Prof Brian Villmoare of the University of Nevada in Las Vegas said the discovery makes a clear link between an iconic 3.2 million-year-old hominin (human-like primate) discovered in the same area in 1974, called "Lucy".
Could Lucy's kind - which belonged to the species
Australopithecus afarensis - have evolved into the very first primitive humans?
"That's what we are arguing," said Prof Villmoare.
But the fossil record between the time period when Lucy and her kin were alive and the emergence of
Homo erectus (with its relatively large brain and humanlike body proportions) two million years ago is sparse.
The 2.8 million-year-old lower jawbone was found in the Ledi-Geraru research area, Afar Regional State, by Ethiopian student Chalachew Seyoum. He told BBC News that he was "stunned" when he saw the fossil.
"The moment I found it, I realised that it was important, as this is the time period represented by few (human) fossils in Eastern Africa."
The fossil is of the left side of the lower jaw, along with five teeth. The back molar teeth are smaller than those of other hominins living in the area and are one of the features that distinguish humans from more primitive ancestors, according to Professor William Kimbel, director of Arizona State University's Institute of Human Origins.
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Prof Chris StringerNatural History Museum, London
"Previously, the oldest fossil attributed to the genus
Homo was an upper jaw from Hadar, Ethiopia, dated to 2.35m years ago," he told BBC News.
"So this new discovery pushes the human line back by 400,000 years or so, very close to its likely (pre-human) ancestor. Its mix of primitive and advanced features makes the Ledi jaw a good transitional form between (Lucy) and later humans."
A computer reconstruction of the skull from Hadar, belonging to the species
Homo habilis, which has been published in Nature journal, indicates that it may well have been the evolutionary descendant of the species announced today.
The researcher involved, Prof Fred Spoor of University College London told BBC News that, taken together, the new findings had lifted a veil on a key period in the evolution of our species.
"By discovering a new fossil and re-analysing an old one we have truly contributed to our knowledge of our own evolutionary period, stretching over a million years that had been shrouded in mystery," he said.
Climate change
The dating of the jawbone might help answer one of the key questions in human evolution. What caused some apes to climb down from the trees and make their homes on the ground.
A separate study in Science hints that a change in climate might have been a factor. An analysis of the fossilised plant and animal life in the area suggests that what had once been lush forest had become dry grassland.
As the trees made way for vast plains, apes found a way of exploiting the new environmental niche, developing bigger brains and becoming less reliant on having big jaws and teeth by using tools.
Prof Chris Stringer of the Natural History Museum in London described the discovery as a "big story".
He says the new species clearly does show the earliest step toward human characteristics, but suggests that half a jawbone is not enough to tell just how human it was and does not provide enough evidence to suggest that it was this line that led to us.
