The australopiths can be divided into an early group of species (sometimes known as gracile australopiths), which arose prior to 3 million years ago; and a later group, known as robust australopiths, which evolved after 3 million years ago. The earlier australopiths -- of which several species evolved between 4.4 million and 3 million years ago -- generally had smaller teeth and jaws. The later robusts had larger faces with large jaws and cheek teeth.
A 5-million-year-old jaw fragment with one molar tooth, found in Kenya, and another jaw with two molars, about 4.5 million years old, may be the oldest australopith fossils. But scientists have not yet agreed on the matter since these fossils are so fragmented and do not tell us about the canine teeth or bipedal walking. Several of the early australopiths are given the genus name Australopithecus. Yet some of the oldest finds of australopith bones, dated about 4.4 million years old, have been given a different name because of their very ancient combination of apelike and humanlike traits. These fossils, first discovered in Ethiopia in 1994, are called Ardipithecus ramidus.
An Ethiopian member of a research team led by paleoanthropologist Tim White discovered the earliest known australopith species in Ethiopia in 1994. These recognizably human fossils were estimated to be about 4.4 million years old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means "root" in the Afar language of Ethiopia, and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other australopiths because it had a very ancient combination of apelike and humanlike traits.
The teeth of Ardipithecus ramidus 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.
In 1965 a research team form Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be 4 million years old, but could not identify the species to which it belonged. It was not until 1994 that a research team, led by paleoanthropologist Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a very primitive species of australopith, which was given the name Australopithecus anamensis. Researchers have since found other A. anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, 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.
Australopithecus anamensis was quite similar to another, much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 3.9 million and 3 million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by paleoanthropologist Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Several hundred fossils of this species have been described from Hadar, 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.
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 were discovered in 1978 by a research team led by paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally.
The controversy about how the australopiths moved has mainly focused on Lucy's species A. afarensis. While Lucy certainly walked upright, she stood only 3.5 feet tall and had longer, more powerful arms than most later human species, which suggests that she was also adept at climbing trees. And while the Laetoli footprints were made by bipedal humans, some scientists have argued that the imprints of the heel, arch, and toes are not exactly like those made by modern human feet. In addition, other fossils from Hadar and Laetoli come from individuals much larger than Lucy, up to 5 feet tall. This has caused controversy over whether the entire set of fossils represents one or two species, although most scientists accept the single-species idea since large and small adults, probably male and female, occurred together at the same site at Hadar.
Another controversy arises from the 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 what is now the Transvaal region of 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. For two decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930s and 1940s, teams led by paleontologist Robert Broom unearthed many more australopith skulls and other bones from the Transvaal sites of Sterkfontein and Swartkrans.
A. africanus generally had a more globular braincase and 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. According to other scientists, however, A. africanus had facial features that mark it on the path to the robust australopiths found later in the same region. Some recent finds from the Transvaal site of Sterkfontein indeed have begun to blur the distinction between the early australopiths and the later robust species. In 1998 a research team led by South African paleoanthropologist Ronald Clarke unearthed an almost complete early australopith skeleton at Sterkfontein. Although it may prove to be a new species, this important find may resolve some of the questions about where A. africanus fits in the story of human evolution.
By 2.7 million years ago, the robust australopiths had evolved. 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, but then mysteriously became extinct by one million years ago. Although the word "robust" originally referred to the larger body once believed to exist in these australopiths, they are now known to have been roughly the same size as A. afarensis and A. africanus. Instead, "robust" accurately describes the very massive molar teeth, face, and skull muscle markings that characterized these species. 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) 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. A similar expansion in the chewing structures can be seen in other groups of plant-eating animals. Microscopic wear on the teeth of P. robustus and P. 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.
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 (the name originally given to the southern robust species). 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).
The earliest known robust species, Paranthropus aethiopicus, had evolved in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, paleoanthropologist Alan Walker discovered the fossil skull that defined this species. It became known as the "black skull" because of the color it had absorbed from minerals in the ground. The skull, dated 2.5 million years old, 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.
Paranthropus boisei, the other well-known East African robust australopith, lived over a large geographic range between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakey discovered the first fossil of this species -- a nearly complete skull at the site of Olduvai Gorge in Tanzania. Paleoanthropologist Louis Leakey, husband of Mary, named the new species Zinjanthropus boisei (Zinjanthropus translates as "East African man"). This skull, which is dated to 1.8 million years ago, has 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. Since the discovery of Zinjanthropus, now recognized as an australopith, scientists have found great numbers of P. boisei fossils in Tanzania, Kenya, and Ethiopia.
The southern robust species, which has the descriptive name Paranthropus robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning "beside man").
The Fate of the Later Australopiths
The youngest fossils of robust australopiths are about 1.2 million years old, 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.