Variation in hominid brain size: How much is due to method?

Brain size represented by cranial capacity (CC) is one of the most frequent ly analysed characters of hominids. Accuracy of individual CC estimates dep ends on completeness of specimens and methods used for reconstruction and m easurement. A file of published estimates of CC of hommids dated from 3.2 M a (million years) to 10 Ka (thousand years) including 606 estimates for 243 specimens was compiled. In the file, 75 specimens are available with multi ple values (3 to15) obtained by various methods and/or by various authors. Using individuals as classes in ANOVA, intraclass variation, which represen ts,error's of estimates, was calculated. For the total sample of multiple e stimates (N = 382) the error variance is 5315 ml(2). The error standard dev iation is 73 ml (coefficient of variation (CV = 7.3%), quite large in compa rison to the actual variation in CC in modern humans, SD = 157 ml (CV = 11. 6%). This large error makes us wonder whether any fossil can he reliably pl aced with respect to a particular "erebral Rubicon" between palaeospecies. Recent discussions concerning cranial capacity of Stw505 are a case in poin t regarding errors in CC estimation. In actual repeated 30 time measurement s on a research quality cast we obtained with various methods (water, seeds , plasticine) CC estimates ranging from 484 to S86 ml. The range of estimat es in the literature is from 515 to 625 ml. When hominid CC by taxon with d ate as a covariate is subjected to ANOVA, taxon is responsible for 5% of th e variance while date is responsible for the main portion, (89%). The relat ionship between CC and date is best characterised as a gradual time trend. It is proven by the ANOVA test for linearity, by Gamma test for trend and b y ASReml fitting of a linear function. The line of best fit to this time tr end is a double exponential curve which explains 90% of the total variance in CC: CC = 306.63 (4.83 (4.83 (0.9995 DATE)) Essentially the same curve fits subsamples of CC dated at less than I Ma an d at 3.2-1.0 Ma. This has several implications for the nature of the Darwin ian process to be reconstructed.