Posterior distributions of log-likelihoods for the non-directional and directional models. Figure a) shows body mass; b) brain mass; c) relative brain size. The log-likelihood of the directional model is shown in red, the non-directional model in blue. The posterior distributions of ancestral state estimates were obtained using uniform priors, two million iterations and a sampling interval of 100 (see Methods). The harmonic means and Bayes Factors of the posterior distributions are given in Table 1.