The enigmatic origin of the human brain

Virtual brain charging early Homo from Dmanisi in the state of Georgia are shown in turquoise. Their structures provide new insights into the evolution of the human brain 1.8 million years ago.

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

Monitoring the occurrence of human-like brain characteristics in hominin fossil records could provide evidence of the timing and process of brain changes and provide insight into the behavior of our ancestors and relatives. Because brain tissues are rarely fossilized, changes in brain size, shape, and organization are collected from brain endocasts (replicas of the inner surface of the brain) (1). After his observations of brain imprints preserved in samples of fossil cranes from Olduvai (Tanzania) (2), paleoanthropologist Phillip V. Tobias stated that “the evolution of hominids has reached a new level of organization … with the emergence of the genus Homo. “Since then, there have been debates about whether the human brain organization emerged at the same time as gender Homo. On page 165 of this issue, Ponce de León and others. (3) dispute this view by proposing it Homo in Dmanisi (foothills of the Georgian Caucasus) 1.85 to 1.77 million years ago (Ma) showed a primitive organization of the brain.

Reconstructing the chronology and manner of brain evolution of hominins requires a good knowledge of subtle changes in areas of the brain. In this respect, the inferior part of the frontal lobe, where Broca’s cap is located, has been at the center of thorough investigations and intensive discussions (4). In addition to its use as a critical landmark for brain reorganization, Broca’s cap plays a fundamental role in the creation and understanding of language, whose evolution is an equally intriguing topic. The broccoli hat of existing people is structurally different from the hat of our closest living relatives, chimpanzees and bonobos. Chimpanzees and bonobos have one special furrow in that region called the frontoorbital sulcus. This is not the case with people who have two vertical furrows instead. In human evolutionary studies, it is assumed that the brains of chimpanzees and bonobos are closer to the primitive conditions for the brain of hominins. Within this context, the “state of a furrow” is interpreted as representing the state of the ancestors.

Despite crucial recent discoveries of German specimens and a revolutionary computer-assisted revision of fossil records (5,, 6), little is known about the early brain Homo. This unfortunate situation can be partly explained by the lack of a complete or at least partial fossil crane from that time period. For example, the alleged earliest human remains from Lady-Geraru (Ethiopia), dated to 2.8 Ma, do not preserve the brain case (7). Early Homo samples for which there is a usable amount of endocrine data (description of the organization of cerebral areas) originate from African and Asian localities that are geologically relatively young (less than 1.8 Ma) (8,, 9), leaving a gap of about 1 Ma in our knowledge of the evolution of the human brain. In terms of frontal organization, this break in the usable fossil record of 1 Ma is crucial. Early hominins that roamed Africa before 2.8 Ma show a relatively primitive organization of this region (4), while prints on later human endo-evidence indicate a derived human condition (5). In this case, the derived organization hypothesis that emerged at the same time as the earliest people could not be rejected.

Evolutionary scenario for frontal lobes of hominins

The evolutionary scenario is based on evidence of brain changes preserved in fossil records and was investigated by Ponce de León and others. Dark gray curves indicate the position of the frontal furrows.

GRAPHICS: N. DESAI /SCIENCE

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Evolutionary scenario for hominin frontal lobes

The evolutionary scenario is based on evidence of brain changes preserved in fossil records and was investigated by Ponce de León and others. Dark gray curves indicate the position of the frontal furrows.

GRAPHICS: N. DESAI /SCIENCE

Through a comprehensive study of a significant collection from Dmanisi and a revision of African and Asian fossil material, Ponce de León and others. cast a new understanding on cerebral organization early Homo in general and Homo erectus (sensu lato), especially challenging the idea of ​​the emergence of a fully derived human brain at the time of the earliest representatives of the genus Homo. The site of hominin fossils in Dmanisi is exceptional due to its geographical (Europe) and chronological (1.85 to 1.77 million) context (10). As such, the fossil deposits of this site document one of the earliest scatterings Homo from Africa. By combining different high-resolution imaging methods (including synchrotron radiation) and three-dimensional modeling techniques (including geometric morphometry), the authors show that endocasta H. erectus at about 1.8 Ma reflects the primitive organization of the frontal lobes, while later H. erectus specimens in Southeast Asia and Africa show a derived state (see figure). Accordingly, the authors suggest that human frontal lobe organization emerged after gender Homo and the earliest displacements from Africa.

The question that will need to be addressed below deals with the nature of basic evolutionary processes. What selection pressure could have been responsible for the reorganization of the frontal lobes? This issue has huge functional and behavioral implications because, in addition to language, it has been shown that Broca’s hat was also certainly involved in tool making (11). Another possibility, which probably deserves more attention, is that there was no selection at all, and the changes that affected Broca’s cap were a byproduct of the reorganization of other areas of the brain and that, ultimately, the appearance of language was the result of exacerbation. new use)12). In this regard, the description of brain imprints in previous endocasts Homo specimens, such as a brain case recently found in Drimolen (South Africa) (13), and future discoveries of new specimens from that time period will be crucial to understanding the evolutionary context of these changes in the brain.

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