half a million years old. Then several fossils were discovered which proved he lived long before that. But this is the oldest bone discovered so far."