A new paper was recently published, and widely reported in the media, about a hominid skull discovered at the Dmanisi site in Georgia in 2005 (Lordkipanidze et al, 2013, Gibbons 2013). The fossil, D4500, is believed to belong to the same individual as a lower jaw fossil, D2600, previously found at the site. The combined skull, designated by the authors as “Skull 5” (the 5th skull from Dmanisi) is almost completely and perfectly preserved, making it one of the most spectacular finds in the entire hominid fossil record. And Dmanisi is rapidly becoming one of the most important sites ever found in the study of human evolution.
Skull 5’s brain volume of 546 cm3 is very small. The other Dmanisi skulls are between 600 cm3 and 730 cm3. (Earlier papers gave the size of the largest one as 780 cm3, but that estimate appears to have been reduced. By comparison, the average modern human brain size is 1350 cm3.) However the fossil also has a large and robust jaw bone, and a large and projecting face. This combination of a very small brain and a large face differs from all other known Homo fossils. The fossil is of a mature adult, and because of the robustness of the skull it is thought to belong to a male.
Scientists are naturally delighted at the discovery of such a superb fossil, but the real impact of Skull 5 comes from the conclusions that the authors have drawn from it.
The Dmanisi fossils are different enough from each other that had they been found at different locations, they might have been classified into different species. Similar differences have been used to create species such as Homo habilis and Homo rudolfensis in the past. The authors believe that the Dmanisi fossils all belong to one species, both because they all come from the same time and place, and because the pattern and amount of variability found between the skulls is similar to that found in populations of modern humans, chimpanzees, and bonobos.
Following that line of reasoning, they conclude that since a similar pattern of variation exists for all early Homo fossils in Africa, and in the absence of any evidence that the supposed different species of Homo were adapted to different ecological niches, the default and most parsimonious assumption should be that all of these fossils belong to a single highly variable lineage (though they recognize that this claim remains to be tested, and alternative scenarios exist). This would mean that Homo habilis, Homo rudolfensis, Homo ergaster and some other more obscure names did not really exist as separate species. The name of that single species would, for reasons of priority, be Homo erectus. Specimens allocated to H. ergaster would then be called Homo erectus ergaster, as a time-limited subspecies. The Dmanisi scientists had previously named a new species, Homo georgicus, for the Dmanisi fossils, but now retract that name and suggest that because the Dmanisi fossils arose from an ergaster population, they should be called Homo erectus ergaster georgicus.