Since the earliest civilizations, humans have manipulated the environments in which they lived. In the first century CE, the Romans used ducts to distribute heated air to a structure's interior. For many centuries, humans have leveled hills and filled valleys in further manipulation of the natural environment. Prior to 1700, available energy sources—largely animal power—limited humans' ability to engineer nature. As mechanical engineers and inventors developed inanimate power sources, they began to engineer environments on a new and vast scale.