Understanding Unearthed: Fossil redefines knowledge about early human evolution and dispersal

A completely intact skull of an early Homo individual has been uncovered in Dmanisi, Georgia. The fossil, which is the best preserved adult early human skull to have ever been found, has completely remoulded  anthropologist’s understanding of species diversity in our early ancestors.

The archaeological site at Dmanisi, Georgia, has previously yielded early hominid skulls but never before has an entirely preserved cranium, together with its lower jaw bone, been found. The fossil is from the early Pleistocene period and inhabited the area approximately 1.8 million years ago, documenting the emergence of Homo out of Africa. Structurally, cranial features of the skull are larger and more prominent than that of previous fossils found in Dmanisi, suggesting that it belonged to a male.

The skull displays morphological features that have never before been observed together. Its small braincase coupled with comparatively large face and jaws distinguish it from all previous cranial Homo fossils that have been found. The fossil has many features that were previously used for defining different proposed Homo species. If the face and braincase of the fossil had been uncovered separately, it is likely that they would have been thought of as belonging to two separate species.

Previous to the find, the facial morphology of early Homo was best represented by two adolescent fossils with underdeveloped features, as well as one senile individual from Dmanisi whose jaws and teeth had been severely modified due to disease. This new fossil enables us to view how the face and lower jaw were positioned relative to the braincase in early Homo. Body mass and stature estimates from the fossil show the first evidence that early adult hominids had small brains but relatively large body mass and skeletal proportions, which are towards the lower limit of current human variation.

The skull, along with previous samples from Dmanisi, has established variation that indicates a possible single evolving lineage of early Homo, rather than the many different species currently proposed by many researchers. Together, the samples found at Dmanisi exhibit normal variation between sexes, and also between individuals, not dissimilar to the variation found in a modern group of humans or apes. The comparative differences in skull shape of the Dmanisi samples are similar to that found between chimpanzee individuals today. This adds to the growing evidence that variation in Homo fossils from the Pleistocene has been misinterpreted as species diversity rather than diversity within a species.

Hominid fossils from the same period, the Pleistocene, in Africa show morphological diversity often thought to represent different species. When looked at with this new evidence from Dmanisi, it appears likely that this is just variation between individuals of the same species, namely Homo erectus. The population that inhabited Dmanisi in the Early Pleistocene, around 1.8 million years ago probably originated from a single evolving lineage of Homo erectus from Africa. Subsequently, hominid specimens from Dmanisi which were previously thought to represent a different potential species of early Homo, Homo ergaster, have now been classified as a sub-species of Homo erectus.

The samples from Dmanisi reinforce the fact that early hominids migrated from Africa to Eurasia and eventually established large populations in Western Asia. However, due to the small braincase found in these samples, we know that this dispersal, likely to be the earliest of the hominids, pre-dated any significant increase in brain-size and therefore intelligence. Instead, it is likely to be features such as the use of tools that enabled increase of dietary meat, changing morphologies of the legs and feet, and increased cooperation between individuals that increased rates of mobility, reproduction and survival of Homo erectus, enabling stable populations outside of Africa to establish.

Ultimately, this newly uncovered skull has provided much needed insights into the evolutionary history, geographical dispersal and population dynamics of our early hominid ancestors. It has been a key finding in the study of human evolution and has redefined Homo erectus as the first global species of human.

Original article: D. Lordkipanidze, M. S. Ponce de Leon, A. Margvelashvili, Y. Rak, G. P. Rightmire, A. Vekua, C. P. E. Zollikofer. A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science, 2013; 342 (6156): 326-330.

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