Human beings belong to the mammalian group known as Primates -- the scientific category that contains over 230 species of lemurs, lorises, tarsiers, monkeys of the Old and New World, and apes. Modern humans, early humans, and other primate species all share many similarities and have some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits and the significance of each development 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, an environment in which the earliest primates evolved. These characteristics 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 in the trees; 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 "wet nosed") primates -- of which the living representatives include lemurs, lorises, and other groups of species -- are all commonly known as prosimians. Strepsirhines are the most primitive of living primates. They share all of the basic characteristics of primates, although their brains are neither particularly large nor complex and they have a more elaborate and sensitive olfactory system (involved in the sense of smell) then do other primates.
The earliest monkeys and apes evolved from ancestral haplorhine (meaning "dry nosed") primates, of which the most primitive living representative is the tarsier. Tarsiers were previously grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinctive 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. Anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Eurasia) groups. The platyrrhine (broad-nosed) monkeys represent the first, and the second is the catarrhine (downward-nosed) monkeys and apes. Humans belong to this second group.
Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among these species. Scientists do not all agree about the appropriate classification of the families within this superfamily. Living hominoids are grouped into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) include humans and, according to some scientists, the great apes. For those who include only humans among the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae.
Traditionally, the term "hominid" has referred to species of humans that evolved after the split between early humans and other ape lineages. But genetic evidence, which shows chimps and humans to be more closely related genetically (and evolutionarily) to each other than to any other ape, supports placing all of the great apes and humans together in the family Hominidae. According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches by about 13 million years ago, belongs to the subfamily Ponginae. The African apes (gorillas, chimpanzees, and humans) are then classified in the subfamily called Homininae (or hominines). And finally, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins.
This classification would be true to the genetic evidence. But it tends to be confusing when learning about the subject, as many similar names (hominoid, hominid, hominine, and hominin) would apply to the different aspects of ape and human evolution. In this article the term "early human" refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the word "hominid" to mean the same thing.
About 98 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ genetically from humans to a greater extent, indicating a more distant evolutionary relationship.
Modern humans have a number of physical characteristics indicative of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation (swinging from branch to branch in trees). Although humans do not brachiate, the general anatomy of that earlier adaptation still remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals.
Human social life, too, shares similarities with that of African apes and other primates -- such as baboons and rhesus monkeys -- that live in large and complex social groups. Group behavior among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life.
However, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible.
The origin of the mammalian group primates is traced back to Plesiadapiformes, the last common ancestors of strepsirhines and other mammals. Plesiadapiformes evolved at least 65 million years ago. They were creatures similar to the modern tree shrews. The earliest primates evolved by about 55 million years ago. The first strepsirhine primates, fossil species similar to lemurs and tarsiers, evolved during the Eocene epoch (about 56 to 34 million years ago). The oldest lineages of catarrhine primates, from which monkeys and apes evolved, are known between 50 and 33 million years ago. A primate known as Propliopithecus (one lineage sometimes called Aegyptopithecus), from the Fayum fossil sites of Egypt, is an archaic-looking catarrhine, and is thought to be what the common ancestor of all later Old World monkeys and apes looked like. So Propliopithecus may be considered an ancestor, or closely related to a direct ancestor, of humans.
Hominoids, or members of the superfamily Hominoidea, evolved during the Miocene epoch (24 million to 5 million years ago). Large ape species had originated in Africa by 23 or 22 million years ago. Among the oldest known hominoids is a group of apes known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans. The ape species Proconsul heseloni lived in dense forests of eastern Africa about 20 million years ago. It was agile in the trees, with a flexible backbone and narrow chest of a monkey, yet capable of wide movement of the hip and thumb as in apes.
Early in their evolution, the large apes underwent several radiations, periods when species originated and became more diverse. After Proconsul had thrived for several million years, a group of apes from Africa and Arabia known as the afropithecines evolved around 18 million years ago and diversified into several species. By 15 million years ago, apes had migrated to Asia and Europe over a land bridge formed between the Africa-Arabian and Eurasian continents, which had previously been separated. Around this time, two other groups of apes had evolved – namely, the kenyapithecines of Africa and western Asia (first known about 15 million years ago) and the dryopithecines of Europe (first known about 12 million years ago). It is not yet clear, however, which of these groups of ape species may have given rise to the common ancestor of African apes and humans.
The First Humans: The Early Australopiths
By at least 4.4 million years ago in Africa, an apelike species had evolved that had two important traits, which distinguished it from other apes: (1) small canine (eye) teeth (next to the incisors, or front teeth) and (2) bipedalism--that is the ability to walk on two legs. Scientists commonly refer to these earliest human species as australopithecines, or australopiths for short. The earliest australopith species known today belongs to the genus Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as "southern ape," in reference to South Africa, where the first known australopith fossils were found.
Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, australopiths ranged widely over the African continent. The Great Rift Valley of eastern Africa, in particular has become famous for its australopith finds because past movements in Earth's crust in this region were favorable to environments in which bones are easily preserved and, later, to exposure of ancient deposits of fossilized bones.
There are many ideas about why the early australopiths split off from the apes, initiating the course of human evolution. Virtually all hypotheses invoke environmental change as an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis.
The savanna hypothesis argues that the Miocene forests of Africa became sparse and broken up between 5 and 8 million years ago due to a cooler and drier global climate. This drying trend led to the separation of an ape population in eastern Africa from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to drier, open savanna environments, which favored the evolution of terrestrial living. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans.
Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, an early australopith jaw similar to A. afarensis has been found in Chad in west-central Africa, 2500 kilometers west of the African rift valley. This find suggests that australopiths ranged widely over the African continent and that East Africa may not have been fully separated from environments further west. Second, there is growing evidence that open savannas were not prominent in Africa until sometime after 2 million years ago.
Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early australopiths evolved in a mosaic of woodland and grassland that offered opportunities for feeding both on the ground and in the trees. Ground feeding then favored regular bipedal activity and, eventually, the evolution of anatomical features of the hip, leg, and foot that assisted this form of locomotion.
The variability hypothesis suggests that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Evidence from early australopith sites, in fact, shows this range of habitats. So the unique appearance of their skeletons may have allowed them the versatility of living in habitats with many or few trees.
Fossils from several different early australopith species that lived between 4 million and 2 million years ago show a variety of adaptations that mark the transition from ape to human. The very early period of this transition, prior to 4 million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features.
Fossils reveal much about the physical build and activities of early australopiths, but little is known about surface physical features, such as the color and texture of skin and hair, or about certain behaviors, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes -- particularly the African apes -- to better understand how early australopiths might have looked and behaved. The study of living apes, therefore, sheds light on how the transition from ape to human might have occurred.
For example, australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains and body sizes in the same range exhibited by the great apes, leading scientists to believe that the australopiths had similar mental capabilities and possibly even social structures.
Most of the distinctly human physical qualities in australopiths related to their bipedal stance. Before australopiths, no mammal had ever evolved an anatomy for habitual upright walking. African apes move around their environments in a variety of ways. They use their arms to climb and to swing through the trees (known as brachiation). They knuckle-walk when on the ground, leaning on the middle parts of their fingers. And sometimes they move on two legs, as when chimpanzees feed on low branches or when gorillas show threat displays. The australopith body was devoted especially to bipedal walking. Australopiths also had small canine teeth, as compared with long canines found in almost all other catarrhine primates.
Other characteristics of australopiths reflected their ape ancestry. Although their canine teeth were not large, their faces stuck out far in front of the braincase. Their brains were about the same size as apes' today, about 390 to 550 cubic cm (24 to 34 cubic in) but were enlarged relative to body size. Their body weight, which can be estimated from their bones, ranged from about 27 to 49 kg (60 to 108 lb.) and they stood about 1.1 to 1.5 m (3.5 to 5 ft) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some australopith species had a large degree of sexual dimorphism -- males were much larger than females -- a trait also found in gorillas, orangutans, and some other primates.
Australopiths also had curved powerful fingers and long thumbs with a wide range of movement. Apes, in comparison, have longer, very strong, even more curved fingers – which are advantageous for hanging and swinging from branches -- but their very short thumbs limit their ability to manipulate small objects. While the fingers were longer than in modern humans, the australopith finger bones were not so long and curved as to suggest arm swinging. It is not yet clear whether these changes in the hand of early australopiths enabled them to use tools in a better way than earlier apes or even modern chimpanzees today.
The anatomy of australopiths shows a number of adaptations for bipedalism. Adaptations in the lower body included the following: The australopith ilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This new shape enabled the hip muscles to steady the body during each bipedal step. The australopith pelvis overall had evolved a more bowl-shaped appearance, which helped support the internal organs during upright stance. The upper legs angled inward from the hip joints, which positioned the knees to better support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. The australopith foot was also reshaped, including shorter and less flexible toes than an ape's, which provided a more rigid lever for pushing off the ground during each step.
Other adaptations occurred above the pelvis. The australopiths’ spine had an S-shaped curve, which shortened the overall length of the torso and gave rigidity and balance when standing. By contrast, apes have a relatively straight spine. The australopith skull also had an important adaptation related to bipedalism. The opening at the bottom of the skull, known as the foramen magnum, where the spinal cord attaches to the brain, was more forward than it is in apes. This position set the head in balance over the upright spine.
Australopiths clearly walked upright on the ground, but paleoanthropologists debate about whether the earliest humans also spent a lot of time in the trees. Certain physical features indicate that they spent at least some of their time in the trees. Such features include their curved and elongated fingers and elongated arms.
Many different explanations have been offered to account for the evolution of upright walking. Some of the ideas include: (1) freeing the hands, which was advantageous for carrying food or tools; (2) improved vision, especially to see over tall grass; (3) reducing the body's exposure to hot sun, which allowed better cooling during the day in an open landscape; (4) hunting or weapon use, which was easier with upright posture; and (5) feeding from bushes and low branches, which was easier when standing and moving upright between closely spaced bushes.
Although none of these hypotheses has overwhelming support, recent study of chimpanzees favors the last one. Chimps move on two legs most often when feeding on the ground from bushes and low branches. Chimps today are not, however, very good at walking in this way over long distances. As the distances between trees or groves of trees became wider during drier periods bipedal behavior in pre-human populations may have become more frequent. Accordingly, a more effective bipedal gait was favored not as an adaptation to savanna living but rather as a way of crossing less favored areas of open terrain. An ability to climb trees continued to be important. This idea may currently be the best explanation for the unique adaptation of the early australopiths: a combination of long, powerful arms, slightly elongated legs, and lower limbs reshaped for upright walking over long distances on the ground.
Compared with apes, humans have very
small canine teeth. Apes, particularly males, have thick, projecting, sharp
canines that they use for displays of aggression and as weapons to defend themselves.
By 4 million years ago, australopiths had developed the human characteristic
of having smaller, flatter canines. Canine reduction might have related to an
increase in social cooperation among humans and an accompanying decrease in
the need for males to make aggressive displays.