Luckily, we know quite a bit about Australopithecus afarensis. Since the 1970s, archaeologists have discovered many specimens of the species in Kenya, Tanzania and Ethiopia. We have much of an adult skeleton and most of an infant skeleton in addition to fragments of limbs, mandibles and crania.
One specimen, nicknamed "Lucy," was about 3.5 feet tall (about 1 meter), and another specimen, named "Kadanuumuu," meaning "big man," was 5 feet to 5.5 feet tall (1.5 to 1.7 meters). As we can see, this species had significant sexual dimorphism, meaning males and females of the species were different sizes, unlike Ardipithecus ramidus for instance. Paleoanthropologists speculate that males and females of the ramidus species were more likely to have shared tasks such as food gathering and childcare, whereas afarensis males were more likely to have competed for dominance. Some argue, however, that the smaller and larger specimens of afarensis don't represent males and females of the same species but rather completely different species [source: Roberts].
From dating layers of volcanic ash, experts estimate afarensis lived about 3.7 million to 3 million years ago. Based on afarensis's teeth, experts guess he ate mostly plants including fruit, leaves and seeds — but also possibly lizards [source: Smithsonian].
The species' skulls reveal a small braincase but large face and jaws. With long arms and curved fingers, the species probably climbed trees. But the thorax, leg shapes and knee joint suggest the species walked upright. All this indicates that afarensis could be a direct ancestor of the genus Homo, and therefore all humans.