"It's already clear that we're seeing the basic grade from which
Australopithecus evolved," he said.
"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 modern humans, 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, kadabba, lived in
Ethiopia at around 5.54 to 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 Ardipithecus
ramidus lived alongside monkeys, mole rats and cow-like grazing
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 Ardipithecus ramidus
fossils in the Middle Awash region of Ethiopia with his colleagues
Gen Suwa and Berhane Asfaw.