“I think many people today never have to actually chew anything all day long,” he says. And humans, he points out, have never had greater access to high-energy processed food than they do now. The study suggested that there is a link between smaller jaws and regularly chewing very high-quality soft food. The stresses of chewing made the upper and lower jaws of the pigs eating hard food grow larger. In one experiment, he fed soft food to one group of pigs, hard food to another. As bones grow, their size and shape respond to biomechanical stresses, so he decided to study the effects of chewing hard versus soft food on the growth and development of the skull in various animal species. About nine years ago, while contemplating why human faces have become smaller in the brief span of recorded history-too short a time for evolution to explain-Lieberman wondered whether the modern diet of soft food might be a contributing factor. (The book draws on the expertise of many scholars as well as his own work.) Experimental biomechanics is one. To probe such mysteries, Lieberman uses a variety of techniques. In short: Why does the head look the way it does? Nevertheless, Lieberman takes up all sorts of interesting questions, such as why humans have big noses (there is a link to running), chins (nobody really knows), small faces that are tucked under the brain, and teeth that are suitable only for eating processed food. Though written so anyone can read it, The Evolution of the Human Head (Harvard University Press) “is not meant to be a bestseller,” he avers. Not only do heads contain many modules (instructions for building an eye to see, for example, or an ear to listen), but each module is itself “intensely integrated in terms of development, structure, and function….Changes to the size, the shape, or the relative timing of development of each of the head’s many modules offer a variety of opportunities for change.” Studying the head’s modules, Lieberman writes, may help us understand why “the human head has changed substantially since our lineage diverged from the chimpanzee’s lineage.” It also provides an opportunity for “exploring how nature tinkers with development in ways that affect function and permit the evolution of complex structures.” “How is it,” he asks, “that something so complicated and so vital can also be so evolvable?” One explanation involves modularity and integration. For example, the roof of the orbits is the floor of the brain-if one changes, they both do. On the other hand, it is where natural selection can and has acted powerfully to make us what we are.” Everything is closely connected. “The head presents an interesting evolutionary paradox,” explains Lieberman, chair of the new department of human evolutionary biology, “because on the one hand it is so complicated that if anything goes wrong, the organism dies. Lieberman’s book considers in detail how each of these functions works from a biomechanical perspective, and then theorizes how each evolved, simultaneously with the others, as part of an integrated whole. “It is the size of a soccer ball, and think what it does”: seeing, hearing, tasting, smelling, balancing, thinking, speaking, chewing, swallowing, breathing, and regulating body temperature. “Many of the things we value most-our big brains, speech, much of our sensory biology-are in the head,” Lieberman explains. The theory that humans evolved to become endurance runners so talented that a team of barefoot hunters on a hot African savanna could actually run a large antelope to ground is based in part on skeletal evidence from the head-a subject about which Lieberman has just published a weighty book, 15 years in the making. It’s bizarre.”īut the apparent dichotomy is bizarre only in the abstract, because Lieberman’s interest in feet-and human endurance running-began with his interest in heads. “I never thought that would happen in my career. “I do seem to end up working on the two ends of the body, and not so much in between,” he muses. These collections neatly bracket the research interests of Daniel Lieberman, human evolutionary biologist: a head-to-toe interest in the human body, its morphology (the science of an organism’s form, including the study of specific structural features), its development, and its evolution. The shoes are likewise varied, from brightly colored Nikes, to fluorescent ASICS, to slipper-like Vibram FiveFingers™ with separated toes-like a glove-intended to mimic barefoot running. The skulls come in a range of sizes and shapes, from the tiny and sharp-toothed, with large orbits for the eyes, to the hefty and humanoid. Beneath shelves of books on the biology of bone, a collection of skulls and running shoes lies in a jumble on a countertop.
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