Fossil hunters working in Ethiopia have unearthed the remains of at least nine primitive hominids that are between 4.5 million and 4.3 million years old.
Fragments of at least nine hominids were found. (Image: Sileshi Semaw)
The fossils, which were uncovered at As Duma in the north of the country, are mostly teeth and jaw fragments, but also include parts of hands and feet.
All finds belong to the same species - Ardipithecus ramidus - which was first described about a decade ago.
Details of the discoveries appear in the latest issue of Nature magazine.
Scientists say features of a pedal phalanx, or foot bone, unearthed at the site confirm the hominid it belonged to probably walked upright like we do.
"It is a very important finding because it does confirm hominids walked upright on two feet definitely 4.5 million years ago," said lead author Sileshi Semaw, of the Craft Stone Age Institute at Indiana University in Bloomington, US.
A. ramidus is also marked out by its diamond-shaped upper canine teeth, which are more humanlike than the "V" shaped upper canines of chimpanzees. But overall, the creature probably looked more like a chimpanzee than a human.
Professor Tim White, of the University of California, Berkeley, agreed it was becoming apparent A. ramidus was an important species that was a very plausible ancestor to later hominids.
"It's already clear that we're seeing the basic grade from which Australopithecus evolved," he told the BBC News website.
The bones come from a site that has yielded many hominid fossils
"The real issues about the earliest hominids are now going to centre on whether we are seeing the same basic creature in Kenya, Ethiopia, and Chad."
The most famous Australopithecus fossils are those of "Lucy", a female skeleton discovered in Ethiopia during the 1970s. The so-called australopithecines are widely thought to have led on to the human lineage.
Ardipithecus could therefore represent an earlier step on the path which led to us, as well as a number of other, extinct hominid species.
The age of the newly described remains was estimated by dating volcanic material found in their vicinity.
"A few windows are now opening in Africa to glance into the fossil evidence on the earliest hominids," Dr Semaw explained.
"We now have more than 30 fossils from at least nine individuals dated between 4.3 and 4.5 million years old."
Another Ardipithecus species, A. kadabba, lived in Ethiopia around between 5.54 and 5.77 million years ago.
Genetic studies have suggested a common ancestor for modern apes and humans may have existed about six million years ago.
Other fossils found at the site show that A. ramidus lived alongside monkeys, mole rats and cow-like grazing animals.
But the authors add that it is not clear exactly in what sort of habitat the hominids lived.
The area where the remains were unearthed would have had features of swamps, springs and streams, as well as regions that experienced seasonal droughts.
Professor White discovered the first A. ramidus fossils in the Middle Awash region of Ethiopia with his colleagues Gen Suwa and Berhane Asfaw.