The earliest fossil remains that may belong to the hominid lineage date to the time period between approximately 7 and 4 million years ago (mya). Most of these fossil remains have been discovered quite recently, within the last ten years or so. Each of the following three genera exhibit a mosaic of characteristics linking them to both the chimpanzee/bonobo and the hominid lineages. It is possible that these fossils belong in the hominid family, or they may be representatives of the ancestors of the chimpanzees/bonobos, or they may represent forms for which there are no modern descendants.
This species is known for only a single skull, found in Chad in Central Africa. It’s dated to about 7 mya. The hominid-like traits include nonhoning canine teeth and a foramen magnum positioned at the base of the skull, indicating that is was (most likely) bipedal. However, the cranial capacity is small and within the range of chimpanzee/bonobo, it has a huge brow ridge, and the enamel on its premolars and molars is thinner than found in other hominids.
Both cranial and post-cranial elements have been found at Lake Turkana, Kenya. The remains date to about 6 mya and some researchers have suggested that Orrorin was bipedal and had nonhoning canine teeth and thick enamel on the molars (both of these are hominid traits). However, the upper canine is large and apelike, throwing some doubt on Orrorin’s hominid status.
These fossils, recovered in Ethiopia, are estimated to be about 4.4 mya. Ardipithecus has a very ancient combination of apelike and humanlike traits. The teeth have a thin outer layer of enamel--a trait also seen in chimps and gorillas, but not in other australopith species or most older fossil apes. This trait suggests a fairly close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for bipedalism.
The First Humans: The Early Australopithecines
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 teeth (next to the incisors, or front teeth) and (2) striding 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 Australopithecus. The name australopithecine translates literally as "southern ape," in reference to South Africa, where the first known australopith fossils (A. africanus) 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.
From Ape to Human
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 bipedal striding (that is, 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 and bonobos 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.
Small Canine Teeth
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.
The australopiths can be divided into an early group of species (sometimes known as gracile australopiths), which arose prior to 3 mya, and a later group, known as robust australopiths, which evolved after 3 mya.
The earlier australopiths -- of which several species evolved between 4.4 - 3 mya – are more primitive, with small body sizes of around 4 feet in height (5 feet in males), generally had smaller teeth and jaws. The later robusts are quite derived (changed from the ancestral form), had larger faces with large jaws and cheek teeth. All the australopiths have thick molar enamel and are bipedal, although many exhibit traits suggesting that they may have spent some time in the trees, or at the very least, more capable of getting into trees than modern hominids. All australopith species (gracile as well as robust) lived only in Africa.
A very primitive species of australopith, dating between about 4.2 million and 3.9 million years. To date, remains of this species have been found only in Kenya. The skull of this species appears apelike (they have parallel tooth rows and honing canines), while its enlarged tibia or lower leg bone, indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking. And to judge from the seeds found with its bones, this chimp-sized hominid was still spending much of its time in woodland and had not yet completely acclimatized to life on the open savannah.
Australopithecus anamensis was quite similar to another, much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 4 million and 3 million years ago. The species was a sexually dimorphic one, he average brain size was about 400 milliliters (about 1 2/3rds cups), they did possess honing canines, but they did have an emphasis on chewing, as demonstrated by the small bony crests on the top of the skull (sagittal crest) that are attachment points for the chewing muscles.
Several hundred fossils of A. afarensis have been described, including a collection representing at least 13 individuals of both sexes and various ages, all from a single site that is dated 3.2 million years old.
The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Her remains indicate she was adept at bipedal locomotion, although her stride length was shorter than comparable humans of similar height. Also, her arms were quite long, with powerful muscle markings, and her fingers were long and curved, suggesting she may have spent some time in the trees.
Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli, a 3.6 million year old site best known for spectacular trails of bipedal human footprints (and the prints of other animals) preserved in a hardened volcanic ash. These footprints provide irrefutable evidence that australopiths regularly walked bipedally.
Some paleoanthropologists claim that A. afarensis was the common ancestor of both later australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between Australopithecus afarensis and another australopith species -- one from southern Africa, named Australopithecus africanus -- makes it difficult to decide which of the two species gave rise to the genus Homo.
Australopithecus africanus thrived in South Africa between about 3.5 million and 2.5 million years ago. The anatomist Raymond Dart described this species -- the first known australopith -- on the basis of a fossil discovered in 1924 at Taung, South Africa. A. africanus generally had a more globular braincase, lacked a sagittal crest, and had less primitive-looking face and teeth than did A. afarensis. Thus. some scientists consider the southern species of early australopith to be a likely ancestor of the genus Homo.
The Later Australopiths
By 2.7 million years ago, the robust australopiths had evolved, but then became extinct by one million years ago. The robust australopiths represent an intriguing group of early humans because they survived for a long time and were quite common compared to other early human species. They had adaptations that differed from the larger-brained populations of Homo who lived at the same time. They had roughly the same body size as A. afarensis and A. africanus, but their facial, cranial, and dental characteristics were quite robust.
The robust australopiths had megadont cheek teeth -- broad, thick-enameled molars and premolars -- which formed a flattened and worn surface. Their incisor teeth, by contrast, were small. An expanded, flattened, and more vertical face accompanied this emphasis on the back teeth. The combination of broad molars and large face was effective in absorbing the stresses of strong chewing. Along the top of the head was a sagittal crest, a raised area of bone along the skull's midline from front to back, where thick muscles that moved the jaw up and down were attached. The bars of bone along each side of the skull (the zygomatic arches or cheek bones) were positioned far to the side, which allowed huge openings for the chewing muscles near where they attached to the lower jaw. Altogether, these traits indicate very powerful and prolonged chewing of food. Microscopic wear on the teeth of A. robustus and A. boisei appear to support the idea of a vegetarian diet. It is thought that the robust australopiths had a diet consisting of tough, fibrous plant food, such as seed pods and underground tubers. However, chemical studies of fossil bones suggest that the southern species may also have eaten animals. (NOTE: A just completed reassessment of the robust species molars suggests that they may not have had a regular diet of hard foods despite their large teeth (see http://www.nsf.gov/news/news_summ.jsp?cntn_id=111457&org=NSF&from=news). Instead, they may actively have avoided eating the very foods they had developed adaptations for when they could find other food sources. Gorillas are know to do this.
Because they share the features of heavy chewing, the robust australopiths appear to represent a distinct evolutionary group of early humans. Many paleoanthropologists have linked the robust species together with a unique genus name, Paranthropus. This classification implies that the first robust species, P. aethiopicus, became separated from the other australopiths and then evolved into P. boisei and P. robustus (the other two robust species). Other researchers have kept the robust species within the genus Australopithecus, stating that the eastern forms (A. aethiopicus and A. boisei) evolved their massive teeth from the early australopiths of the region (perhaps A. afarensis), whereas the southern species (robustus) evolved independently from A. africanus. If this type of parallel evolution occurred, the robust species would form two separate side branches of the human family tree. Due to alternative views such as this, the robust species are often known by more than one name (such as Australopithecus boisei and Paranthropus boisei).
Australopithecus (Paranthropus) aethiopicus
The earliest known robust species, it had evolved in eastern Africa by 2.7 million years ago. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. P. aethiopicus shares some primitive features with A. afarensis -- that is, features that originated in the earlier East African australopith. This may indicate that P. aethiopicus evolved from A. afarensis.
Australopithecus (Paranthropus) boisei
Paranthropus boisei, the other well-known East African robust australopith, lived over a large geographic range (Tanzania, Kenya, and Ethiopia) between about 2.5 million and 1 million years ago. This species had the most specialized features of all the robust species. It has a massive, wide, and dished-in face that was capable of withstanding extreme chewing forces, and its molars are four times the size of those in modern humans.
Australopithecus (Paranthropus) robustus
The southern robust species lived between about 2 million and 1.3 million years ago in the same region that was home to A. africanus.
The Fate of the Later Australopiths
The youngest fossils of robust australopiths date to about 1.2 million years ago (although there is one specimen which dates to about 800,000 years ago), which suggests that they became extinct by around then. At about that time world climate began to fluctuate in a different pattern, and that may have reduced the food supply on which the robust species depended. Interaction with other early humans, such as Homo erectus, has been suggested as another reason for their extinction, although no compelling evidence exists of direct contact between these species. Competition with several other species of plant-eating monkeys and pigs, which thrived in Africa in the time, may have been an even more important factor. Still, the reasons why the robust australopiths became extinct, after such a successful time, are unknown.