Did humans shape their environment or did the environment shape humans?
In general, we can say that both of these process happened to some degree. However, it is likely safe to say that people have shaped their environment more than the environment has shaped people.
There are some ways in which the environment has shaped people. Paleontologists suggest that it might have been environmental changes that caused the first human ancestors to start walking upright. There are some people, such as natives of the Arctic, whose body shapes have been affected by their environments. Environments have helped to shape cultures. The Greeks developed the system of city states in part because of the mountainous terrain of their country. All of these are examples of the environment shaping people.
But people have shaped the environment still more. The most recent, and perhaps greatest, example of this is how we are causing global warming today. There have been other examples as well. The coming of human beings to the Americas led to mass extinctions as people hunted animals that were not adapted to their presence. The same happened in Australia. People have cut down forests, dammed rivers, and reclaimed land from the sea. In all of these ways, and many more, people have had more of an effect on the environment than the environment has had on them.