Abstract
Interactions of the brain and cranium in archaic populations remain poorly understood. Hominin fossils from Middle Pleistocene localities in Africa and Europe have been allocated to one or more species distinct from Homo erectus, the Neanderthals and modern humans, based on the assumption that characters of the vault and face are developmentally independent. However, it is possible that increased frontal width, parietal lengthening, midvault expansion and occipital rounding all reflect encephalisation occurring within the H. erectus lineage. If specimens from Broken Hill and Elandsfontein (in southern Africa) and Sima de los Huesos and Petralona (in Europe) differ from H. erectus only in brain volume, then it will be difficult to distinguish and diagnose Homo rhodesiensis or Homo heidelbergensis adequately. In this study, correlation analysis showed that the brain fails to influence vault breadth within either H. erectus or the mid-Pleistocene sample. Instead, the (large) cranial base has a major effect on width. Variation in brain volume is not associated with frontal flattening. In H. erectus and in individuals such as Bodo and Petralona, the massive face seems to override the brain as a determinant of frontal curvature. Small H. erectus crania have rounded occipitals, whilst larger individuals show greater flexion. Later hominins do not follow this trend, and encephalisation cannot explain the occipital rounding that is present. Few of the vault characters considered diagnostic for the mid-Pleistocene fossils can be attributed to increasing brain volume. The situation is complex, as of course the brain must influence some traits indirectly. The cranial base is also an instrument of change. Discriminant analysis identified basicranial breadth as critical to distinguishing individuals such as Broken Hill, Sima de los Huesos and Petralona from H. erectus.
Highlights
Beginning with the Broken Hill discoveries in 1921, many hominin fossils have been found at mid-Pleistocene localities in Africa
An increase in brain size is one of several keys to the transformation of the skull occurring in Middle and Late Pleistocene Homo
Endocranial volume is significantly greater in the mid-Pleistocene group than in H. erectus, and this change influences vault length, vertex height and sagittal arc lengths
Summary
Beginning with the Broken Hill discoveries in 1921, many hominin fossils have been found at mid-Pleistocene localities in Africa. First applications of U-series dating to a speleothem present in the lower part of the Sima stratigraphic sequence suggested a date of >350 ka.[5] More recent sampling from the same speleothem has produced high resolution U-series dates averaging 600 ka, and a conservative minimum estimate for the age of the fossils is said to be 530 ka.[6] Another hominin from Petralona in Greece is like the Broken Hill specimen in many aspects of vault shape – in height, breadth and massive construction of the upper face and cheek; orientation of the infraorbital region; and several measures of facial projection.[7,8,9] The same is true for the less complete cranium from Arago Cave in France. Acceptance of this hypothesis implies that populations of H. erectus did not change appreciably prior to the evolution of Neanderthals in Europe and modern humans in Africa, probably after 300 ka
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