Jaw structures suggest that at least three homo species once roamed the African plains. (from Nature.com)

Fossilized skulls show that at least three distinct species belonging to the genus homo existed between 1.7 million and 2 million years ago, settling a long-standing debate in palaeoanthropology.
A study published this week in Nature focuses on homo rudolfensis, a hominin with a relatively flat face, which was first identified from a single large skull in 1972. Several other big-skulled fossils have been attributed to the species since then, but none has included both a face and a lower jaw. This has been problematic: in palaeoanthropology, faces and jaws function like fingerprints for identifying a specimen as a particular species (which is indicated by the second word in a Linnaean title, such as ‘rudolfensis’), as opposed to the broader grouping of genus (the first word, as in ‘ homo’).

Without complete skulls, it has been difficult to reach a consensus on whether specimens attributed to H. rudolfensis are genuinely members of a distinct species, or actually belong to other homo species that lived around the same time, such as homo habilis or  homo erectus. Understanding how many different homo species there were, and whether they lived concurrently, would help to determine whether the history of the human lineage saw fierce competition between multiple hominins, or a steady succession from one species to another.
But the latest result has dissipated much of this uncertainty. It concerns three fossils — two lower jaws and a juvenile’s lower face — that were found in a desert area called Koobi Fora in northern Kenya. The team that pulled them out of the ground, led by Meave Leakey, a palaeontologist at the Turkana Basin Institute in Nairobi, describes how the dental arcade, the arch created by the teeth at the front of the mouth, is nearly rectangular, just like the palate structure of the 1972 skull. By contrast, the average modern human mouth has a curved dental arcade.
Further evidence comes from the juvenile’s face, which tellingly has cheek bones joining the palate quite far forward. “It was such an extended excitement as the juvenile face slowly emerged from the encasing rock, and its similarity to the 1972 specimen became so striking,” recalls Leakey.