LD 350-1
Fossilized mandible
LD 350-1 is the earliest known specimen of the genus Homo, dating to 2.75–2.8 million years ago (mya), found in the Ledi-Geraru site in the Afar Region of Ethiopia. The specimen was discovered in fossiliferous silts 10 m (33 ft) above the Gurumaha Tuff section of the site by Ethiopian palaeoanthropologist Chalachew Seyoum on 29 January 2013. It is currently unassigned to a species, and it is unclear if it represents the ancestor to H. habilis and H. rudolfensis which appear in the fossil record around 2.4 mya.
Description
LD 350-1 is a partial adult left mandible including the canine, both premolars, and all three molars. In terms of overall size, the specimen is within the range of what is seen in small Australopithecus afarensis specimens, and LD 350-1 seems to be a transitional form between Australopithecus and Homo. However, the specimen's anatomy strongly diverges from australopithecines and more closely aligns with Homo: the mental foramina are not located on a depression, it has a symphyseal keel (a line of bone jutting out at the midline of the jaws), the jawbone maintains a more or less constant depth whereas it is deepest under the premolars in some Australopithecus, and there are several differences regarding the tooth crowns. This specimen confirms that Homo dental and jaw anatomy diverged from those of Australopithecus very early on. LD 350-1, like other archaic humans, seems to have lacked a distinct chin.
Classification
The origin of Homo 2.8–2.5 mya is accompanied by climatic changes, but because other Homo specimens are not known from this time period, it is unclear if this was indeed the causal factor. Because of the strong dental divergence exhibited in LD 350-1, it may be that the initial split was caused by a change in diet.
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