Encephalization and allometric trajectories in the genus Homo: evidence from the Neandertal and modern lineages

Abstract

The term "encephalization" is commonly used to describe an enlargement in brain size, considered as either absolute endocranial volumes or relative values in relation to body size. It is widely recognized that a considerable endocranial expansion occurred throughout the evolution of the genus Homo. This article aims to evaluate whether this phenomenon was the outcome of distinct evolutionary lineages, reaching similar brain expansions but through different trajectories. Endocranial morphology was studied in a sample of fossil hominines by multivariate approaches using both traditional metrics and geometric morphometrics. The analysis was focused on the transition from a generalized archaic pattern within the genus Homo to the modern morphology and compared with changes that occurred along the Neandertal lineage. The main result was the identification of two different evolutionary trajectories, in which a similar expansion in endocranial size has been reached by different changes in shape. Along the Neandertal lineage we observed maintenance of an "archaic" endocranial model, in which a large amount of variability is based on a single allometric trend. By contrast, when modern endocasts were compared with nonmodern ones, we found important differences apparently led by a parietal expansion. In this light, the origin of our species may have represented the opportunity to surpass the constraints imposed on encephalization by the ontogenetic pattern shared by nonmodern Homo representatives.

Fig. 1.

The virtual reconstruction of the cranium and endocast of SCP1 is shown together with the plot of the first two PCs computed on nine metric variables taken on the endocasts (a) (see Fig. 5) and the unweighted pair-group method using arithmetic averages procedure cluster analysis based on the Euclidean distance matrices obtained from the absolute values (b) and the relative values (c). Cophenetic correlation coefficients are 0.74 and 0.85, respectively. •, archaic specimens; ▪, Neandertals; ▴, modern humans.

Fig. 2.

Results of the PC analysis computed on the 11-landmark configuration (see Fig. 6) superimposed by the Procrustes procedure. (a) PC1 vs. PC2 (total variance explained = 51%). (b) Shape changes along the PC1 in left lateral (Upper) and superior views. (c) Shape changes along the PC2 in left lateral and superior views. Note the configuration in the left lateral view superimposed on the stereolithography of the SCP1 endocast. Solid and dashed links show the shifting of the configuration toward positive and negative values, respectively, of the morphological vectors.

Fig. 3.

Pairwise comparison between 23-landmark averaged configurations. (a) Archaic vs. Neandertal. (b) Archaic vs. modern (solid links, archaic average configuration; dashed links, Neandertal and modern average configuration).

Fig. 4.

Left lateral (Upper) and rear (Lower) views of the endocast virtual reconstructions of Atapuerca SH5 (Left), Atapuerca SH4 (Center), and SCP1 (Right). The upper (parasagittal) parietal areas are flattened in the SH4/5 endocasts, with a consequent sloped and angled (tent-like) profile in posterior view. In contrast, the SCP1 endocast shows a more rounded shape without any marked step parasagittally to the superior sagittal sinus.