This photo of the LD 350-1 mandible was taken steps from where it was found by graduate student Chalachew Seyoum. | Kaye Reed
A piece of an ancient jawbone discovered in eastern Africa suggests that the genus Homo, to which all modern humans belong, arose almost half a million years earlier than previous research had indicated.
This fossil, known as LD 350-1, was unearthed from the Ledi-Geraru research area at the Afar Regional State in Ethiopia in 2013. Researchers say that the 2.8 million-year-old jawbone exhibits a unique blend of primitive Australopithecus traits and more modern Homo features.
Brian Villmoare from the University of Nevada, Las Vegas and colleagues from around the world analyzed the fossil mandible and its five intact teeth, while Erin DiMaggio from Pennsylvania State University and another international team of researchers described the geological setting in which the fossil was discovered.
Their findings are published together in the 6 March issue of the journal Science.
Chalachew Seyoum, an Ethiopian graduate student from Arizona State University who discovered the fossil, said his country has been the center of human evolution studies since the 1960s. "So many important hominin fossils have been discovered there, including Lucy, Selam, and Ardi, which have been attracting researchers all over the world and answering critical questions in human evolution," Seyoum said at a 4 March press teleconference. "This new fossil from Ledi-Geraru…is also one of these discoveries."
Until now, the earliest credible fossil evidence of the genus Homo was dated to about 2.3 or 2.4 million years ago. Older hominin fossils have generally been attributed to the Australopithecus genus, but a limited fossil record between 2.0 and 3.0 million years ago has made it difficult to discern how the transition from Australopithecus to Homo occurred.
The Ledi-Geraru site is one of many important human fossil and stone tool sites in Ethiopia. | Erin DiMaggio
"Prior to 3 million years ago, we have a good fossil record of early Australopithecus, such as Lucy's species in eastern Africa, A. afarensis, a small-brained upright biped, which is often considered a remote human ancestor," explained William Kimbel from Pennsylvania State University during the teleconference. "By around 2 million years ago, we have multiple overlapping species of early Homo, including H. habilis, H. rudolfensis, and early H. erectus. These are species with larger brains, slimmed-down jaws and teeth, and stone tool technology."
After studying the LD 350-1 fossil in detail, the researchers suggest that although the mandible's age and location place it close to Australopithecus afarensis, its teeth align it more closely with the early Homo species.
"What is special about this jaw is not only the date, which is much older than any specimen of Homo known until now, but its unique combination of traits — from the height of the mandible to the shape of the teeth — that make it clearly transitional between Australopithecus and Homo," said Villmoare. "The fact that it has features that so clearly ally it with Homo by 2.8 million years ago helps us narrow the time of transition and suggests that the transition itself was relatively rapid."
Analysis of the fossil's sedimentary surroundings suggested that the site was mostly mixed grasslands and shrubs when the mandible was deposited. There was also a lake and rivers in the area with hippos, crocodiles, and fish, according to the researchers.
"The fossils that we have recovered at the time of LD 350-1 indicate an extremely open environment, similar to that of the Serengeti Plains today — a region with grasses as far as the eye could see," explained Kaye Reed from Arizona State University. "What we cannot say until we have more sediments in the time period before 2.8 million years ago and after 2.95 million years ago is whether there was a trend toward this more open and likely arid habitat that is present during the time of our earliest Homo."
"What we do know is that early Homo could live in this fairly extreme habitat, and that apparently Lucy's species, A. afarensis, could not," Reed said.