The Fossil in the Silt
In the Ledi-Geraru research area of Ethiopia's Afar Region, the ground is a fractured and arid badlands. On January 29, 2013, an Ethiopian graduate student named Chalachew Seyoum was surveying a low hill when he spotted a fossil tooth eroding from the silt. Further investigation revealed it was part of a hominin mandible. This fossil, designated LD 350-1, consists of the left side of an adult lower jaw with five teeth: the canine, two premolars, and all three molars.
The discovery's placement in the geologic record was almost perfect. It was found in sediments just 10 meters above a layer of volcanic tuff. Scientists used argon-argon dating on the volcanic ash layers above and below the fossil to establish its age with confidence. The results placed the jawbone between 2.75 and 2.8 million years old. This finding pushed back the known fossil record for the genus Homo by about 400,000 years, dropping it squarely into a period with a very sparse fossil record. Before this discovery, the oldest fossils attributed to Homo were about 2.3 to 2.4 million years old.
A Mosaic of Old and New
The LD 350-1 mandible displays a mix of traits from its predecessors and successors. The overall shape and the primitive, sloping chin are reminiscent of the much earlier Australopithecus afarensis, the species of the famous "Lucy" fossil, which was also found in the Afar region. However, the dental anatomy aligns it firmly with the genus Homo. The molars are slim, the premolars are symmetrical, and the jaw has a more uniform depth compared to the deeper jaws of Australopithecus. These features are characteristic of later species like Homo habilis.
This combination makes LD 350-1 an example of a transitional fossil. It shows an evolutionary stage, preserving features from an ancestral group alongside newly-emerged traits of a descendant lineage. The fossil suggests that the dental and jaw anatomy of Homo diverged from that of Australopithecus very early in its history.
Analysis of associated animal fossils—including prehistoric antelope, elephants, and crocodiles—shows the environment 2.8 million years ago. It was a predominantly open or mixed grassland and shrubland, with some rivers and lakes. This landscape was drier and more open than the more wooded environment that "Lucy" inhabited hundreds of thousands of years earlier. This supports the idea that climate change and the spread of grasslands were major factors in the evolution of our own genus.