Close-up images of LD 350-1 lower jaw
William Kimbell
The fossil record of hominids between three million years ago and two million years ago is thin, leaving a gap in our understanding of human evolution. Prior to three million years ago (called mega-annum or Ma), there were many fossils of it Australopithecus, the ape-like humanoids including the famous Lucy. By 2 Ma there are multiple overlapping fossils clearly belonging to the genus Homo, the group that includes modern humans and all of our most direct ancestors. As compared to Australopithecusthese creatures had larger brains, smaller jaws and stone technology.
The gap is important, says Brian Villmoare, a physical anthropologist who analyzes facial features in fossils. This is the period when our ancestors made the important transition to the more adaptive patterns we see Homo.
The discovery of a fossilized Homo jawbone, published in Science this week sheds light on this chasm in human evolutionary history. The fossil has features that classify it as belonging to the genus Homo, but it is about 2.8 million years old. This makes it 400,000 years older than the oldest evidence of it Homo previously found.
The jawbone was discovered in the Ledi-Gararu research area in Ethiopia by Chalachew Seyoum, a graduate student at Arizona State University who worked at the research site in his home country of Ethiopia. He found a fragment of the jawbone on the surface of the mound he was surveying, and the obvious fracture of the fragment made him realize there could be more pieces in the area. After further searching, he found pieces that fit together perfectly.
The new find, labeled LD 350-1, shares some features with jawbones from Australopithecus afarensis, especially its size. Because fossils of A.afarensis was found nearby and dated to about 3 Ma, the researchers investigated the possibility that it could be late Australopithecus fossil of this species rather than an early one Homo fossil.
However, LD 350-1 carries several trademark features Homo, including the wear pattern on the teeth, the shape of the molars and symmetrical premolars. These features, which are distinctive features of Homo fossils, indicate that the specimen is a transitional fossil between Australopithecus And Homo, the researchers write. But they say there isn’t enough information in a single jaw to identify it as belonging to a particular species. It has similarities with homo habilisbut it also has some more primitive features that suggest it does not belong to this genus.
To determine the age of LD 350-1, the researchers used radiometric dating techniques to determine the age of the rock layer in which the fossil was found. This placed it between 2.8 and 2.75 Ma. “The discovery of a 2.8 million year old rock that contains fossils, especially hominin fossils, is really exciting,” said Erin DiMaggio, principal investigator of a second Science document describing the geological and ecological features of the Ledi-Gararu area. “Many of the rocks from this time eroded long ago, so this discovery not only sheds light on human lineage, but also the geological setting and environment in which early Homo lived.”
The environment of that time is important, explains Kaye Reed, a co-author of the paper led by DiMaggio. “Many researchers suggest that climate change caused evolutionary changes in a variety of mammals, including hominins.” Specifically, the region is thought to have changed from a wetter, forested area to a drier and more open plain. This would have created selection pressure for new and different characteristics of the hominins that lived there at the time. “Now we can reconstruct the environment in which these species lived,” says Reed.
More fossils, including those of other mammals, were recovered in the sediment layer containing LD 350-1. Many of these fossils show adaptations seen in animals living in arid climates. Overall, the evidence indicates that the area would have been open grassland at that time, with rivers and lakes but few trees; it would have been an environment similar to the modern Serengeti plains.
This discovery raises many more questions, says Villmoare, and the researchers are continuing to explore the area, looking for further evidence. It is necessary to investigate the exact nature of the transition, says William Kimbel. “For example, it is also not known whether the early changes in the teeth and jaws were accompanied by changes in other systems, such as the brain or technology,” he explains. At some point, the ancestor of the Homo species evolved larger brains, but whether this change was in effect as early as 2.8 Ma is unknown.
Further evidence that we should be seeing significant transient gay fossils in this era comes from a paper in Naturealso published this week, detailing the automated reconstruction of a previously identified Homo jaw based on a partial fossil dated to 1.8 Ma. The reconstruction revealed that the jaw had unexpectedly primitive features, suggesting Homo habilis arose before 2.3 Ma — timing consistent with the LD 350-1 find.
Combined, these discoveries significantly narrow the time frame in which field workers now have to focus their search for these answers, Kimbel says.
Similar questions remain for the geologists investigating the climate change thought to have driven mammalian evolution in the region. Until there is more evidence, it’s impossible to tell if there was a general trend in the region toward more arid and open habitat, Reed says, or if this was just a local phenomenon.
“What we do know is that Homo lived here, but Australopithecus didn’t,” says Reed.
Science2015. DOI: 10.1126/science.aaa1343 (About DOIs).