The enigmatic origin of the human brain

Virtual fillings of the brain sacs of early Homo of Dmanisi, Georgia, is displayed in turquoise. Their structures offer new insights into the evolution of the human brain 1.8 million years ago.

IMAGE: Mr. PONCE DE LEÓN EN CH. ZOLLIKOFER / UNIVERSITY OF ZURICH

Detecting the emergence of human cerebral features in the hominin fossil record can provide evidence of the timing and process of brain changes and a look at the behavior of our ancestors and family members. Because brain tissue rarely fossilizes, changes in brain size, shape, and organization are obtained from brain endocytes (replicas of the inner surface of the meninges) (1). After his observation of brain pressures preserved in fossil cranial samples from Olduvai (Tanzania) (2), the paleoanthropologist Phillip V. Tobias said that ‘hominid evolution has reached a new level of organization … with the rise of the genus Homo. ”Since then, there have been debates about whether human brain organization coexisted with the occurrence of the genus. Homo. On page 165 of this issue, Ponce de León et al. (3) disputes this view by proposing it Homo in Dmanisi (foothills of the Georgian Caucasus) showed 1.85 to 1.77 million years ago (Ma) a primitive organization of the brain.

Reconstructing the chronology and mode of hominin brain evolution requires a good knowledge of subtle changes in brain areas. In this respect, the subordinate part of the front lobe, where Broca’s hat is located, was the focus of thorough investigations and intense discussions (4). In addition to its use as a critical beacon for brain reorganization, Broca’s cap plays a fundamental role in language production and comprehension, of which evolution is an equally interesting topic. The Broca’s hat of existing people differs structurally from that of our immediate family members, the chimpanzees and bonobos. Chimpanzees and bonobos have a distinct front in the region called the pre-bite sulfus. This is not the case with people who have two vertical furrows. In human evolutionary studies it is assumed that the brain of chimpanzee and bonobo better approaches the primitive state for the hominin brain. Within this context, the “single ancestor state” is interpreted as the representation of the ancestral state.

Despite important recent discoveries of germinating samples and groundbreaking computer-aided review of the fossil record (5, 6), is little known about the brain of early Homo. This unfortunate situation can be partly explained by the scarcity of a complete or at least partial fossil skull from that period. For example, the alleged earliest human remains of Ledi-Geraru (Ethiopia), dated 2.8 Ma, do not preserve the stroke (7). The early Homo samples for which an extensive amount of endocast data exist (description of the organization of the cerebral regions) are from Africa and Asia which are relatively young (less than 1.8 Ma) (8, 9), which leaves a gap of about 1 Ma in our knowledge of evolution of the human brain. As far as frontal organization is concerned, this breakthrough of 1 Ma in the usable fossil record is of crucial importance. Early hominins that roamed Africa before 2.8 Ma show a relatively primitive organization of this region (4), while prints on later human endocastes indicate a derived human condition (5). In this case, the hypothesis of a derivative organization that arises at the same time as the earliest people cannot be discounted.

Through their comprehensive study of the remarkable composition of Dmanisi and their revision of the African and Asian fossil material, Ponce de León et al. sheds new understanding on the cerebral organization of early Homo in general, and Homo erectus (late night) in particular, the challenge of the idea of ​​the emergence of a fully derived human brain at the time of the earliest representatives of the genus Homo. The hominin fossil site of Dmanisi is exceptional for its geographical (Europe) and chronological (1.85 to 1.77 Ma) contexts (10). As such, fossil deposits from this environment are one of the earliest distributions of Homo from Africa. By combining different methods of high-resolution imaging (including synchrotron radiation) and three-dimensional modeling techniques (including geometric morphometry), the authors show that the endocast of H. erectus about 1.8 Ma reflects a primitive organization of the frontal lobes, while later H. erectus samples in Southeast Asia and Africa show a derivative state (see figure). Consequently, the authors suggest that human frontal lobanization originated after the genus Homo and the earliest distributions from Africa.

The question that needs to be addressed next is about the nature of the underlying evolutionary processes. What kind of selection pressure was possibly responsible for the reorganization of the frontal lobes? This question has enormous functional and behavioral implications because, apart from language, it has been shown that the Broca hood was certainly also involved in the manufacture of tools (11). The other possibility, which probably deserves more attention, is that there was no selection at all, and that changes that affected the tracking of the Broca were a by-product of the reorganization of other brain regions, and that eventually the emergence of language was the result of a withdrawal (previously existing anatomical features co-opted for a new use) (12). In this respect, the description of the brain prints of the endocastes of earlier Homo specimens, such as the brain box recently found at Drimolen (South Africa) (13), and future discoveries of new samples from that period will be essential to understanding the evolutionary context of these brain changes.