Scientists have discovered a fragment of an ancient jawbone belonging to a potentially new species revealing that the human family may have emerged nearly half a million years earlier than previously believed.

The specimen, discovered in Ledi-Geraru in Ethiopia, is 2.8 million years old, which means that it is 400,000 years older than what experts thought the human family has first emerged. Prior to the find, the oldest fossil associated with the genus Homo was an upper jaw dated 2.35 million years ago and was also found in Ethiopia.

Brian Villmoare, from the University of Nevada in Las Vegas, said that the discovery offers the first insight into what is considered the most important transitions in human evolution. He said that it makes a clear association between Lucy, the iconic hominin that was discovered in 1974, as there is the possibility that Australopithecus afarensis, Lucy's species evolved into the first primitive humans.

"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," said William Kimbel, the director of the Institute of Human Origins at Arizona State University.

The newly discovered fossil is the left side of a lower jaw and comes with five teeth. Its back molar teeth, a feature that distinguish humans from their more primitive ancestor, are smaller compared with those of other hominins that lived in the area.

Don Johanson, one of the discoverers of the Lucy fossils said that the research is closing the gap between Lucy and humans and the more advanced Homo ancestors. He also said that the Ledi-Geraru fossil implies that there were possibly two or three Homo lineages.

The age of the jawbone could also answer one key question regarding human evolutions: what could have led to some of our primitive ancestors to transition from living in the trees to making their homes on the ground.

Another study suggested that climate change may have contributed to this phenomenon. The analysis hinted that what used to be lush forests became dry grasslands and as human-like primates exploited new environmental niche, they developed bigger brains and became less reliant on their big jaws and teeth when they started to use tools.

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