First extinction event?

A claim frequently made by climate change skeptics is that humans cannot create a sufficient imbalance in nature to cause climate change. They deny that the current mass extinction is related in any way to human activity. Is it not curious that an early, perhaps first, extinction event was precipitated by the anaerobic metabolism of microscopic life which released sufficient free oxygen into the atmosphere to allow aerobic life, like mammals, to evolve (see here and here)? It would appear that living creatures can unwittingly change their environment. The question for humans is can/will we do so wittingly.