Fossil teeth and jawbones found in eastern Ethiopia are bridging a gap between two species of human ancestors that lived about four million years ago.

The fossils are from the most primitive species of Australopithecus, called Au. anamensis, and date from about 4.1 million years ago.

Fossil teeth, found in Ethiopia, are from the upper jaw of Au. anamensis, a hominid ancestor of humans. (Photo courtesy Tim White/Brill Atlanta)

Researchers say the teeth are a link between this species of Australopithecus, sometimes called ape-man, and earlier hominids.

Seven species have been named in the genus Australopithecus, considered to be the direct ancestor of the genus Homo, which includes humans.

The most famous Australopithecine fossil – discovered in 1974 – was found in the same area of Ethiopia's Afar desert. The "Lucy" species, called Au. afarensis, lived between 3.6 and three million years ago.

"This discovery closes the gap between the fully blown Australopithecines and earlier forms we call Ardipithecus," said study co-author Tim White of the University of California, Berkeley, in a statement.

The new fossils fall into the million-year gap in the fossil record between Ardipithecus and Australopithecus, and shed some light on how one genus led to the other.

The researchers say the fossil teeth are an intermediate between the smaller teeth of Ardipithecus and the large cheek teeth of Australopithecus, which allowed the latter to eat tougher food, such as plant roots and tubers.

"Australopithecus became a superior omnivore ... adapting it better to times of scarcity during periods of extended drought," White said.

The first of the fossils – teeth and an upper jawbone – were found in 1994. The research team, including scientists from the United States, Ethiopia, Japan and France, found additional fossils in 2000, 2003 and 2005. Their findings appear in Thursday's issue of the journal Nature.

The fossils represent the remains of about eight individuals, the scientists said, and include the largest hominid canine tooth found so far.

Many of the fossilized teeth were completely shattered, so the fragments had to be separated from the sand and painstakingly reassembled.

They were found in the Middle Awash area of the Afar desert, about 230 kilometres northeast of Ethiopia's capital, Addis Ababa.