Australopithecus afarensis
- 演化之聲
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Age
Neogene, Pliocene（Zanclean-Piacenzian）
3.7-3 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Australopithecus
Species: Australopithecus afarensis
Morphological description
Australopithecus afarensis had a lower body already well adapted for bipedal walking, whereas the upper body still retained many features suited to climbing, making this species one of the most representative transitional forms in the evolutionary history of the hominin lineage.
Its lower jaw remained prognathic, and its brain was still small, placing it closer to apes than to the later members of the genus Homo. At the same time, however, the dentition and jaws already showed the general pattern seen in early hominins.
In limb proportions, Australopithecus afarensis had relatively long arms and comparatively short femora. Evidence from Lucy (A.L. 288-1) indicates that its upper-to-lower limb proportions fell between those of modern humans and living non-human apes. Even so, the pelvis, femur, knee, tibia, and foot all show structural adaptations for bipedalism, allowing body weight to remain stable during single-leg support and enabling the lower limb to swing efficiently in the anteroposterior direction, that is, within the sagittal plane. The arrangement of the distal tibia and talus, the mechanics of the knee and ankle joints, and the forward-oriented weight-bearing structure of the foot all likewise indicate a clear capacity for habitual bipedal locomotion.
The pectoral girdle, clavicle, scapula, and upper limb nevertheless retained primitive traits that remained closer to the ape condition, indicating that this species had not completely lost functions related to climbing. Both juvenile and adult specimens support the view that some degree of arboreal activity probably persisted throughout life.
Etymology
Genus name Australopithecus: from the Latin australis, meaning "southern", and the Greek pithekos, meaning "ape"; thus, "southern ape".
Species name afarensis: named after the Afar region of Ethiopia; the suffix -ensis means "from", "belonging to", or "originating in".
Biological description
Australopithecus afarensis lived in eastern Africa about 3.7 to 3.0 million years ago and is currently one of the most central and best-documented species in the study of early hominins. Its fossils come mainly from Hadar and Dikika in Ethiopia and from Laetoli in Tanzania. These include the famous partial skeleton of Lucy, the abundant associated remains from Hadar, and the fossil footprint trail at Laetoli.
If one considers only the legs, knees, pelvis, and feet, there is little doubt that Australopithecus afarensis was a habitual biped, and the Laetoli footprints provide especially strong support for this interpretation. The upper body, however, suggests that these hominins probably still climbed trees frequently, and that this behavior may have persisted from youth into adulthood. This has led to the view that the lower limbs became highly specialized earlier, whereas the upper limbs and pectoral girdle were slower to lose their arboreal functions.
In terms of diet, early hominins were once often imagined as feeding mainly on the fruits and soft tissues of C3 plants, but carbon isotope studies have shown that Australopithecus afarensis does not fit such a narrow picture. It also consumed a substantial proportion of C4 and CAM plant resources. In other words, it did not feed only in wooded settings, but also exploited foods from grasslands, wetland margins, sedge-dominated habitats, and dry open environments. Its feeding behavior was therefore more flexible than once assumed. In addition, cut-marked animal bones have been found at Dikika, raising the possibility that Australopithecus afarensis may at least occasionally have used animal resources as well.
The environments in which it lived were part of a savanna mosaic, made up of many adjoining microhabitats, including relatively closed woodland or open woodland, bushland, wetland margins, grassland, and more open areas with low vegetation. Australopithecus afarensis was therefore neither a species confined to forests nor a fully specialized grassland dweller. It is better understood as a flexible hominin that moved among different resource patches and fed across a range of settings. This ecological flexibility was probably one of the reasons it persisted successfully for such a long span of time.
Whether Australopithecus afarensis was our direct ancestor remains unresolved. One reason is that evolution involved multiple closely related lineages that overlapped in time and were geographically interwoven rather than arranged in a simple linear sequence. Given the evidence currently available, the more cautious and more reasonable interpretation is that Australopithecus afarensis occupied a position very close to the origin of the genus Homo and was a successful, widespread, and long-lasting early hominin species.
In addition, recent work suggests that juveniles of this kind of early hominin may have depended on adult care more strongly than do living great apes. Although their brains were still small and their brain shape remained ape-like, some studies indicate that the brain, foot, and locomotor system of Australopithecus afarensis required a longer period of development than in modern apes. This implies that aspects of a more prolonged developmental pattern had already begun to emerge before any major increase in brain size took place.
The fossil sample of Australopithecus afarensis also shows marked differences in body size. In some individuals, skeletal elements such as the femur are noticeably larger and more robust. This pattern is often interpreted as evidence of sexual dimorphism.
（Author: Shui-Ye You）
References
Alemseged Z. (2023). Reappraising the palaeobiology of Australopithecus. Nature.
Farrell HN and Alemseged Z. (2025). Clavicular evidence for continued arboreality in Australopithecus afarensis. Journal of Human Evolution.
Kimbel WH and Delezene LK. (2009). "Lucy" Redux: A Review of Research on Australopithecus afarensis. Am J Phys Anthropol.
Wynn JG et al. (2013). Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia. PNAS.
Wynn JG et al. (2016). Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia. Journal of Human Evolution.
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