This large group of species comprises both the gracile and the robust australopithecines.
Recently some scientists have suggested that some species presently assigned to the Homo clade would be better placed in Australopithecus – an example of how rapidly our understanding of our evolutionary past is changing, and of the reviews, discussion and disagreements that characterise scientific research.
The earliest known australopithecine is Australopithecus anamensis, which lived between 4.2 and 3.9 million years ago. This has teeth and jaws that strongly resemble those of older fossil apes. However, they were very likely bipedal (based on the structure of the tibia) and had human-like upper limbs. Maeve Leakey’s research team suggests that anamensismay be ancestral to all later hominids.
The best-known member of this species is “Lucy” , discovered in 1974 by Don Johanson & Tom Gray and estimated to be around 3.2 million years old (afarensis lived from 3.9 to 3 million years ago). This is an important find as the skeleton is remarkably complete for its age (40% complete by some estimates, but this does not include the bones of the hands and feet), providing a wealth of data about her size, posture, and gait. A range of other finds, including the 13 individuals of the “First Family”, give supporting information, and the famous Laetoli footprints have also been attributed to this species.
Afarensis remains indicate that the species was strongly sexually dimorphic, with males much larger than females. The remains indicate that afarensis heights ranged from 107 to 152 cm, and cranial capacity from 375 cc to 550cc (AL 444-2, a large adult male). This may give us some clues about social behaviour in anamensis, since modern apes with a high degree of sexual dimorphism are polygynous.
The face and cranium of afarensis was ape-like: a prominent brow ridge, low forehead, and a prognathous muzzle that lacked a chin. The teeth are intermediate between ape and human: the molars are large and the canines, though much smaller than those of living apes, are larger and more pointed than those of humans. The shape of the dental arcade lies between the human parabolic form and the apes’ rectangular shape, and the foramen magnum, while further forward than in apes, is not directly under the cranium as in humans.
However, their postcranial skeleton is far closer to that of modern humans. The pelvic, leg, and foot bones clearly show that this species was bipedal, though not well adapted for running. While the finger & toe bones are curved and longer than in humans, a feature that most scientists consider to be evidence that afarensis still spent time in the trees, their hands are otherwise human-like. (A recent study suggests that afarensis’ wrist bones still show some adaptations for knuckle-walking.)
Our knowledge of Australopithecus afarensis has been extended with the description of a juvenile afarensis (Alemsegedet al. 2006). The complete skull and partial skeleton (around 50% complete) are probably those of a female who was around 3 years old when she died.
The find is particularly exciting in that it yields information lacking in other afarensis remains: an endocranial cast of the cranium; shoulder blades and collarbones – permitting an estimate of the extent of arm movement in this species; and a hyoid bone, which shows that the voice box had an ape-like structure.
Scientists have always wondered how much time afarensis spent in the trees. The form of the shoulder blade, the fact that the arms could be swung above the head, and the presence of long, curved fingers all suggest that the species was at least partly aboreal.
Until recently our knowledge of early hominids suggested that there was a single early-middle Pliocene lineage, of whichA. afarensis is the best example. However, in 2001 Maeve Leakey’s research team reported the find of a well-preservedcranium that they placed in a new genus: Kenyanthropus platyops.
This fossil is described as a mosaic, with a combination of ape-like and hominid features. On the ape side, it has a small ear opening, thick enamel on its molar teeth, a cranial capacity of about 400cc (the same as a chimp’s), and a flat nose. However, its face has a number of novel features not seen in the other gracile australopithecines: a flat face, vertical cheek region, and small brow ridge and cheek teeth.
Some researchers dispute the classification of this specimen into a new genus, noting that the cranium is severely distorted and that many of the features used to define this genus are simply a result of the distortion.
This species, Australopithecus garhi, which its discoverers felt could be intermediate between A. afarensis & earlyHomo, was described in 1999 from fossils found in the Awash region of Ethiopia. The 2.5 million-year-old fossils were found in association with animal bones – which had marks on them that appeared to be from stone tools. Very simple stone tools were found in a nearby site of the same age.
The fossils of Australopithecus garhi comprise a partial cranium and a fragment of another, partial skeletons and other postcranial bones, and two mandibles. (The remains may or may not be all from the same species.) Many features of the skull, including the small cranial capacity of 450cm3, are very similar to A. afarensis. There was a sagittal crest and an ape-like dental arcade. The molars and canines were extremely large – an unexpected feature if garhi is to be regarded as ancestral to Homo. The proportions of the limb bones are of interest: the femur-to-humerus ratio was like that of modern humans, but the ratio of forearm-to-humerus was like that of a chimpanzee.
Raymond Dart’s “Taung child”, consisting of teeth, jaws, a complete set of facial bones, and an endocranial cast, was the first australopithecine fossil to be found. The fact that all its milk teeth are present, and the state of its cranial sutures, suggests that this individual was about 3 years old when it died. Although his description of the Taung infant as a hominid was originally questioned, the subsequent discovery of many adult africanus fossils confirmed his classification. As a species, africanus lived between 3.3 and 2 million years ago. Like afarensis, it showed strong sexual dimorphism, and bones of the feet, legs, pelvis and spine show that it was bipedal. However, both body size and cranial capacity (420 – 500cc) were slightly larger in africanus, which also had larger molar teeth, smaller canines, and a fully parabolic dental arcade.
Australopithecus anamensis, afarensis, and africanus, and Kenyanthropus platyops are collectively known as gracile australopithecines, because of their relatively light, slender build. This is by comparison to the “robust” australopithecines: all the gracile species were still more robust than modern H. sapiens.
Together with A. aethiopicus and boisei, robustus is one of the “robust” australopithecines. (Some authors have placed these three species in the genus Paranthropus.) All of them had large jaws, heavily built skulls, sagittal crests, and thick enamel on their molar teeth.
Robustus lived between 2 and 1.5 million years ago. While its body size was similar to that of A. africanus, it had a larger, more robust cranium (average capacity 530cc) and very large molars in its large lower jaw. Its face was also large, with no forehead, and many specimens had sagittal crests in addition to their big brow ridges. The combination of these ridges and crests with the large jaws and molar teeth suggest that this species must have eaten coarse, tough food requiring much chewing.
Interestingly, while tool use was regarded for many years as a characteristically human feature, robustus may have been one of the first hominids to use tools. This interpretation was placed on bones found with robustus fossils as the worn ends of these bones suggest they may have been used for digging.
This is the oldest of the three robust australopith species, living between 2.6 and 2.3 million years ago. There is one major fossil, the “Black Skull” , so named because it had been stained by minerals in the soil. Some researchers consider it an ancestor of both robustus and boisei (see Figure 1). At 410cc its cranial capacity is little more than that of a chimpanzee, and posterior parts of the skull are similar to those of A. afarensis. But it also has the very heavily built face and jaws of the other robust species, and the largest sagittal crest ever seen in a hominid.
A. boisei, originally named Zinjanthropus boisei, is the most robust of all the robust australopithecines. Louis Leakey nicknamed it “Nutcracker Man” because of its huge molar teeth – some up to 2cm across. Its cranial capacity is similar to that of robustus, around 530cc, while its face and jaws are even more massively built. The “hyper-robust” nature of this species suggests it was highly specialised to chew hard, low quality foods. It’s been suggested that boisei became extinct because it was so highly adapted to a specific ecological niche, and could not evolve fast enough to adapt when the environment changed.