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The enigmatic origin of the human brain



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Virtual fillings of early brains Homo from Dmanisi, Georgia, appears in turquoise. Their structures provide new insights into human brain development 1

.8 million years ago.

PICTURE: M. PONCE DE LEÓN AND CH. ZOLLIKOFER / UNIVERSITY OF ZURICH

Tracing the emergence of human cerebral features in hominin fossil records could provide evidence for the timing and process of brain change and provide insight into the behavior of our ancestors and relatives. Because brain tissue rarely fossilizes, changes in brain size, shape, and organization are obtained from brain endocastes (replicas of the inner surface of the brain) (1). After his observations of brain imprints preserved in fossil cranial samples from Olduvai (Tanzania) (2), paleoanthropologist Phillip V. Tobias said that “hominid evolution reached a new level of organization … with the advent of the genus Homo. “There have since been debates about whether human brain organization arose at the same time as the appearance of the genus Homo. On page 165 of this issue, Ponce de León et al. (3) challenges this view by proposing it Homo in Dmanisi (the foot of the Georgian Caucasus) 1.85 to 1.77 million years (Ma) ago showed a primitive organization of the brain.

Reconstructing the chronology and state of hominin brain development requires a good knowledge of subtle changes in brain areas. In this respect, the subordinate part of the frontal lobe, where Broca’s hood is located, has been the focus of thorough investigations and intense discussions (4). In addition to its use as a critical landmark for brain reorganization, Broca’s hood plays a fundamental role in language production and comprehension, the development of which is an equally exciting topic. Broca’s cap of preserved humans differs structurally from our closest living relatives, chimpanzees, and bonobos. Chimpanzees and bonobos have a distinct furrow in this region called the frontoorbital sulcus. This is absent in humans who instead have two vertical furrows. In human evolutionary studies, it is believed that the chimpanzee and bonobo brain more closely approximate the primitive state of the hominin brain. Within this context, the “single-furrow condition” is interpreted as the condition of representative ancestors.

Despite important recent discoveries of German samples and pioneering computer-assisted revision of fossil records (5, 6), is little known about the brain from an early age Homo. This unfortunate situation can be partly explained by the scarcity of complete or at least partially fossil crania from that period. For example, the presumed earliest human remains from Ledi-Geraru (Ethiopia), dated to 2.8 Ma, do not retain braincase (7). The early Homo samples for which there is a usable amount of endocast data (description of the organization of the brain regions) are from African and Asian sites that are geologically relatively young (less than 1.8 Ma) (8, 9), which leaves a gap of approx. 1 Ma in our knowledge of human brain development. In terms of frontal organization, this 1 Ma breach in the usable fossil record is crucial. Early hominins that migrated in Africa before 2.8 Ma show a relatively primitive organization of this region (4), whereas imprints on later human endocasts indicate a derived human condition (5). In this case, the hypothesis of a derivative organization that arose at the same time as the earliest humans could not be discounted.

An evolutionary scenario for hominin frontal lobes

The evolutionary scenario is based on evidence of brain changes preserved in the fossil record and studied by Ponce de León et al. Dark gray curves indicate the location of the frontal sulci.

GRAPHIC: N. DESAI /SCIENCE

Through their extensive study of the remarkable collection of Dmanisi and their revision of the African and Asian fossil material, Ponce de León et al. shed new understanding of cerebral organization of early Homo in general and Standing man (meaningful summer) particularly challenging the idea of ​​the emergence of a fully derived human brain at the time of the earliest representatives of the genus Homo. The fossil bearing site of Dmanisi is unique for its geographical (Europe) and chronological (1.85 to 1.77 Ma) coherence (10). As such, fossil deposits at this site document one of the earliest dispersal of Homo out of Africa. By combining different methods of high-resolution imaging (including synchrotron radiation) and three-dimensional modeling techniques (including geometric morphometry), the authors demonstrate that the endocast of H. erectus at about 1.8 Ma reflects a primitive organization of the front paws, whereas later H. erectus samples in Southeast Asia and Africa show a derived state (see figure). Therefore, the authors suggest that the human organization of the frontal lobe arose after the genus Homo and the earliest spreads out of Africa.

The question to be addressed next is about the nature of the underlying evolutionary processes. What kind of selection pressure may have been responsible for the reorganization of the frontal lobes? This question has enormous functional and behavioral implications because, apart from language, it has been demonstrated that Broca’s hood was also certainly involved in toolmaking (11). The second possibility, which probably deserves more attention, is that there was no choice at all, and changes affecting Broca’s cap were a by-product of the reorganization of other areas of the brain, and that ultimately the emergence of language was the result of an exaptation (previously existing anatomical features co-selected for a new application) (12). In this respect, the description of the brain is an imprint of previous endocasts Homo copies, e.g. the brain box recently found in Drimolen (South Africa) (13), and future discoveries of new samples from that period will be crucial to understanding the evolutionary context of these brain changes.


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