Part IV: Pleistocene Epoch
Homo species indeterminate (2.8 mya)
A possible new species of Early Homo was recently discovered. Villmoare et al. (2015) reported on what they are calling Homo species indet. (“indeterminate”). The half mandible (LD 350-1) was discovered by Chalachew Seyoum in 2013 at the Ledi-Geraru site in the Afar region of Ethiopia. If indeed the jaw is part of the Homo lineage, it extends the origin of our genus to 2.8 mya, 400 kya older than the oldest H. habilis specimen, i.e. AL 666-1 from the Hadar site in Ethiopia where Lucy was discovered. Villmoare et al. (2015) believe that the specimen represents a transitional form between Au. afarensis and Homo habilis. Support for their supposition includes the combination of australopith and Homo-like characteristics seen in the mandible and teeth, as well as the fact that it is from the same area as Au. afarensis that is known to have survived there until 3 mya. They report on a fragmentary mandible from the Koobi Fora site in Kenya (KNM-ER 5431) that exhibits a similar combination of australopith and Homo morphology (see Villmoare et al. 2015 for references).
The anterior of the mandible exhibits the most primitive characteristics, along with some Homo-like characteristics. Homo characteristics include narrower molars with more derived dimensions and cusp morphology, and the canine/first premolar configuration is less ape- or australopith-like, as evidenced by wear patterns. They conclude that deviation in the teeth and jaws, from the australopith condition, occurred early in the Homo lineage.
DiMaggio et al. (2015) reported on the paleoenvironment of the site. The area was a mosaic environment consisting of open grass- and scrubland, gallery forest, and lakes and/or rivers. They see a faunal turnover 2.8–2.6 mya in the strata, in accordance with the area becoming more open and likely arid with an increase in grazers. They cite evidence for climatic and vegetation shifts resulting from “rifting processes and extensive volcanism [that] altered the architecture of sedimentary basins” (see DiMaggio et al., p. 1, for references). Their work fills an important gap in the paleoenvironmental history of the region.
The fact that Au. garhi is younger and their teeth are more robust leads the researchers to conclude parsimoniously that Au. garhi is not part of our ancestry, as earlier and later forms of Homo had more gracile teeth and jaws. The same would also hold true for Au. sediba due to their much later appearance in the fossil record at ~2 mya. However, Hawks et al. (2015) argue that the osteometrics (bone measurements) and anatomical features do not support firm taxonomic placement in genus: Homo. They believe that they have ample evidence showing that Au. sediba is closely related to early species of Homo (see Chapter 21).