March 04, 2015
In 1964 Louis Leakey and his colleagues announced the new species Homo habilis, or Handy man, as the earliest known member of our human evolutionary lineage. Central to the newly recognised species was the fossil Olduvai Hominid 7 (OH 7 for short), which consists of a lower jaw, parts of a braincase and hand bones of a single individual. These bones had been found in 1.8 million-year-old layers exposed in Olduvai Gorge, Tanzania.
The challenge in the intervening 50 years has been to work out which other fossils also belong to Homo habilis, as a key to understanding the early evolution of the human lineage. However, the preservation of the original Handy man fossil OH 7 has been a major hindrance; the shape of its lower jaw is distorted and the partial braincase consists of many broken fragments. To address this problem head-on the researchers used computed tomography (CT) and state-of-the-art 3D imaging technology to digitize, tease apart and reassemble the pieces virtually in the computer.
After weeks of painstaking reconstruction OH 7 re-emerged showing an unexpected mixture of features. The lower jaw turns out to be remarkably primitive in shape; it has a long and narrow dental arch which resembles that of the much older species Australopithecus afarensis (‘Lucy’), rather than closer related species such as Homo erectus. In contrast, the reconstructed braincase of OH 7 is not primitive at all, showing that the brain size was larger than previously estimated and similar to that seen in Homo erectus. For the first time an undistorted OH 7 could now be compared with other fossils, leading to two important conclusions.
Firstly, large differences in jaw shape among early Homo fossils show that three different species existed between 2.1 and 1.6 million years ago: Homo habilis, Homo erectus and Homo rudolfensis. “Sophisticated statistical analyses reveal differences in the shape of the jaw between these early human species that are sometimes as large as between humans and chimpanzees” says Philipp Gunz of the Max Planck Institute for Evolutionary Anthropology and one of the lead authors of the study. In the past differences in brain size were often considered important to characterize species of early Homo. However, the new analyses show that the three species cannot be distinguished by their brain size, in contrast to their telling differences in facial appearance.
Secondly, the new findings provide insights into the evolutionary origins of Homo habilis and the genus Homo in general. An upper jaw bone from Ethiopia, known as AL 666-1 and dated to 2.3 million years ago, is commonly thought to represent a potential ancestor or early representative of Homo habilis. However, this fossil is now shown to be too modern human-like in shape to be ancestral to Homo habilis with its primitive jaw. Instead, the two fossils from Ethiopia and Tanzania appear to represent separate evolutionary lineages that likely split well before 2.3 million years ago, with a common ancestor that has remained elusive until this week.
A report in the journal Science describes the discovery of a 2.8 million-year-old lower jaw from Ledi-Geraru, Ethiopia, which now provides the earliest evidence of the genus Homo (Science Express, March 5th; embargo conditions apply). The fossil, known as LD 350-1, makes a good ancestor for Homo habilis and other species of early Homo. “By digitally exploring what Homo habilis really looked like we could infer the nature of its ancestor, but no such fossils were known. Now the Ledi-Geraru jaw has turned up as if ‘on request’, suggesting a plausible evolutionary link between Australopithecus afarensis and Homo habilis”, concludes Fred Spoor of University College London and the Max Planck Institute of Evolutionary Anthropology.