The remains of at least nine primitive human ancestors up to 4.5 million years old have been discovered by paleontologists working in Ethiopia.

The Ardipithecus ramidus fossils show significant ape-like characteristics of the ancient hominids, who lived in woodlands rather than the open savanna.

The discovery was made at As Duma in the Afar region of Ethiopia, about 500 kilometres northeast of Addis Ababa. The site has been a rich source of significant fossils for anthropologists.

Fossil of a hominid lower jaw bone believed to be about 4.5 million years old. (Courtesy: Sileshi Semaw)
Fossil of a hominid lower jaw bone believed to be about 4.5 million years old. (Courtesy: Sileshi Semaw)

The fossils are mostly teeth and jaw fragments, with some hand and feet bones, Spanish and American researchers report in Thursday's issue of the journal Nature.

Scientists say Ardipithecus is the earliest hominid after the lineage split to chimpanzee. They hope the finds will help fill in some gaps in their understanding of early human evolution.

The canine teeth are small and blunt like those of other human ancestors, while the other teeth, such as the molars, resemble those of great apes.

Hominid fossils from Gona, Ethiopia date to 4.5-4.3 million years ago. (Copyright: GPRP 2004; Courtesy: Sileshi Semaw)
Hominid fossils from Gona, Ethiopia date to 4.5-4.3 million years ago. (Copyright: GPRP 2004; Courtesy: Sileshi Semaw)

Wear on the teeth suggests the hominids were herbivores, the team reported after a four-year dig that started in 1999.

Paleoanthropologist Sileshi Semaw of Indiana University Bloomington and colleagues also found evidence the human ancestors lived near a menagerie of antelope, rhinos, monkeys, giraffes and hippos.

Back then, the region was much wetter than today and experienced seasonal droughts, the team said.

Although the researchers aren't clear about what sort of habitat the hominids lived in, remains from the area suggest it included swamps, springs and streams.

The individuals date to between 4.3 million and 4.5 million years old, according to geological and isotope tests.

Other specimens of A. ramidus suggest its skull was aligned on top of its spinal column, instead of in front as in apes. The difference points to an ability to walk upright.