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Daniel LiebermanA modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Compared to H. erectus, H. sapiens, or humans, have larger brains and different faces, and they mature slower, reproduce faster, and carry more fat. Compared to most mammals, humans produce fewer offspring with higher survival rates. Humans take 18 years to mature, and human mothers reproduce twice as often as apes and can care of more than one dependent offspring at a time.
H. erectus dispersed from Africa to Europe and Asia. Researchers hypothesize that H. erectus lived in small bands inhabiting large territories and that their populations increased relatively rapidly. Lieberman notes it’s strange that H. erectus dispersed into temperate regions in the Ice Age, given the extreme low cyclical temperatures.
Dispersed groups become isolated, and isolated groups are subject to unique natural selection. H. erectus groups varied in size and evolved into distinct species of archaic Homo including H. heidelbergensis, the Denisovans, and Neanderthals. Genetic research shows humans interbred sparsely with the Denisovans and Neanderthals. These archaic Homo had larger brains, lived as hunter-gatherers, made advanced tools, and learned to control fire.
Neanderthals diverged from modern humans up to 800,000 years ago. They lived in Europe and Asia up until 30,000 years ago, and they were intelligent hunter-gatherers adept at surviving in cold environments. Their brains and faces were larger than humans’, they cooked and made a variety of stone tools, and experts hypothesize Neanderthals would still exist if it were not for Homo sapiens. Archaeological records suggest Neanderthals did not exhibit modern behaviors like art, transporting materials, or fishing.
Hominin brains almost doubled in size during the Ice Age. Large brains have cognitive benefits but energetic costs, which may be why they have not evolved in other species. Human brains grow faster and longer than other apes’ brains, and they consume 20% to 25% of the body’s energy while resting but account for 2% of the body’s weight. Brains are fragile, and large brains make birth more difficult and dangerous. Researching archaic Homo brains is challenging as most benefits, such as cooperative behaviors and survival knowledge, aren’t preserved in fossil records.
Most mammals have three developmental stages: infancy, juvenile, and adolescence. Humans have a fourth stage—childhood—which occurs between the infant and juvenile stages and lasts from ages 3 to 7. During childhood, a person is weaned but unable to eat and procure food independently. The childhood stage allows human mothers to reproduce twice as often as apes. The juvenile and adolescent stages of development are extended in humans to about 12 years, perhaps to aid the development of the complex human brain.
Human brains need constant energy, which is why humans evolved to carry more fat. Fat is concentrated and stored energy, and humans can turn carbohydrates into fat. It is critical for supplying the brain with constant energy and for fueling endurance. Like other soft tissues, fat is not preserved in fossils, so experts cannot judge when hominins’ fat storage increased. This capacity to store fat has become harmful to many modern humans.
Archaic Homo procured the energy to grow their brains and bodies by obtaining more food and spending less energy on other bodily functions, creating an energy surplus. Cooking, tool use, and cooperation impacted the energy surplus contributing to a positive feedback loop: When energy needs were met, humans could spend more energy on reproducing, growing, and learning, which boosted their abilities to create surpluses. This feedback loop may explain the gradual increase in brain, body size, and cognitive skills.
However, this growth trend was not universal. H. floresiensis, called “the Hobbit,” was a dwarf hominin that lived on the Flores archipelago of Indonesia. Gigantism and dwarfism are common on islands due to a lack of competition and resource limitations, respectively. Researchers are uncertain whether H. floresiensis evolved from H. erectus or H. habilis. Homo sapiens, the last surviving hominin, evolved 200,000 years ago.
Lieberman reflects on the 1970s Tasaday hoax in which Manuel Elizalde paid villagers to pose as archaic hunter-gatherers. The story was popular because it suggested humans are inherently peaceful, moral, and healthy. Similarly, Marshall Sahlins in his Stone Age Economics (1972) called hunter-gatherer lifestyles affluent. Lieberman counters that such lifestyles were neither “idyllic” nor “dreadful,” and that they required ample work and cooperation. However, he agrees that hunter-gatherers had fewer demands and more free time than living humans.
Genetic studies show H. sapiens evolved at least 200,000 years ago in Africa and that they dispersed 100,000 to 80,000 years ago (129). Humans are “genetically homogenous,” meaning individual populations contain almost all genetic variations—“you could wipe out the entire population of the world except for, say, Fiji or Lithuania and still retain almost every human genetic variation” (129). Studies of DNA fragments found in fossilized remains show that humans and Neanderthals diverged 500,000 years ago, that the species are genetically similar, and that they interbred around 50,000 years ago. Humans also interbred with Denisovans. Humans first reached the Middle East 150,000 to 80,000 years ago, but their populations seem to cease there and reappear 50,000 years ago. They reached Asia 60,000 years ago, Europe, New Guinea, and Australia 40,000 years ago, and the Americas 30,000 to 15,000 years ago (131). As H. sapiens arrived in new territories, other hominin species went extinct.
H. sapiens vary from archaic Homo in that they have smaller, retracted faces, spherical skulls, true chins, narrower hips, less muscular shoulders, more significant lumbar spine curves, narrower torsos, and shorter heel bones. Archaeological findings show human behavior was significantly different, too. H. sapiens practiced symbolic behaviors by 70,000 years ago during the Middle Paleolithic, while Neanderthals practiced only rare symbolic behavior. Advancing human culture, referred to as “Upper Paleolithic culture” (135), emerged around 50,000 years ago.
Human tool-making progressed, and human diets expanded to include more animal-based foods, including seafood, leading to increasing populations. The most striking changes were the development of art and the emergence of cultural variations between individual groups. Cultural change has been constant since the Upper Paleolithic. The ability to innovate via culture is what separates humans from other hominins.
Neanderthal brains were a similar size to human brains, but variations in skull shapes suggest they were structured differently. Humans have a larger neocortex, prefrontal cortex, and temporal and parietal lobes, accounting for human’s unique cognition and spherical skulls. Human brains may also be “wired” differently, but this is currently speculation, and researchers are searching for genetic evidence.
Human success was likely impacted by advanced communication abilities. Humans can precisely control their mouths and tongues, and their retracted faces and laryngeal structures help them produce clear, diverse vocalizations. However, human’s throat structures make them more susceptible to choking.
Researchers agree that behavioral modernity is manifested through human’s unique ability to innovate and share culture. Cultural evolution impacts biological evolution but is stronger and faster because cultural traits, or memes, can be intentional and can be disseminated efficiently. Learned behaviors alter the body’s environment and condition, and memes steer natural selection over time. This is seen in cultural buffering, where cultural practices create evolutionary barriers—e.g. wearing clothes prevents physical adaptation to cold temperatures. Culture can also trigger biological adaptations—“For instance, the invention of stone tools and projectiles made possible selection for increased manual dexterity and the ability to throw with power and accuracy” (147).
Lieberman returns to the question of what humans are “adapted for,” suggesting humans evolved to be bipedal hunter-gatherers with endurance, diverse diets. He identifies adaptability and culture as human’s defining qualities. Humans’ adaptability resulted in physical and cultural variations that eventually led to the widespread development of agriculture and permanent settlements. Given the severe consequences of agrarian lifestyles, Lieberman questions if they were a mistake.
As Lieberman’s conversation progresses to later Homo species, including Homo sapiens, The Evolution of the Human Body and Health and Evolutionary Mismatches and Modern Disease become more prominent. The appearance of these themes is mostly implicit, as Lieberman is still focusing primarily on the progression of human evolution. However, they serve to connect the different parts of the book while maintaining the chronological flow of Parts 1 and 2.
The Evolution of the Human Body and Health is developed through humans evolving the capacity to store more fat relative to other ape species. According to Lieberman, this capacity evolved to supply human brains with a constant supply of energy and to fuel human endurance. In hunter-gatherer circumstances, the ability to store ample amounts of fat is an adaptation; however, in modern environments, the evolved trait loses its adaptiveness. That the modern environment has led to the prevalence of diet- and fat-related mismatch diseases is implied through Lieberman’s discussion of chimp and human brains:
In absolute numbers, your brain costs 280 to 420 calories per day, whereas a chimpanzee’s brain costs about 100 to 120 calories per day. In our modern world of energy-rich food, you can supply this amount of energy with about one donut per day, but a donut-deprived hunter-gatherer needs to forage an extra six to ten carrots to get the same number of added calories (109).
The comparison between donuts and carrots is representative of the mismatch between human evolution, which equipped humans to survive on natural foods, and modern human environments, which are replete with high-calorie foods. Lieberman recognizes this thematic element, but he remarks that fat storage will be covered in depth in later chapters, foreshadowing the future development of Evolutionary Mismatches and Modern Disease.
The link between Evolutionary Mismatches and Modern Disease is also explored from a more social perspective. Lieberman notes that “our brains evolved to cope with social networks of about 100 to 230 people” (110). He also describes “contemporary stresses—commuting, the threat of losing one’s job, getting into college, saving for retirement” (127), which are cited through a suggestion that hunter-gatherer lifestyles are more beneficial. These remarks imply that some modern stress arises from a mismatch between humans’ evolved social capacities and modern social demands. Stress is a well-known contributing factor to numerous health conditions, many of which Lieberman lists as mismatch diseases.
While Lieberman does not spend much time focusing on stress, other prominent scientists who take evolutionary approaches to modern diseases, including Robert Sapolsky in his Why Zebras Don’t Get Ulcers, support the notion that humans are not necessarily well-adapted to their social environments. This concept further aids in helping readers distinguish between cultural evolution and humans’ biological social evolution, which are difficult to separate. The distinction between social capacity and modern social environments encourages readers to independently consider further health complications which are not present in the text.