The Evolution of the Human Head by David E. Lieberman

“Almost every particle entering your body, either to nourish you or to provide information about the world, enters via your head,” writes David E. Lieberman.  So how does something so important to survival not only grow, but also evolve, and what changes took place to make our heads so very different from all other mammals? Lieberman combines the disciplines of anatomy, biomechanics and paleoanthropology to review the development of the human head and what factors may have influenced its design. Geared towards a university audience, The Evolution of the Human Head is divided into three sections allowing the reader to choose whether he or she wants to jump to a specific subject or read the text as a whole. The first two sections of the book focus on the bones and soft tissues of the human head while the last section focuses on the fossil record and the hominins that led to modern humans.

One would think that making any change to something as complex as the head would be disastrous to the survivability of the animal in question, but the integration and adaptability of the bones of the skull allow for a multitude of opportunistic changes to its design. Harmful changes would have been very quickly removed from the population because those individuals would not have survived long enough to reproduce while the beneficial changes would have been passed on to the next generation.

Because heads perform so many critical functions, it is possible that variations in the head are more likely to have effects on fitness than variations in other regions of the body. Put differently, there are more targets of selection in the head than elsewhere because the head participates in more functions.

Modifications to the skull can affect how one perceives the world. Our ability to see, hear, taste, and smell have all changed during the millions of years that have separated us from our last common ancestor with the African great apes. Lieberman looks at the changes in the human head, compared to our hominin predecessors and to modern chimpanzees, to determine what our derived characteristics are and how they may have helped or hindered the fitness of our ancestors. Take, for instance, our descended larynx; the feature that makes our speech and language abilities possible, but at the same time increases the risk of choking while feeding. Clearly, either the descended larynx, or another feature of the head that happens to affect the location of the larynx, such as the cranial base flexion, offered our ancestors something that increased fitness enough to outweigh the risk of premature death via choking. Unfortunately, as Lieberman discusses, soft tissues and social behaviors are not preserved in the fossil record. Lieberman does, however, discuss research that has been conducted on the energy requirements for mammalian brains, and the difference in human dietary requirements compared to those of other primates, to come to some understanding about what soft tissue changes might have occurred in hominins and what might have made it possible for us to develop such large brains.

An adult modern human needs to consume about 400-650 kcal of energy a day just for the brain’s upkeep; in contrast, an adult chimpanzee brain needs only 120-150 kcal/day, and a typical early H. erectus brain (about 800 cm³) would have needed about 280 kcal/day. Aiello and Wheeler (1995) have suggested that humans offset the increased cost of a large, expensive brain by decreasing the size of another very expensive tissue, the gut. Whereas average adult humans have a brain that is 851 g heavier than expected for a primate of our body mass, we have guts that are 781 g lighter than expected relative to body mass. Because gut size affects the rate of nutrient absorption, Aiello and Wheeler suggest that this trade-off also required an increase in dietary quality, such as more meat and fat.

Archeological evidence has also supported the idea of increased meat consumption among hominins, such as Homo erectus, as evidenced by their tool use and the signs of cut marks on animal bones dated to the same period.

The Evolution of the Human Head is a fantastic introduction to the evolution of a very complex and vital part of our bodies and while a complete discussion of all of the topics touched on in this book would fill volumes, Lieberman includes suggested readings in each section of the book for those who are interested in learning more.

Reviewed by Jennifer Campbell

The Evolution of the Human Head by Daniel E. Lieberman
Belknap Press, 2011
Cloth, 768 pp, $39.95
ISBN-13: 9780674046368