Human Evolution

I

Introduction

Human Evolution, lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people evolved over a period of at least 6 million years.

One of the earliest defining human traits, bipedalism—walking on two legs as the primary form of locomotion—evolved more than 4 million years ago. Other important human characteristics—such as a large and complex brain, the ability to make and use tools, and the capacity for language—developed more recently. Many advanced traits—including complex symbolic expression, such as art, and elaborate cultural diversity—emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa—chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas—share a common ancestor that lived sometime between 8 million and 6 million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 million and 2 million years ago come entirely from Africa.

Most scientists distinguish among 12 to 19 different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species—probably the majority of them—left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

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Early humans first migrated out of Africa into Asia probably between 2 million and 1.7 million years ago. They entered Europe somewhat later, generally within the past 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.

The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.

II

The Process of Evolution

All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring—that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

The mechanism for evolutionary change resides in genes—the basic units of heredity. Genes affect how the body and behavior of an organism develop during its life. The information contained in genes can change—a process known as mutation. The way particular genes are expressed—how they affect the body or behavior of an organism—can also change. Over time, genetic change can alter a species’s overall way of life, such as what it eats, how it grows, and where it can live.

Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population—a group of organisms of the same species that share a particular local habitat. Many factors can favor new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behavior. Over long periods, evolution dramatically transformed humans and their ways of life.

Geneticists estimate that the human line began to diverge from that of the African apes between 8 million and 5 million years ago (paleontologists have dated the earliest human fossils to at least 6 million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago.

III

Characteristics, Classification, and Evolution of the Primates

Humans belong to the scientific order named Primates, a group of over 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities as well as some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, as well as the significance of each step in human evolution.

All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives.

The scientific classification of primates reflects evolutionary relationships among individual species and groups of species. Strepsirhine (meaning 'turned-nosed') primates—of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians—evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning 'simple-nosed') primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors.

Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans—who share many traits not found in other primates—together make up the suborder Anthropoidea. Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups.

A

Strepsirhines

Strepsirhines are the most primitive types of living primates. The last common ancestors of strepsirhines and other mammals—creatures similar to tree shrews and classified as Plesiadapiformes—evolved at least 65 million years ago. The earliest primates evolved by about 55 million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about 55 million to 38 million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large or complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates.

B

Haplorhines

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