Encephalization and allometric trajectories in the genusHomo: 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.

Evolution of the base of the brain in highly encephalized human species

“…Our results may fi t with these observations. Figure 1 , indicating roughly parallel distributions of archaic and modern human scatters, likely refl ects some of these general diff erences, and resembles plots of Bruner et al 6 Diff erent scaling also seems consistent with recent studies of endocranial development showing that Neanderthal brains grew differently early in ontogeny, and probably prenatally, when compared with modern humans 7 . Evolutionary diff erences comprise the entire craniofacial system 7,50,51 .…”

“…1 ) refl ects the importance of size increase and allometry in brain and evolution of the base of the brain. Distributions of archaic and modern humans are roughly parallel refl ecting diff erent scaling patterns shown previously 6 . Evolutionary trajectories are indicated by lines originating close to early Homo heading towards the centres of the distribution.…”

“…4d,f ; for example, early Homo ; Supplementary Movie 2 ). Retention of low temporal pole position and relative length could be seen as structural elements of Bruner et al ' s 6 hypothesis of an archaic pattern of Neanderthal encephalization.…”

“…4b ). Frontal lobe widening, known for higher encephalized humans 6,12 , produces a rectangular outline of the frontal lobe base in H. sapiens . Maximum breadth of the basal frontal lobes is located close to the anterior half of the ACF ( Figs 3,4b ) at the level of the cribriform plate.…”

“…Bruner et al 6 suggested that Neanderthals possess -in generalan encephalized (enlarged) version of a primitive Homo brain by being at the endpoint of an archaic allometric trajectory, signifi cantly diff erent from the modern human brain scaling trajectory 6 . On the other hand, a more localized study revealed also the width of the Neanderthal frontal lobes to be non-allometrically increased 12 .…”