Australopithecus afarensis is an extinct hominin species and an early human relative. Its skull remains offer insight into early human evolution. Studying these ancient cranial structures provides information about this distant ancestor’s physical characteristics and adaptations, illuminating the evolutionary path to modern humans.
Discovery of Key Australopithecus afarensis Skulls
Understanding Australopithecus afarensis advanced through fossil discoveries in East Africa, primarily in Ethiopia. The most widely known find is “Lucy” (AL 288-1), a partial skeleton unearthed in 1974 by Donald Johanson in Hadar, Ethiopia, dating back approximately 3.2 million years ago. While Lucy’s skeleton is largely post-cranial, associated cranial fragments from Hadar contributed to defining the species.
The “First Family” site (AL 333) in Hadar, Ethiopia, yielded over 200 hominin fossils in 1975, representing at least 13 individuals, including four children. This collection provided insights into population variation and social structure within A. afarensis. A significant find was “Selam,” the Dikika child (DIK-1/1), a complete juvenile skeleton discovered in 2000 in Dikika, Ethiopia. This 3.32-million-year-old specimen, believed to be a young female, included an almost complete skull, offering significant detail on the species’ development.
Distinctive Features of the A. afarensis Skull
The Australopithecus afarensis skull exhibits a blend of features. The braincase, or cranium, was relatively small, with an estimated volume ranging from 385 to 550 cubic centimeters, comparable to that of a chimpanzee. It featured a low and sloping forehead, lacking the prominent forehead seen in later human species. A pronounced brow ridge was also present above the eye sockets.
The facial structure of A. afarensis was prognathic, meaning the face projected outwards, particularly around the mouth and jaw. The skull also shows evidence of strong muscle attachments for chewing, indicating a powerful masticatory system. Prominent cheekbones further contributed to the robust facial appearance.
The dentition of A. afarensis included large molars with thick enamel, suggesting an adaptation for processing tough or abrasive foods. While the canines were smaller than those of apes, they were still larger and more pointed than human canines, showing an intermediate morphology. The overall shape of the dental arcade was somewhat U-shaped, a characteristic shared with apes, rather than the parabolic arch typical of modern humans. The presence of a nuchal crest, a ridge of bone at the back of the skull, provided attachment for strong neck muscles, supporting the head.
What the Skull Reveals About A. afarensis
The features of the Australopithecus afarensis skull offer insights into the species’ biology and lifestyle. The small brain size suggests cognitive abilities more akin to apes than to modern humans. Studies of endocasts, which are internal molds of the braincase, indicate that the organization of the A. afarensis brain was ape-like, lacking certain features uniquely evolved in humans. However, the “Selam” skull revealed that brain development in A. afarensis occurred over a prolonged period, similar to the extended childhood brain growth seen in humans, albeit with an ape-like brain structure.
The robust dental features, including large molars and thick enamel, suggest a diet primarily composed of tough, fibrous plant material, such as grasses, fruits, and leaves. While plant consumption was dominant, fossil animal bones with cut marks found in Dikika, attributed to A. afarensis, indicate that meat may have also been a component of their diet.
The position of the foramen magnum, the opening at the base of the skull where the spinal cord connects, provides indirect evidence for locomotion. In A. afarensis skulls, the foramen magnum is positioned more anteriorly (forward) than in quadrupedal apes, supporting the conclusion that the species was bipedal. This reinforces evidence from post-cranial fossils, such as Lucy’s skeleton, which demonstrates habitual upright walking. The A. afarensis skull also shows sexual dimorphism, with male skulls being considerably larger and more robust than female skulls.
Placing the A. afarensis Skull in Human Evolution
The Australopithecus afarensis skull occupies a significant position in the evolutionary narrative, showcasing a mosaic of ape-like and human-like characteristics. This combination of features makes it a transitional fossil, bridging the gap between earlier ape-like ancestors and later, more human-like hominins.
When compared to older hominins like Ardipithecus ramidus, A. afarensis displays a more pronounced shift towards bipedal adaptations in its cranial base, even though the foramen magnum’s forward inclination is not as pronounced as in humans. However, A. afarensis retains more primitive features in its skull and dentition relative to its successors.
The differences between A. afarensis and later hominins, particularly those in the genus Homo, are marked. Homo species exhibit an increase in brain size, a reduction in facial prognathism, and further changes in dental morphology, such as smaller canines and a more parabolic dental arcade. A. afarensis is considered a direct ancestor to later Australopithecus species and potentially to the genus Homo, illustrating the evolutionary trajectory that led to larger brains and more human-like facial structures.