The hominin fossil record 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 many fossils of Australopithecus, the apelike hominins that included 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 our most direct ancestors. Compared to Australopithecus, these creatures had larger brains, smaller jaws, and stone technology.

The gap is an important one, says Brian Villmoare, a physical anthropologist who analyzes facial characteristics of fossils. This is the period during which our ancestors made the important transition to the more adaptive patterns seen in Homo.

The discovery of a fossilized Homo jawbone, published in Science this week, sheds light on this gap in human evolutionary history. The fossil has features that classify it as belonging to the genus Homo, but it's approximately 2.8 million years old. This makes it 400,000 years older than the oldest evidence of 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 was working at the research site in his home country of Ethiopia. He found a fragment of the jawbone on the surface of the hill he was surveying, and the clean break of the fragment led him to realize that there could be more pieces in the surrounding area. After further searching, he found pieces that fit together perfectly.

The new find, labeled LD 350-1, shares some characteristics with jawbones from Australopithecus afarensis, especially its size. Because fossils from A. afarensis had been found nearby and dated to approximately 3 Ma, the researchers explored the possibility that it could be a late Australopithecus fossil from this species rather than an early Homo fossil.

However, LD 350-1 bears multiple trademark features of Homo, including the wear pattern on the teeth, the shape of the molars, and symmetrical premolars. These features, which are distinguishing characteristics of Homo fossils, indicate that the specimen is a transitional fossil between Australopithecus and Homo, write the researchers. But they say there is not enough information in a single jaw to identify it as belonging to a particular species. It has similarities to Homo habilis, but it also has some more primitive characteristics that suggest it does not belong to this species.

To determine the age of LD 350-1, the researchers used radiometric dating techniques to establish 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 record that contains fossils, and especially hominid fossils, is really exciting,” explains Erin DiMaggio, lead researcher on a second Science paper that details the geological and environmental characteristics of the Ledi-Gararu area. “Many of the rocks from this time were eroded long ago, so this discovery sheds light not just on human lineage, but also on the geologic setting and environment in which early Homo lived.”

The environment of the time is important, explains Kaye Reed, a co-author on the paper led by DiMaggio. “Many researchers suggest that climate change drove evolutionary change in a variety of mammals, including hominids.” Specifically, the region is thought to have changed from being a wetter, forested area to a drier and more open plain. This would have created selection pressures for new and different characteristics in the hominids living 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 from the sediment layer containing LD 350-1. Many of these fossils display adaptations seen in animals living in arid climates. Altogether, the evidence indicates that the area at the time would have been open grassland, with rivers and lakes but few trees; it would have been an environment comparable to the modern Serengeti plains.

This discovery raises many more questions, says Villmoare, and the researchers are continuing to explore the area, searching for further evidence. It’s necessary to explore the exact nature of the transition, says William Kimbel. For example, "it’s also unknown whether the early changes in the teeth and jaws were accompanied by changes in other systems, such as brain or technology,” he explains. At some point, the ancestor of Homo species developed a larger brain, but whether this change was already in place 2.8 Ma is unknown.

Further evidence that we should see key transitional Homo fossils in this era comes from a paper in Nature, also published this week, detailing the computerized reconstruction of a previously identified Homo jaw based on a partial fossil dated to 1.8 Ma. The reconstruction found that the jaw had unexpectedly primitive characteristics, which point to Homo habilis having originated before 2.3 Ma—timing consistent with the LD 350-1 find.

Combined, these discoveries significantly narrow the time range in which fieldworkers should now focus their search for these answers, says Kimbel.

Similar questions remain for the geologists exploring the climate change that is thought to have driven the evolution of mammals in the region. Until there is more evidence, it’s impossible to say whether there was a general trend in the region toward a more arid and open habitat, says Reed, or whether this was just a local phenomenon.

“What we do know is that Homo lived here, but Australopithecus could not,” Reed says.

Science, 2015. DOI: 10.1126/science.aaa1343 (About DOIs).