Brain size of Homo floresiensis and its evolutionary implications

Abstract

The extremely small endocranial volume (ECV) of LB1, the type specimen of Homo floresiensis, poses a challenge in our understanding of human brain evolution. Some researchers hypothesize dramatic dwarfing of relative brain size from Homo erectus presumably without significant decrease in intellectual function, whereas others expect a lesser degree of brain diminution from a more primitive, small-brained form of hominin currently undocumented in eastern Asia. However, inconsistency in the published ECVs for LB1 (380-430 cc), unclear human intraspecific brain-body size scaling and other uncertainties have hampered elaborative modelling of its brain size reduction. In this study, we accurately determine the ECV of LB1 using high-resolution micro-CT scan. The ECV of LB1 thus measured, 426 cc, is larger than the commonly cited figure in previous studies (400 cc). Coupled with brain-body size correlation in Homo sapiens calculated based on a sample from 20 worldwide modern human populations, we construct new models of the brain size reduction in the evolution of H. floresiensis. The results show a more significant contribution of scaling effect than previously claimed.

Figure 1.

Physical replicas (three-dimensional prints) of horizontally sectioned cranium of LB1 (a) before and (b) after cleaning and clay-based reconstruction. (Online version in colour.)

Figure 2.

Virtual endocast of LB1 showing reconstructed areas. Anterior, posterior, right lateral, left lateral, basal and superior views (clockwise). Grey, intact original bone surface (untreated); light blue, clay reconstruction of relatively simple surfaces; blue, clay reconstruction of relatively complex structures (with minute digital correction when necessary); orange, anatomical foramina, and cracks and other damages treated digitally. See text for methodological details.

Figure 3.

Relationship between the ECV and FHD. Open circle, H. sapiens (male); filled circle, H. sapiens (female); rhombus, mean data for the fossil hominin samples; x, individual data for Australopithecus; +, individual data for premodern Homo. Possible range of the ECV is indicated for the small A. afarensis individual (A.L. 288-1). Estimated ranges of the FHD for the two early Indonesian H. erectus samples (Sangiran Upper and Lower) are indicated by thick broken lines (3.81–3.91 in logarithmic form; see text for more details). The diagonal lines indicate expected brain size reductions from the H. habilis and Sangiran Lower (earliest Indonesian H. erectus) conditions following the scaling relationship for male (dotted lines) and female (solid lines) H. sapiens based on the RMA regressions. (Online version in colour.)