Australopithecus (Greek for "southern ape"); pronounced AW-strah-low-pih-THECK-us
Plains of Africa
Late Pliocene-Early Pleistocene (4-2 million years ago)
Size and Weight:
Varies by species; mostly about four feet tall and 50-75 pounds
Bipedal posture; relatively large brain
Although there's always the possibility that a stunning new fossil discovery will upset the hominid apple cart, for now, paleontologists agree that the prehistoric primate Australopithecus was immediately ancestral to genus Homo-which today is represented by only a single species, Homo sapiens. (Paleontologists have yet to pin down the exact time when the genus Homo first evolved from Australopithecus; the best guess is that Homo habilis derived from a population of Australopithecus in Africa about two million years ago.)
The two most important species of Australopithecus were A. afarensis, named after the Afar region of Ethiopia, and A. africanus, which was discovered in South Africa. Dating to about 3.5 million years ago, A. afarensis was about the size of a grade-schooler; its "human-like" traits included a bipedal posture and a brain slightly bigger than a chimpanzee's, but it still possessed a distinctly chimp-like face. (The most famous specimen of A. afarensis is the famous "Lucy.") A. africanus appeared on the scene a few hundred thousand years later; it was similar in most ways to its immediate ancestor, although slightly bigger and better adapted to a plains lifestyle. A third species of Australopithecus, A. robustus, was so much bigger than these other two species (with a bigger brain as well) that it's now usually assigned to its own genus, Paranthropus.
One of the most controversial aspects of the various species of Australopithecus is their presumed diets, which is related intimately to their use (or non-use) of primitive tools. For years, paleontologists assumed that Australopithecus subsisted mostly on nuts, fruits, and hard-to-digest tubers, as evidenced by the shape of their teeth (and the wear on tooth' enamel). But then researchers discovered evidence of animal butchering and consumption, dating to about 2.6 and 3.4 million years ago, in Ethiopia, demonstrating that some species of Australopithecus may have supplemented their plant diets with small servings of meat-and may (emphasis on the "may") have used stone tools to kill their prey.
However, it's important not to overstate the extent to which Australopithecus was similar to modern humans. The fact is that the brains of A. afarensis and A. africanus were only about a third the size of those of Homo sapiens, and there's no convincing evidence, aside from the circumstantial details cited above, that these hominids were capable of using tools (though some paleontologists have made this claim for A. africanus). In fact, Australopithecus seems to have occupied a place fairly far down on the Pliocene food chain, with numerous individuals succumbing to predation by the meat-eating megafauna mammals of their African habitat.