Australopithecus

Australopithecus were upright-walking apes. They lived in Africa, not in Australia. Their mosaic of characteristics is an important piece of evidence for evolution.

Description
Australopithecus walked upright. This is shown by their centrally placed foramen magnum, the structure of their backbones and of their knees and pelvises. Many of them also had feet like those of modern humans, without prehensile toes. They had smaller canine teeth than other apes, and what they had of them lacked sharp points. They had no diastema for housing the canine teeth of the opposite jaw. Most importantly, their jaws were bow-shaped like humans, not rectangular like apes. However, their jaws and molars were huge, and their brains were small like those of apes in general. They were short, little more than a meter tall (which Homo habilis was too).

Evolutionary role for humans
The fact that late Australopithecus lived at the same time as early Homo, and retained their small ape brains, means that bigger brains were not an inevitable consequence of anything related to upright walking. This debunks many theories of human brain evolution, including:

¤Intelligence required for making better tools enabled by free hands.

¤Social consequences of being able to carry things (e.g. gifts) requiring greater social intelligence.

¤Social consequences of invisible female genitalia (i.e. hidden ovulation) requiring greater social intelligence.

¤The difficulty of walking on two legs (as clearly demonstrated by robot studies as it is) requiring greater brainpower.

Despite all of this, humans obviously inherited most of their physical form from Australopithecus, and Australopithecus remains an important piece of evidence for human kinship with apes and African roots.

While it is obvious that Australopithecus walked upright, it is unclear why. Since one or a few sentinels in a group of apes are enough to warn the others, apes living on a grassland could take turns walking upright to look over the grass for predators. This would only require each ape to walk upright for short periods of time, which is no problem for most Great Apes (possibly barring the extremely quadrupedal gorillas). So what caused Australopithecus to evolve permanent bipedalism remains unknown. But no matter how, it was from them we inherited it.

And after all, there are other primate examples of weird transitions of locomotion. There is no obvious reason why some primates, at the transition from monkeys to apes, shifted from walking on and jumping between the branches to climbing hanging from and swinging between them either.