Australopithecus - Evolutionary Role

Evolutionary Role

The fossil record seems to indicate that Australopithecus is the common ancestor of the distinct group of hominids, now called Paranthropus (the "robust australopiths"), and most likely the genus Homo which includes modern humans. Although the intelligence of these early hominids was likely no more sophisticated than modern apes, the bipedal stature is the key evidence which distinguishes the group from previous primates who are quadrupeds. The morphology of Australopithecus upsets what scientists previously believed, namely, that large brains preceded bipedalism.

If A. afarensis was the definite hominid which left the footprints at Laetoli, it strengthens the notion that A. afarensis had a small brain but was a biped. Fossil evidence such as this has made it clear that bipedalism far predated large brains. However, it remains a matter of controversy how bipedalism first evolved millions of years ago (several concepts are still being studied). The advantages of bipedalism allowed hands to be free for grasping objects (e.g. carrying food and young), and allowed the eyes to look over tall grasses for possible food sources or predators. However, many anthropologists argue that these advantages were not large enough to cause the evolution of bipedalism.

A recent study of primate evolution and morphology noted that all apes, both modern and fossil, show skeletal adaptations to upright posture of the trunk, and that fossils such as Orrorin tugenensis indicate bipedalism around 6 million years ago, around the time of the split between humans and chimpanzees indicated by genetic studies. This suggested that upright, straight-legged walking originally evolved as an adaptation to tree-dwelling. Studies of modern orangutans in Sumatra showed that these apes use four legs when walking on large stable branches, swing underneath slightly smaller branches, but are bipedal and keep their legs very straight when walking on multiple small flexible branches under 4 cm diameter, while also using their arms for balance and additional support. This enables them to get nearer to the edge of the tree canopy to get fruit or cross to another tree.

It is suggested that the ancestors of gorillas and chimpanzees became more specialised in climbing vertical tree trunks, using a bent hip and bent knee posture which matches the knuckle-walking posture they use for ground travel. This was due to climate changes around 11 to 12 million years ago that affected forests in East and Central Africa so that there were periods when openings prevented travel through the tree canopy, and at these times ancestral hominids could have adapted the upright walking behaviour for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle-walking.

However, the view that human ancestors were knuckle-walkers is now questioned since the anatomy and biomechanics of knuckle-walking in chimpanzees and gorillas are different suggesting this ability evolved independently after the last common ancestor with the human lineage. Further comparative analysis with other primates suggests these wrist bone adaptations support a palm based tree walking.

Radical changes in morphology took place before gracile australopiths evolved; the pelvis structure and feet are very similar to modern humans. The teeth have small canines, but australopiths generally evolved a larger post-canine dentition with thicker enamel.

Most species of Australopithecus were not any more adept at tool use than modern non-human primates, yet modern African apes, chimpanzees, and most recently gorillas, have been known to use simple tools (i.e. cracking open nuts with stones and using long sticks to dig for termites in mounds), and chimpanzees have been observed using spears (not thrown) for hunting.

However, some have argued that A. garhi used stone tools due to a loose association of this species and butchered animal remains.