As you can see, the skeletal system has evolved greatly over time. The picture on the left is ancient human skeleton, or an Australopithecus Afarensis, is from about 3.2 million years ago. The rib cage is wider than current humans, the skull is longer, and it has less of a brain capacity. The hands are almost the same size as its feet, as oppose to a human's smaller hands and larger feet. The fact is, skeletons and organisms are constantly changing and evolving. Back in the past, there were different things that a human had to do and have to survive. The large hands and long forearms helped the Afarensis climb trees or run away faster to escape predators or things that may
have threatened its survival. Now, humans are taller, smarter, and are capable of so much more. We no longer are in need of large hands and forearms, so over the course of many thousands of years. We developed to have a large brain capacity, since we could no longer escape. Early humans probably faced many challenges, but it was those with the largest brains, the most fit for their current environment, that were the ones that survived. These descendants of those early humans are today know as Homo Sapiens. That, people, is evoulution.
Here's A Video on How Australopithecus Afarensis Would Have Lived in the Wild!