Scientists in northeastern Ethiopia said Saturday that they have discovered the skull of a small human ancestor that could be a missing link between the extinct Homo erectus and modern man.

The hominid cranium -- found in two pieces and believed to be between 500,000 and 250,000 years old -- ''comes from a very significant period and is very close to the appearance of the anatomically modern human,'' said Sileshi Semaw, director of the Gona Paleoanthropological Research Project in Ethiopia.