Scientists at Indiana University and other institutions have unearthed skeletal fossils of a human ancestor believed to have lived between 4.3 and 4.5 million years ago, the US National Science Foundation (NSF) said Wednesday.

The fossils, which are believed to come from nine individuals of the species Ardipithecus ramidus, will help scientists piece together the mysterious transformation of primitive chimp-like hominids into more human forms, scientists said.

The fossils were retrieved from the Gona Study Area in northern Ethiopia, a site known for the excavation of the oldest stone tools ever discovered.

Several Ethiopian dig sites have yielded hominid fossils from that time period. Plant and animal fossils indicate that these early humans lived in a low-lying area with swamps, springs, streams, and volcanic centers, with a mosaic of woodlands and grasslands.

"While biomolecular evidence helps us to date the timing of major events in the evolution of apes and humans, there is no substitute for fossils when it comes to trying to picture the anatomy and behavioral capabilities of our early relatives," Mark Weiss, program officer at the NSF said.

"The late Miocene-early Pliocene is a particularly important era as it was roughly at that time that our ancestors and those of the chimpanzee parted company. Each new fossil helps to tell a bit more of the story of these early stages in human origins."

The dig team led by Sileshi Semaw also reported new evidence that suggests those human ancestors lived in close quarters with a menagerie of antelope, rhinos, monkeys, giraffes and hippos in northern Ethiopia that was far wetter than it is today. Research is continuing to determine which habitats those hominids preferred.

Despite millions of years that separate us, modern humans have a few things in common with Ardipithecus ramidus.

Fossils from Gona and elsewhere suggest that the ancient hominid walked on two feet and had diamond-shaped upper canines, not the "v"-shaped ones chimps use to chomp. However, the hominids would appear a lot more chimpanzee-like than human outwardly.