Until now, the oldest member of the Homo genus was thought to be Homo habilis ("handy man") who lived some 2.3 million years ago. But the new fossil, described in a pair of papers published Wednesday in Science, is significantly older, and seems to link more recent members of the genus to the primates that preceded them.
"One of the most important time intervals for understanding the emergence of our evolutionary lineage, Homo, is the period between 3 [million] and 2 million years," William Kimbel, author of one of the studies and director of the Institute of Human Origins at Arizona State University, said during a news teleconference on Wednesday. "Ironically, it is one of the least well-known time periods in the human fossil record."
The period is important because it represents the gap between the time when Australopithecus (the genus that includes individuals such as the famous Lucy, a member of the species A. afarensis) flourished and when Homo hit the scene. But ironically, while researchers can find clear evidence of Australopithecus 3 million years ago and multiple, co-existing species of Homo 2 million years ago, they don't usually find much in between.
This new fossil isn't just special because it falls into the right date range, however. The piece of lower jaw, which was found in the Afar region of Ethiopia, shares similarities with both Australopithecus and Homo. The researchers report that a sloping chin links the set of teeth to the ape-like Australopithecus, but narrow, symmetrical molars and jaw proportions place it clearly in the Homo genus.
According to the geologists who dated the fossil by analyzing the volcanic ash and sediment it was found in, these early hominids lived in a climate not unlike today's Serengeti -- dry with lots of grass, probably with rivers, but few trees.
There's some evidence that climate change around 2.5 million to 3 million years ago may have left the region extremely dry and difficult to live in, and some have suggested that this event led to the appearance of hominids as Australopithecus struggled.
It now looks as though the first examples of Homo probably came before dramatic climate shifts, so it's hard to say how much this may have had to do with the emergence of the first members of the genus.
"What we do know is that early Homo could live in this fairly extreme habitat," Arizona State professor and study author Kaye Reed said, "and that apparently Lucy’s species, Australopithecus afarensis, could not."
In a related paper published Wednesday in Nature, researchers present a new reconstruction of Homo habilis, the former oldest known example of the genus. In what's believed to be a more accurate reconstruction of the skull of the species, the shape was surprisingly primitive. This may further indicate that the first ancestor of the genus lineage appeared quite early, giving the genus time to diversify into multiple species -- some more distinctly "human" from the ape-like Australopithecus than others -- by the time habilis came along.
These discoveries, the researchers said during the teleconference, help narrow down the period during which archaeologists should search for clues about the transition from ape to man.
"Of course this specimen raises many more questions than it answers, and those questions will only be resolved by further fieldwork," study leader Brian Villmoare of the University of Nevada at Las Vegas said during the news conference. "Hopefully, with future discoveries in our ongoing fieldwork, we will be able to speak to many of those questions."