Well, you have to understand the change they are talking about here is gradual change over long periods of time, sometimes millions of years. For example, I answered a question a few weeks ago about a distinct split between the two dominant branches of homo sapiens, one containing man, gorillas, chimpanzees, and orangutans, while the other contained what are known as the old world monkeys. The basic difference was the existence of a functional, prehensile tail, useful if you were still living the bulk of your life in tree tops. At some point, the branch that contained man came out of the tree tops and made a go of it on the ground, becoming ground-dwellers. So the need for a tail diminished, and over time, it became smaller and more nonexistent. There is an old saying, "if you don't use it, you lose it." That would apply nicely here, to the evolution of a line of homo sapiens without functional tails.
evolution is the change done by organisms over time to adapt better changes in thier body as to achieve better survivement. animals which cannot live in the current habitat develope themselves for example their organs , skin, stategy, colour, energy consumption food etc so, that these newly developed organs and features may help him to survive at his best.