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Chapter 4. Becoming Human: The Origin and Diversity of Our Species. To What Group of Animals Do Humans Belong?. Biologists classify humans as Homo sapiens , members of the primates— a subgroup of mammals.
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Chapter 4 Becoming Human: The Origin and Diversity of Our Species
To What Group of Animals Do Humans Belong? • Biologists classify humans as Homo sapiens, members of the primates— a subgroup of mammals. • Biological species are defined by reproductive isolation and designated by a two part name including genus (Homo) and species (sapiens). • Other primates include lemurs, lorises, tarsiers, monkeys, and apes. • Studying the anatomy and behavior of other primates helps us understand how and why early humans developed as they did.
When and How Did Humans Evolve? • Present evidence suggests that humans evolved from small African apes between 5 and 8 million years ago. • Bipedalism, or walking on two feet, was the first change to distinguish the human evolutionary line. • Several million years after the evolution of bipedalism, brain size began to expand, along with development of cultural activities such as making stone tools. • The earliest stone tools date to between 2.5 and 2.6 million years ago, coinciding with the appearance of the first members of the genus Homo in the fossil record.
Is the Biological Concept of Race Useful for Studying Physical Variation in Humans? • Biologically defined, race refers to subspecies, and no subspecies exist within modern Homo sapiens. • The vast majority of biological variation within our species occurs within populations rather than among them.
Paleoanthropologists and Primatologists • Paleoanthropologists are anthropologists specializing in the study of human evolutionary history. • Primatologists are specialists in the behavior and biology of living primates and their evolutionary history.
Evolution Through Adaptation • Evolution refers to changes in the genetic makeup of a population over generations. • Genes are basic physical units of heredity that specify the biological traits and characteristics of each organism.
Evolution Through Adaptation • Evolution takes place through adaptation, a series of beneficial adjustments of organisms to their environment. • Adaptation is the cornerstone of the theory of evolution by natural selection, originally formulated by English naturalist Charles Darwin in 1859. • In this theory, individuals with characteristics best suited to a particular environment survive and reproduce with greater frequency than individuals without those characteristics.
Human Adaptations and Culture • Humans relied increasingly on culture as an effective way of adapting to the environment. • They figured out how to manufacture and utilize tools. • They organized into social units that made food-foraging more successful;. • They learned to preserve and share their traditions and knowledge through the use of symbols that ultimately language.
Humans and Other Primates • Humans are one of 10 million species on earth, 4,000 of which are fellow mammals. • Species are populations or groups of populations having common attributes and the ability to interbreed and produce live, fertile off spring. • The human species is a kind of primate, a subgroup of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. • Among fellow primates, humans are most closely related to apes—chimpanzees, bonobos, gorillas, orangutans, and gibbons.
Classifying Primates • Early scientific struggles to classify great apes, and identify and weigh the similarities and differences between them and humans, is reflected in early European renderings of apes, including this 18th-century image of a chimpanzee portrayed as a biped equipped with a walking stick.
Anatomical Adaptations • Ancient and modern primate groups possess a number of anatomical characteristics: • Generalized set of teeth, suited to insect eating but also fruits and leaves. • Depth perception • Intensified sense of touch • Binocular stereoscopic vision
Anatomical Adaptations • Brain is large, heavy in proportion to body weight, and very complex • Skeleton has adaptations for upright posture and flexibility of limb movement. • Fewer offspring born to each female and a longer period of infant dependency.
Jaws: Reptiles and Mammals • The jaw of reptiles contains a series of identical teeth. If a tooth breaks or falls out, a new tooth will emerge. • Mammals possess precise numbers of specialized teeth, each with a particular shape characteristic of the group, as indicated on the chimpanzee jaw.
Primate Vision • Anthropoid primates possess binocular stereoscopic vision. • Binocular vision refers to overlapping visual fields associated with forward facing eyes. • Three-dimensional vision comes from binocular vision and the transmission of information from each eye to both sides of the brain.
Behavioral Adaptations • Primates adapt to their environments not only anatomically but also through a wide variety of behaviors. • Young apes spend more time reaching adulthood than most other mammals. • During their growth and development, they learn the behaviors of their social group. • Two closely related African species of chimpanzee: common chimpanzees and bonobos, provide models to reconstruct the behavior of evolving humans
Chimpanzee and Bonobo Behavior • Among chimps, the largest social unit is the community, fifty or more individuals who inhabit a large geographic area. • Chimps are usually found ranging singly or in small subgroups consisting of adult males, or females with their young, or males and females together with young. • While strength and size contribute to an animal’s rank in the community, the rank of its mother, largely determined through her cooperative social behavior also plays a role.
Nutrition and Hunting: Chimpanzees • Jane Goodall’s fieldwork among chimpanzees revealed they sometimes kill small invertebrate animals for food, and also hunt and eat monkeys. • Hunting is not done purely for dietary purposes, but for social and sexual reasons as well. • Fertile females are more successful than others at begging for meat, and males often share the meat after copulation. • Males use their catch to reward friends and allies, gaining status in the process.
Nutrition and Hunting: Bonobos • Recent research shows that bonobos in Congo’s rainforest supplement their diet by hunting. • Among bonobos hunting is primarily a female activity. • Female hunters regularly share carcasses with other females, but less often with males. • Even when the most dominant male throws a tantrum nearby, he may still be denied a share of meat. • Discriminatory sharing among female bonobos is also evident when it comes to other foods such as fruits.
Sexual Practices: Chimpanzees • For chimps, sexual activity occurs only when females signal their fertility through genital swelling. • Dominant males try to monopolize females, although cooperation from the female is usually required for this to succeed. • An individual female and a lower-ranking male sometimes form a temporary bond, leaving the group together for a few private days during the female’s fertile period. • Dominant males do not necessarily father all of the off spring in a social group.
Female Chimpanzee Genital Swelling • Female chimpanzees display their fertility through swelling of the genitalia at the time of ovulation. • In contrast to humans and bonobos, animals with time limited displays are sexually receptive only during these times of fertility.
Sexual Practices: Bonobos • Bonobos do not limit their sexual behavior to times of female fertility, bonobo female genitals are perpetually swollen. • Concealed ovulation in bonobos may play a role in the separation of sexual activity for social reasons and pleasure from the biological task of reproduction. • Primatologists have observed every possible combination of ages and sexes engaging in an array of sexual activities, including oral sex, tongue-kissing, and massaging each other’s genitals. • The primary function of most of this sex is to reduce tensions and resolve social conflicts.
Chimpanzee and Bonobo Childhood Development • The young chimp or bonobo learns by observation, imitation, and practice how to interact with others and manipulate them for his or her own benefit. • Young primates learn to match their interactive behaviors according to each individual’s social position and temperament. • Anatomical features such as a free upper lip allow varied facial expression, contributing to greater communication among individuals. • Young chimpanzees also learn to how to make and use tools.
Human Ancestors • Humans are classified as hominoids, the broad-shouldered tailless group of primates that includes all living and extinct apes and humans. • Humans and their ancestors are distinct among the hominoids for bipedalism, walking upright on two feet. • Genetic and biochemical studies have confirmed that the African apes—chimpanzees, bonobos, and gorillas—are our closest living relatives.
Common Primate Ancestors • Based on molecular similarities and differences, a relationship can be established among various primate groups.
Human Ancestors • Between 5 and 8 million years ago, humans, chimpanzees, and gorillas began to follow separate evolutionary courses. • Chimpanzees diverged into two separate species: the common chimpanzee and the bonobo. • Early human evolutionary development followed a path that produced only one surviving bipedal species: Homo sapiens.
The First Bipeds • During the early Pliocene, 5 million years ago, the genus Australopithecus appeared in Africa. • Australopithecines include a diverse group of bipedal species with small brains in proportion to body size. • One of the other australopithecine species appears to be a direct ancestor of the genus Homo.
Australopithecine Fossil Locations • Australopithecine fossils have been found in South Africa, Malawi, Tanzania, Kenya, Ethiopia, and Chad.
Advantage of Bipedalism • A way to cope with heat stress. • Allowed them to gather food and transport it to a place of safety for consumption. • Mothers were able to carry their infants safely. • They could reach food on trees too flimsy to climb. • Allowed them to travel far without tiring.
Advantages of Bipedalism • Food and water were easier to spot. • More likely to spot predators before they got too close for safety. • Hands freed from locomotion provided protection by allowing them to brandish and throw objects at attackers.
Early Homo • Increased meat consumption was important for human evolution. • Failure to satisfy protein requirements can lead to stunted growth, malnutrition, starvation, and death. • Without sharp teeth, our ancestors needed sharp tools for butchering carcasses. • The earliest identify able stone tools have been found in Africa often in the same geological strata as the earliest Homo fossils.
Early Homo and Tools • Stone flakes and choppers mark the beginning of the Lower Paleolithic, the first part of the Old Stone Age, from about 200,000 to 2.6 million years ago. • Flakes were obtained from a “core” stone by striking it with stone or against a large rock. • The flakes that broke off had sharp edges, effective for cutting meat and scraping hides. • Leftover cores were made into choppers, used to break open bones.
Early Stone Tools • The earliest stone tools dated to the beginning of the Lower Paleolithic between 2.5 and 2.6 million years ago were discovered by Ethiopian paleoanthropologist Sileshi Semaw at Gona, in the west-central Afar region of Ethiopia. The 2.6 million-year-old Gona flake is a cutting tool with sharp edges.
Homo habilis • “Handy man.” • The first fossil members of the genus Homo appearing 2.5 million years ago, with larger brains and smaller faces than australopithecines.
Tools, Food and Brain Expansion • Scenarios about behavioral adaptation in early Homo, such as the relationship among tools, food, and brain expansion, propose a feedback loop between brain size and behavior. • Over time, large-brained individuals contribute to successive generations, so the population evolves to a larger-brained form.
Tools, Food and Brain Expansion • Natural selection for increases in learning ability has led to the evolution of larger and more complex brains over about 2 million years. • Bipedalism set the stage for the evolution of large brains and human culture by freeing the hands for tool making and carrying of resources or infants.
Homo erectus and the Spread of the Genus Homo • Shortly after 2 million years ago, at a time that Homo habilis and Oldowan tools had become widespread in Africa, a new species, Homo erectus, appeared on that continent. • Evidence of H. erectus fossils almost as old as those discovered in Africa have been found in the Caucasus Mountains of Georgia, South Asia, China, the island of Java, and western Europe. • Fossil evidence suggests some differences within and among populations of H. erectus inhabiting regions of Africa, Asia, and Europe.
Homo erectus Sites • Sites, with dates, at which Homo erectus remains have been found. Arrows indicate the proposed routes by which Homo spread from Africa to Eurasia.
H. erectus and the Ice Age • Emergence of H. erectus coincided with the beginning of the Pleistocene epoch (Ice Age), which spanned from 10,000 to almost 2 million years ago. • During this period, Arctic cold conditions and snowfall in the earth’s northern hemisphere created ice sheets that covered much of Eurasia and North America. • These periods often lasted tens of thousands of years, separated by intervening warm periods. • During much of this time sea levels were much lower than today, exposing large surfaces now under water.
H. erectus and the Ice Age • Of all the epochs in the earth’s 4.6 billion-year history, the Pleistocene is the period in which humans—from H. erectus to H. sapiens— evolved and spread all across the globe. • Our early human ancestors were challenged to make biological and cultural adaptations in order to survive and reproduce. • The principle of natural selection was at work on humans favoring the perpetuation of certain characteristics within particular environmental conditions.
H. Erectus • H. erectus had a body size and proportions similar to modern humans, though with heavier musculature. • Differences in body size between the sexes diminished compared to earlier bipeds, perhaps to facilitate successful childbirth. • H. erectus’ average brain size fell within the higher range of H. habilis and within the lower range of modern human brain size. • The dentition was fully human, though relatively large by modern standards.
Homo Erectus Fossil • One of the oldest—at 1.6 million years—and most complete fossils of Homo erectus is the “strapping youth” from Lake Turkana, Kenya: a tall and muscular boy who was already 5 feet 3 inches tall when he died at about age 13.
H. erectus Tools • The Oldowan chopper was replaced by the more sophisticated hand axe. • The hand axes, shaped by regular blows giving them a larger and finer cutting edge than chopper tools, were probably all purpose implements for food procurement and processing, and defense. • H. erectus also developed cleavers and scrapers to process animal hides for bedding and clothing.
Use of Fire • Fire allowed our human ancestors to continue activities after dark and provided a means to frighten away predators. • It supplied them with the warmth and light needed for cave dwelling, and it enabled them to cook food. • Cooking detoxifies poisonous plants and allows important vitamins, minerals, and proteins to be absorbed from the gut rather than passing unused through the intestines. • When our human ancestors learned to use fire they dramatically increased their geographic range and nutritional options.
The Beginnings of Homo sapiens • At various sites in Africa, Asia, and Europe, a number of fossils have been found that date between roughly 200,000 and 400,000 years ago. • The best population sample, bones of about thirty individuals of both sexes and all ages comes from Atapuerca, a 400,000-year-old site in Spain. • These bones show a mixture of characteristics of Homo erectus with those of early Homo sapiens.
“Pit of the Bones” • In a cave beneath a hillside in Atapuerca, Spain, lies the Sima de los Huesos (“Pit of the Bones”). • The bottom of the pit is crammed with animal bones, including cave bears, lions, foxes, and wolves. • Thousands of early human fossils dating back 400,000 years have been found here. • The well-preserved remains come from at least 28 individuals and comprise the greatest single cache of ancient Homo erectus fossils in the world.
Neandertals • An extremely muscular people living from approximately 30,000 to 125,000 years ago in Europe and southwestern Asia. • With brains of modern size, Neandertals possessed faces distinctively different from modern humans. • Their large noses and teeth projected forward. • They had bony brow ridges over their eyes. • On the back of their skull, there was a bunlike bony mass for attachment of powerful neck muscles.
Mousterian Tradition • Tool-making tradition of the Neandertals and their contemporaries of Europe, western Asia, and northern Africa. • Named after the Neandertal site of Le Moustier, France. • Tools included hand axes, flakes, scrapers, borers, wood shavers, and spears. • Mousterian peoples buried their dead, cared for the disabled, and made objects for symbolic purposes.
Neandertals • As this face-off between paleoanthropologist Milford Wolpoff and his reconstruction of a Neandertal shows, the latter did not differ all that much from modern humans of European descent.
