<DIV></DIV>It's a question that has engaged climate scientists for more than a decade: Not <I>if</I> human activity impacts the environment, but <I>when</I> did that impact start to be significant? Was it in the mid-18th century, as long believed, with the Industrial Revolution producing huge amounts in greenhouse gas emissions? Or did it actually begin thousands of years earlier, as a result of the discovery and spread of agricultural practices?