Human Biology is a diverse field incorporating disciplines such as genetics, physiology, anatomy, and anthropology, all with their own preoccupations and methodological constraints. If anything unites such wide-ranging interests, it may be the biological implications of everyday human activity and its variation across individuals and populations. Traditionally, human biologists have measured human activities in naturalistic settings. Increasingly, the measurements have included detailed physiological and genetic markers, substrates that can be interpreted in evolutionary terms. The principle of allocation to explain trade-offs between the reproductive system, growth and development, and bodily maintenance has been critical to such interpretations, as has the use of energy as the critical currency in assessing such trade-offs.
On the basis of the logic of allocation, we might apportion our research efforts according to the amount of time humans spend in particular activities. The most recent American Time Use Survey reports that Americans spend 8.7 h each day sleeping, 4.0 hours working, 1.8 hours eating and preparing food, and only 0.7 hours socializing and communicating. On the basis of these numbers, we might allocate approximately one-third of our research effort to studying sleep, a one-sixth to work, less than a one-twelfth to food and eating, and a meager 1/33 to social activity.
Of course, such a time allocation approach is not really practical and more importantly it is misleading. Not only does it fail to take into account the different energetic costs and benefits of each activity but many human activities are actually multitasking with impact on several different biological functions. Another alternative, illustrated in this issue, is to more fully incorporate the study of the brain within human biology. The human brain is highly energetically dependent organ, allowing for the consideration of trade-offs between brain and body in terms of energetic cost. In addition, the human brain is the source of the multitasking social capacity that allows humans to sometimes bend the simple allocation rules posited by life history theory.
The articles in this issue present an up to date view of recent research on the human brain, and its potential contribution to a human biology that links brain and body. John Allman's Pearl lecture on “The Von Economo Neurons” provides a comprehensive look at von Economo neurons (VENs) and their potential role in the importance of somatic information, especially that from the gut, in human social decision making. The article provides a compelling case for the importance of VENs in the complex social behavior of human and the great apes. Of additional interest is evidence that neuromedin B (NMB), a protein marker of VENs, is present in analogous regions of the mouse brain. These results suggest that the VENs in humans and the great apes are not novel, but a higher level elaboration of neural mechanisms deeply embedded in our mammalian heritage. The article by Cheryl Stimpson and colleagues, “Biochemical Specificity of von Economo Neurons in hominoids,” provides more detailed results on VENs in humans and the great apes. Here the results indicate differences in the frequency of activating transcription factor 3, interleukin 4 receptor (ILP4) markers between humans and the great apes suggesting further refinement in VENs over the course of hominid evolution.
Other articles take a more explicitly energetic approach to the human brain. Achim Peter's contribution, “The Selfish Brain: Competition for Energy Resources,” demonstrates how the principle of allocation can be applied to the brain's energy needs. His concept of “brain pull” is supported by clearly delineated physiological mechanisms by which the brain maintains its own energy supply. His conclusion that adequate “brain pull” may be a critical marker of organismal homeostasis over the life span will be of particular interest to human biologists interested in aging and the human life span. “The Evolution of Meningeal Vascular System in the Human Genus: from Brain shape to Thermoregulation” by Emiliano Bruner and colleagues combines endocranial angiotomography in living humans and thermic modeling to investigate brain thermoregulation. The finding that the middle meningeal artery exhibits little blood flow in modern human adults is a real shocker. It has important implications for hominid brain evolution, where this artery has figured prominently in discussions of thermoregulation. Benjamin Campbell's article, “Adrenarche in comparative perspective,” compares developmental trajectories of adrenal hormones, cortical glucose utilization, and neurodevelopment across rodents, primates, and humans to contextualize human adrenarche in evolutionary terms. The results suggest that postnatal increases in the adrenal hormones DHEA and DHEAS could provide neuroprotection for slow developing brain regions to ensure the neuroplasticity needed to integrate the neural representation of a growing body with emerging prefrontal function.
Last but not least, Eric Vallender's article, “Comparative genetic approaches to the evolution of human brain and behavior,” takes a forward looking perspective on the ability of genetic evidence to help unravel the mysteries of human brain evolution. He notes that the on-going explosion of primate genomic sequencing will be a boon for understanding the human brain in comparative perspective. At the same time, because allelic variation in genes related to the human brain may only provide information back to 200,000 ybp, new forms of genetic variation will need to be explored to fully take advantage of the genetic revolution.