Homo brain size and encephalization data

Brain size (DeSilva et al., 2023) and encephalization (Stibel, 2021) data were taken from prior studies (Tab 1). These data were then updated where more recent information was available for date, sex, geography, and taxonomy. Sources for brain size measurements, taxonomy, and dates are included in the data file. Juvenile, immature, deformed, and fragmentary cranium without enough elements to make a proper estimate of adult brain weight were eliminated from the sample. Estimated brain size was derived from 800 cranial capacity measures using a formula that permits direct comparisons using a least-squares regression (R2 = 0.995) of 27 primate species (brain mass = 1.147 x cranial capacity0.976) (Ruff et al., 1997). Eleven specimens are noted as either having possible skull deformity (n = 1) (Rightmire, 2009) or contested taxonomy (n = 10) (Arsuaga, et al., 2014). H. floresiensis and H. naledi are included but noted as questionable as to whether these species are part of the genus Homo (Tattersall, 2023). The final sample includes 690 H. sapiens and 99 individuals that have been previously categorized across eight extinct paleospecies: H. erectus (n = 44), H. heidelbergensis (n = 15), H. floresiensis (n = 1), H. habilis (n = 6), H. longi (n = 1), H. naledi (n = 3), H. rudolfensis (n = 3), and H. neanderthalensis (n = 26).  

The encephalization sample (tab 2) is comprised of 79 H. sapiens and 10 archaic individuals. Because of the highly correlated relationship between overall body and brain size (Henneberg and Steyn, 1993), encephalization measurements were only taken from specimens that had sufficient cranial and postcranial remains to estimate brain and body mass for each individual. For 86 specimens, body mass estimates were derived based on the latest formula for femoral head (FH) estimations (body mass = 2.262 × FH – 38.7) (Ruff et al, 2018). For three individuals (Arene Candide 1, San Teodoro 1, and San Teodoro 3), where femoral head or proximal tibia estimates were not available, stature/bi-iliac was utilized to estimate body mass (body mass = 0.522 x ST + 1.809 x BI – 75.5) (Ruff et al., 1997). Estimated encephalization for each of the 89 specimens were derived from a formula accounting for surface area (Jerison, 1973) and metabolic turnover (Martin, 1981) as follows: EQ = brain mass/(11.22 × body mass0.76).