Brain cooling via emissary veins: Fact or fancy?

The diploic venous system in Homo neanderthalensis and fossil Homo sapiens: A study using high-resolution computed tomography

OBJECTIVES

The diploic venous system has been hypothesized to be related to human brain evolution, though its evolutionary trajectory and physiological functions remain largely unclear. This study examines the characteristics of the diploic venous channels (DCs) in a selection of well-preserved Homo neanderthalensis and Upper Paleolithic Homo sapiens crania, searching for the differences between the two taxa and exploring the associations between brain anatomy and DCs.

MATERIALS AND METHODS

Five H. neanderthalensis and four H. sapiens fossil specimens from Western Europe were analyzed. Based on Micro-CT scanning and 3D reconstruction, the distribution pattern and draining orifices of the DCs were inspected qualitatively. The size of the DCs was quantified by volume calculation, and the degree of complexity was quantified by fractal analyses.

RESULTS

High-resolution data show the details of the DC structures not documented in previous studies. H. neanderthalensis and H. sapiens specimens share substantial similarities in the DCs. The noticeable differences between the two samples manifest in the connecting points surrounding the frontal sinuses, parietal foramina, and asterional area.

DISCUSSION

This study provides a better understanding of the anatomy of the DCs in H. neanderthalensis and H. sapiens. The connection patterns of the DCs have potential utility in distinguishing between the two taxa and in the phylogenetic and taxonomic discussion of the Neandertal-like specimens with controversial taxonomic status.

Absence of selective brain cooling in unrestrained baboons exposed to heat

To test whether baboons are capable of implementing selective brain cooling, we measured, every 5 min, the temperature in their hypothalamus, carotid arterial bloodstream, and abdominal cavity. The baboons were unrestrained and exposed to 22°C for 7 days and then to a cyclic environment with 15°C at night and 35°C during the day for a further 7 days. During the latter 7 days some of the baboons also were exposed to radiant heat during the day. For three days, during heat exposure, water was withheld. At no time was the hypothalamus cooler than carotid arterial blood, despite brain temperatures above 40°C. With little variation, the hypothalamus was consistently 0.5°C warmer than arterial blood. At high body temperatures, the hypothalamus was sometimes cooler than the abdomen. Abdominal temperature was more variable than arterial blood and tended to exceed arterial blood temperature at higher body temperatures. Hypothalamic temperature cooler than a warm abdomen is not evidence for selective brain cooling. In species that can implement selective brain cooling, the brain is most likely to be cooler than carotid arterial blood when an animal is hyperthermic, during heat exposure, and also dehydrated and undisturbed by human presence. When we exposed baboons to high ambient temperatures while they were water deprived and undisturbed, they never implemented selective brain cooling. We conclude that baboons cannot implement selective brain cooling and can find no convincing evidence that any primate species can do so.

Meat‐Adaptive Genes and the Evolution of Slower Aging in Humans

The chimpanzee life span is shorter than that of humans, which is consistent with a faster schedule of aging. We consider aspects of diet that may have selected for genes that allowed the evolution of longer human life spans with slower aging. Diet has changed remarkably during human evolution. All direct human ancestors are believed to have been largely herbivorous. Chimpanzees eat more meat than other great apes, but in captivity are sensitive to hypercholesterolemia and vascular disease. We argue that this dietary shift to increased regular consumption of fatty animal tissues in the course of hominid evolution was mediated by selection for "meat-adaptive" genes. This selection conferred resistance to disease risks associated with meat eating also increased life expectancy. One candidate gene is apolipoprotein E (apoE), with the E3 allele evolved in the genus Homo that reduces the risks for Alzheimer's and vascular disease, as well as influencing inflammation, infection, and neuronal growth. Other evolved genes mediate lipid metabolism and host defense. The timing of the evolution of apoE and other candidates for meat-adaptive genes is discussed in relation to key events in human evolution.

Dopamine and the origins of human intelligence

A general theory is proposed that attributes the origins of human intelligence to an expansion of dopaminergic systems in human cognition. Dopamine is postulated to be the key neurotransmitter regulating six predominantly left-hemispheric cognitive skills critical to human language and thought: motor planning, working memory, cognitive flexibility, abstract reasoning, temporal analysis/sequencing, and generativity. A dopaminergic expansion during early hominid evolution could have enabled successful chase-hunting in the savannas of sub-Saharan Africa, given the critical role of dopamine in counteracting hyperthermia during endurance activity. In turn, changes in physical activity and diet may have further increased cortical dopamine levels by augmenting tyrosine and its conversion to dopamine in the central nervous system (CNS). By means of the regulatory action of dopamine and other substances, the physiological and dietary changes may have contributed to the vertical elongation of the body, increased brain size, and increased cortical convolutedness that occurred during human evolution. Finally, emphasizing the role of dopamine in human intelligence may offer a new perspective on the advanced cognitive reasoning skills in nonprimate lineages such as cetaceans and avians, whose cortical anatomy differs radically from that of primates.