A new paper published in Science details a 1.8 million year old skull. The skull find has stirred up debate amongst palaeoanthropologists, as the authors of the new paper have asserted that the hominid skull shows that Homo habilis, Homo rudolfensis and Homo erectus are all part of a single evolving lineage that led to modern humans. Other scientists disagree however, saying there is still evidence that at least three distinct species of humans co-existed in Africa.

The new cranium, discovered in Dmanisi, (D4500), together with its mandible (D2600), represents the world’s first completely preserved hominid skull from the early Pleistocene. The cranium has a small brain case at 546 cubic centimetres and has a large prognathic face, meaning that its jaws project beyond the upper part of its face. It seems to have close structural similarities (morphological affinities) with the earliest known Homo fossils found in Africa.

The Dmanisi sample is now composed of five crania, and shows direct evidence for wide variation within the early Homo populations - but crucially, within the same species. This variation within the Homo populations is similar to that seen within modern Pan (chimpanzee) groups.

The authors conclude that the diversity seen in the African fossil record around 1.8 million years ago most likely reflects variation between groups of a single evolving lineage rather than species diversity. That single lineage is Homo erectus, with specimens previously attributed to H. ergaster reclassified as a chronosubspecies, H. erectus ergaster. As the Dmanisi population most likely originated from an Early Pleistocene (2.58 – 0.78 million years ago) expansion of the H. erectus lineage from Africa, the authors place it within H. e. ergaster and formally designate it as H. e. e. georgicus, referring to the samples’ geographic location. H. habilis and H. rudolfensis fossils require further testing to determine whether they too belong to a single evolving Homo lineage. Identifying the hominid groups and identifying variation within the populations will aid in understanding the evolution and dispersal of early Homo.

Not all palaeoanthroplogists agree with the authors of this new paper. A previous paper sought to confirm taxonomic diversity in early Homo. The Nature paper showed that three newly discovered fossils, aged between 1.78 and 1.95 million years (Myr) old, showed that there were two contemporary species of early Homo, in addition to Homo erectus, in the early Pleistocene of eastern Africa. This finding added further support to the classification of a skull found in 1972 as a separate species of human, Homo rudolfensis. The skull was the only example of this species which contributes to the contention over its lineage.

A co-author of the Nature study, Fred Spoor, told BBC News that Lordkipanidze et al.’s analysis of the cranium describing the shape of the face and braincase was in broad and sweeping terms, and that those Homo sapiens are not defined using such a broad overview. Very specific characteristics had been used to define H. erectus, H. habilis and H. rudolfensis, and these were not were not indicated by the landmarks that the team used.

It is clear from this recent finding and previous work that the Dmanisi site still has much more to offer in discoveries of our lineage.