A brief article in The Economist relays some evidence a palaeoclimatologist has recently put forward that anthropogenic global warming began 5000-7000 years ago, as a result of slash-and-burn agriculture spreading throughout Europe and Asia. Or to be less exciting, ice core samples show that CO2 and methane levels started rising 5000-7000 years ago, and since it's known that agriculture was spreading widely at that time, Dr. William Ruddiman of UV Charlottesville (among others) argues that early agriculture may be to blame. Although the world population 5000 years ago was obviously much smaller, the efficiency of agriculture was so much lower (Ruddiman estimates per capita land use was 10x higher than in recent recorded history.
The idea of early societies causing heavy environmental tolls is not new. There's fairly wide support for the idea that deforestation and over-mining contributed to the collapse of the Bronze Age cultures in the Mediterranian. However, the idea that "global warming" started with the late neolithic is kind of charming. Please consider adopting a more hunter-gatherer lifestyle!
More practically, this strikes me as underlining that there is not some single, sacred, stability point which industrial civilization has destroyed. We humans and our planet have always had an effect on each other, and it's virtually impossible for us to avoid that. The course of wisdom lies in trying to avoid making more impact than necessary (while not setting unrealistic goals or stiffling development) and being prepared to deal with unwanted effects that may occur.