Cerebral asymmetries common to humans and African apes

Emmanuel Gilissen (Vertebrates Section, Mammalogy Service), Antoine Balzeau (researcher at the CNRS, France) and Dominique Grimaud-Hervé (lecturer at the National Museum of Natural History, France) carried out a comparative study amongst present-day humans, African apes and fossil humans. The study demonstrates for the first time the presence of cerebral asymmetries in all the hominids. This implies that the last common ancestor of the African ape and modern human also had an asymmetric brain. This result modifies our understanding of the cognitive capacities of prehistoric humans.

Illustration of the protocol used to quantify the "petalias” (click to enlarge)

Petalias

The human brain has structural asymmetries. These have aroused the curiosity of scientists for the past 150 years because of their possible relationship with our cognitive capacities in general and, more specifically, our capacity for language and the fact that we are left or right-handed. One anatomical character in particular could be linked to these functions: asymmetries of the anterior and posterior extremities of the cerebral hemispheres. These asymmetries are known as “petalias”. An extension of the right hemisphere in relation to the left hemisphere (or conversely) can be observed.

The first quantitative study of petalias

For the first time, researchers have carried out a quantitative study of these asymmetries on very large samples of skulls of African apes, present-day human and fossil hominids (from Australopithecus to Neanderthals). With the aid of imaging methods, it is possible to reconstruct in three dimensions “virtual” models of the cranium and the endocranium, which stores all the imprints left by the brain on the internal surface of the cranium. In fossils the endocranium is the only material available for the study of the shape of the brain because the brain is not conserved.

The “last common ancestor” had an asymmetrical brain

The results of this study show the presence in all hominids, including apes, of a shared pattern of directional asymmetry. This implies that these characters were probably inherited from the last common ancestor of the African apes and modern human.

Simplified hominid evolutionary tree and shared neuroanatomical asymmetries (click to enlarge)

Implications for the knowledge of the behaviour of fossil hominids

These results have major implications as regards the possible relationship between cerebral asymmetries and the functional capacities of fossil hominids. It was for example suggested that a left occipital petalia (behind) combined with a right frontal petalia (in front) indicates that a fossil was right-handed and had a capacity for language. However, it has now been demonstrated that apes also have this type of asymmetry, even if they have different manual preference modalities and do not have the articulated language of modern human. This illustrates the dissociation between certain patterns of behaviour (language, manual lateralisation etc.), which are particular to the human lineage, and the structural asymmetries of the brain, which are not.

Since 5 January 2012 the study is available on the PLoS One website:
http://dx.plos.org/10.1371/journal.pone.0029581