Process of encephalization in hominid evolution: preliminary results of biostatic analysis of brain size phylogenetic changes

In order to produce an evolutionary interpretation of the rate and mode of encephalization in hominid evolution, we present a new statistical approach which should be able to describe more precisely the individual phases of relative brain size evolution and to explore the possibility of different trends at a geographical or phylogenetic level. We start from the hypothesis that brain size evolution has not been linear, because indications exist that the evolution of the brain in the hominid lineage cannot be considered an allometric change. Therefore we need more advanced statistical procedures than the widely used linear regression methods. We have collected from literature endocranial capacity estimates pertaining to 166 fossil specimens dating from 3.3 million to about 10 thousand years ago. Sources of additional variation, such as the quality of fossil remains, the reliability of estimates of their brain size and of the sexual diagnosis of the specimens, have also been taken into account. We have performed a preliminary standardization of brain size estimates with estimates of body size, in order to study changes in encephalization instead of simply endocranial capacity alone. Once the sample has been divided in taxonomic groups an increment of encephalization between two groups index has been created. A kernel density estimation to evaluate the distribution of such an index has been performed. Finally a non parametric regression, such as smoothing spline regression, has been used to investigate on relationship between time and encephalization. The results indicate that brain size evolved at a different rate and velocity in individual phases of hominid evolution. Two stases seem to fit well with the fossil evidence: (1) in early Homo, and (2) in very late Homo erectus and archaic Homo sapiens (or Homo heidelbergensis). However, a geographical analysis provides an indication of a more gradualist trend. Our results suggest that the introduction of new statistica