The size of scalable brain components in the human evolutionary lineage: With a comment on the paradox of Homo floresiensis

The discovery of a diminutive, small-brained hominin skeleton (LB1) from the Pleistocene of Flores, Indonesia, seems to present a paradox concerning the interpretation of overall brain size in an evolutionary context. This specimen forms the holotype of a purportedly new hominin species, Homo floresiensis. As inferred from the archaeological record, it has been suggested that this species of Homo, existing as recently as 12,000 years ago, engaged in sophisticated cultural behaviors with an adult brain size equivalent to that seen in modern chimpanzees and one that in modern humans would be defined as high degree microcephaly and always associated with idiocy. The alternative explanation for these behaviors at the observed brain size would require that H. floresiensis deviate from existing patterns of primate brain scaling at either a macroscopic or microscopic level. Here we develop predictive equations and confidence intervals for estimating the size of various brain components in the human evolutionary lineage by calculating scaling relationships among overall brain size and 11 components of the primate brain using phylogenetically independent contrasts (PIC) methods. Using these equations, paleoanthropologists can: (a) estimate brain component size (and confidence intervals) for any primate in the fossil record if overall brain size is known; and (b) calculate some reasonable outside limits as to how far species-specific departures from allometric constraints (i.e., brain reorganization) can be taken in assessing human brain evolution. We conclude that if the original assessment of LB1 is correct, i.e., that it samples a population from a new species of Homo, H. floresiensis, that was capable of Homo sapiens-like cultural attributes (fire, blade manufacturing, etc.), while having a chimpanzee-sized brain, then we are faced with the paradox that 1 cm 3 of H. floresiensis brain could not be functionally equivalent to 1 cm(3) of a modern human or modern chimpanzee brain. (c) 2007 Elsevier GmbH. All rights reserved.