University of Tennessee ecologists Paul and Hazel Delcourt argue in their new book, Prehistoric Native Americans and Ecological Change, that we have underestimated the varied impacts American Indians have had on the natural environment the past 15,000 years.
Eastern North America, for example, was not a "virgin" forest when Europeans arrived 500 years ago. Native Americans altered and even managed the environment in many ways. The evidence comes from the testimony of early European pioneers as well as archaeological and paleoecological studies.
This was a main thesis of 1491. It's been kicking around for some time, but it seems to be starting to become more accepted.