Evolutionary cerebrovascular consequences of upside-down postural verticality of the anthropoid fetus have been largely overlooked in the literature. This working hypothesis-based, narrative review postulates that the rapid evolution of the human brain has been promoted by fetal head-down position due to maternal upright and semi-upright posture. Habitual vertical torso posture is a feature not only of humans, but also of monkeys and non-human apes that spend considerable time in sitting position. Head-down position of the fetus may have caused physiologic craniovascular hypertension that stimulated expansion of the nourishing intracranial vessels and acted as an epigenetic physiological stress, which enhanced neurogenesis and eventually, along with other selective pressures, led to the progressive anthropoid brain enlargement. This article collaterally opens a new insight into the conundrum of high cephalopelvic proportions (i.e., the tight fit between the pelvic birth canal and fetal head) in phylogenetically distant lineages of monkeys, lesser apes, and humans. Low cephalopelvic proportions in non-human great apes could be accounted for by their energetically efficient horizontal nest-sleeping and consequently by their larger body mass compared to monkeys and lesser apes that sleep upright. It can be further hypothesized that brain size varies in anthropoids according to the degree of exposure of the fetus to postural verticality. The supporting evidence for this postulation includes a recent finding that in fossil hominins cerebral blood flow rate increased faster than brain volume. The approaches for testing the hypothesis include experimentation with avian embryo position in vitro, comparative cephalometry of vertex-breech twins, and Doppler assessment of the fetal cerebral circulation in maternal upright and horizontal posture. The current report opens a perspective for further related research on circadian postural behavior, obstetrics, and fetal postural cranial hemodynamics in humans and other primates. It may also be possible to expand the introduced hypothesis into a universal theory of epigenetically enhanced embryonic/fetal cerebral neurogenesis as a driver of evolutionary encephalization.

species devote between 2% and 8% of basal metabolism to the brain. In primates, however, the percentage is much higher—in humans it rises to 20–25%. The The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for special senses such as vision, hearing, and olfaction. Being the most specialized organ, it is responsible for receiving information from the sensory nervous system, process The brains of humans and other primates contain the same structures as the brains of other mammals, but are generally larger in proportion to body size. The encephalization quotient (EQ) is used to compare brain sizes across species. It takes into account the nonlinearity of the brain-to-body relationship. Humans have an average EQ in the 7-to-8 range, while most other primates have an EQ in the 2-to-3 range. Dolphins have values higher than those of primates other than humans, but nearly all other mammals have EQ values that are substantially lower. Most of the enlargement of the primate brain comes from a massive expansion of the cerebral cortex, especially the prefrontal cortex and the parts of the cortex involved in vision. The visual processing network of primates includes at least 30 distinguishable brain areas, with a complex web of interconnections. It has been estimated that visual processing areas occupy more than half of the total surface of the primate neocortex. The prefrontal cortex…

A programme on the evolution and cognitive abilities of humans and primates. About what we share and what sets us apart. The primatologists Josep Call and Miguel Llorente participate, the veterinary Rosa Garriga and the doctor in Prehistory, Marina Mosquera.