9.6: Early Hominins - Genus Australopithecus

Genus Australopithecus (“southern ape”) was first used in 1924 by Raymond Dart for the “Taung Child,” a juvenile Au. africanus specimen from the quarry site of Taung, in South Africa.  It had a slender build and was bipedal, but had robust arm bones and, like other early hominins, may have spent significant time in trees. Its brain was larger than that of A. afarensis  at 500 cubic centimeters, which is slightly less than one-third the size of modern human brains.

Figure \(\PageIndex{1}\): This adult female Australopithecus afarensis skeleton, nicknamed Lucy, was discovered in the mid 1970s. (credit: “120”/Wikimedia Commons)

Australopithecus is a genus of hominin that evolved in eastern Africa approximately 4 million years ago and went extinct about 2 million years ago. This genus is of particular interest to us as it is thought that our genus, genus Homo, evolved from a common ancestor shared with  Australopithecus  about 2 million years ago.

Australopithecus  had a number of characteristics that were more similar to the great apes than to modern humans. For example, sexual dimorphism was more exaggerated than in modern humans. Males were up to 50 percent larger than females, a ratio that is similar to that seen in modern gorillas and orangutans. In contrast, modern human males are approximately 15 to 20 percent larger than females. The brain size of Australopithecus  relative to its body mass was also smaller than modern humans and more similar to that seen in the great apes.

Figure \(\PageIndex{2}\): The skull of (a) Australopithecus afarensis, an early hominin that lived between two and three million years ago, resembled that of (b) modern humans but was smaller with a sloped forehead and prominent jaw.

A key feature that Australopithecus  had in common with modern humans was bipedalism, although it is likely that Australopithecus also spent time in trees. Hominin footprints, similar to those of modern humans, were found in Laetoli, Tanzania and dated to 3.6 million years ago. They showed that hominins at the time of Australopithecus  were walking upright.

Figure \(\PageIndex{3}\): Laetoli footprint cast. “Australopithecus afarensis footprint” by Tim Evanson is licensed under CC BY-SA 2.0.

There were a number of Australopithecus  species, which are often referred to as australopiths. A lot is known about the early species Australopithecus afarensis, which lived between 3.9 and 2.9 million years ago. This species demonstrates a trend in human evolution: the reduction of the dentition and jaw in size. A. afarensis (Figure) had smaller canines and molars compared to apes, but these were larger than those of modern humans. The famous Laetoli footprints are attributed to Au. afarensis (see Figure). They provided support for the then controversial idea of habitual bipedalism, as well as the species’ presence in a more open environment.

Its brain size was 380–450 cubic centimeters, approximately the size of a modern chimpanzee brain. It also had prognathic jaws, which is a relatively longer jaw than that of modern humans. In the mid-1970s,

the fossil of an adult female A. afarensis  was found in the Afar region of Ethiopia and dated to 3.24 million years ago (Figure). The fossil, which is informally called “Lucy,” is significant because it was the most complete australopith fossil found, with 40 percent of the skeleton recovered.

Figure \(\PageIndex{4}\): Laetoli and A. Afarensis recreation.” “Laetoli recreated” by Wapondaponda is licensed under CC BY-SA 3.0.

With the discovery of Australopithecus afarensis, “Lucy,” (3.2 mya) (see Figure) in 1974 by Donald Johanson’s crew at the site of  Hadar in the Afar Depression of Ethiopia, paleoanthropology gained momentum and the rush was on in East Africa to find more evidence of human origins. Certainly Louis  and Mary Leakey recognized the importance of the Great Rift Valley, but Johanson “upped the ante” with his 3.2 mya find. In addition, since Lucy’s skeleton was almost 40% complete (making it one of the six most complete fossilized hominin skeletons older than 100 kya), much could be said about her anatomy and locomotor capabilities.

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