L’évolution du cerveau humain : de la matière grasse à la matière grise

Abstract

L’évolution du cerveau humain suscite l’intérêt des scientifiques depuis longtemps. Ainsi l’augmentation de la taille du cerveau au cours de la période de modernisation pré-humaine a été associée à l’apparition d’aptitudes remarquables comme la capacité à fabriquer des outils, la bipédie, la chasse et le raffinement de l’interaction sociale, les talents artistiques et le développement du langage. Bien entendu, avec la taille c’est aussi la croissance de certaines parties du cerveau (notamment le cortex) qui entrent en jeu. Quel a été l’élément déterminant de cette évolution ? Il semble bien qu’aujourd’hui comme naguère (il y a cinq ou six millions d’années), l’apport nutritif constitue un facteur clé. La disponibilité et l’ingestion d’aliments riches en acides gras fournissent une suite d’éléments explicatifs susceptibles d’étayer une théorie selon laquelle les individus les plus adipeux (fattest) se seraient les mieux adaptés (fittest)1. En somme, l’environnement aurait eu un effet sur les capacités de survie de certains individus par rapport à d’autres.

The circumstances of human brain evolution are of central importance to accounting for human origins, yet are still poorly understood. Human evolution is usually portrayed as having occurred in a hot, dry climate in East Africa where the earliest human ancestors became bipedal and evolved tool-making skills and language while struggling to survive in a wooded or savannah environment. At least three points need to be recognised when constructing concepts of human brain evolution : (1) The human brain cannot develop normally without a reliable supply of several nutrients, notably docosahexaenoic acid, iodine and iron. (2) At term, the human fetus has about 13 % of body weight as fat, a key form of energy insurance supporting brain development that is not found in other primates. (3) The genome of humans and chimpanzees is <1 % different, so if they both evolved in essentially the same habitat, how did the human brain become so much larger, and how was its present-day nutritional vulnerability circumvented during 5-6 million years of hominid evolution ? The abundant presence of fish bones and shellfish remains in many African hominid fossil sites dating to 2 million years ago implies human ancestors commonly inhabited the shores, but this point is usually overlooked in conceptualizing how the human brain evolved. Shellfish, fish and shore-based animals and plants are the richest dietary sources of the key nutrients needed by the brain. Whether on the shores of lakes, marshes, rivers or the sea, the consumption of most shore-based foods requires no specialized skills or tools. The presence of key brain nutrients and a rich energy supply in shore-based foods would have provided the essential metabolic and nutritional support needed to gradually expand the hominid brain. Abundant availability of these foods also provided the time needed to develop and refine proto-human attributes that subsequently formed the basis of language, culture, tool making and hunting. The presence of body fat in human babies appears to be the product of a long period of sedentary, shore-based existence by the line of hominids destined to become humans, and became the unique solution to insuring a back-up fuel supply for the expanding hominid brain. Hence, survival of the fattest (babies) was the key to human brain evolution.