As you read the following quote from Stephen Jay Gould, think about how he compares upright posture to the enlarged brain.
Upright posture is the surprise, the difficult event, the rapid and fundamental reconstruction of our anatomy. The subsequent enlargement of our brain is, in anatomical terms, a secondary epiphenomenon, an easy transformation embedded in a general pattern of human evolution. As a pure problem in architectural reconstruction, upright posture is far-reaching and fundamental, an enlarged brain superficial and secondary. But the effect of our large brain has far outstripped the relative ease of its construction.
(From "Our Greatest Evolutionary Step," in The Panda's Thumb. [London: Penguin, 1980])
A second major adaptive advantage that appeared later in human evolution was bigger brains. Fossil evidence allows us to trace the development of the brain as it increased threefold over the past 3 million years. Early hominids such as the australopithecines had brains the size of modern apes (400 to 500cc). Homo habilis, with a brain of about 650cc, was probably the first hominid discovered to make and use stone tools. As brain size increased, new capabilities evolved as well, giving these early humans abilities to adapt to and modify their environments. Tools became more sophisticated and eventually humans developed culturally as well as biologically.
Another earlier hominid, Homo erectus (with an approximate brain size of 900cc), was the first to develop humanlike culture. Homo erectus used tools, including hand axes, made fires, and were the first hominid species believed to have spread from Africa into Asia. Modern humans, Homo sapiens (with brains ranging from 1200 to 1600cc), have even more sophisticated capabilities, probably due to neurological developments within the brain rather than size alone. Brain size gives only limited information about the internal structure and capabilities of the brain. One later hominid species, Homo neanderthalensis, had a brain size of over 1300cc, but is considered to have been much less sophisticated than, and possibly even driven to extinction by, modern humans.