"A. afarensis", then, is an obligate bipedal hominid. Its brain size was very similar to that of modern chimpanzees and gorillas. So, in one important respect it was like the genus "Homo" since it was bipedal. That means it was adapted to living in open woodland and savannah, not tropical rain forest. On the other hand, it had a brain about the size of an ape. This situation, where different traits of an animal evolve at different rates, is known as mosaic evolution.