Aside from discussions of the Anthropocene, it is generally considered that in Britain & Europe at least, forest clearance would
have been the first detectable effect, probably preceding agrriculture and carried out by Mesolithic hunter-gatherers using fire as a way of enhancing certain components of vegetation and/or facilitating hunting. The evidence of fire comes from charcoal deposits in peat layers - but of course not all fire is started by man, so it's all rather speculative.

Nobel Prize winner Paul Crutzen has coined the phrase 'the Anthropocene' for the period of earth's history from which it has been significantly affected by people. He dates it back to the Industrial Revolution (18th C).

However, it is debated. Specifically, it has been dated as far back as 6000 BC with the commencement of agriculture, from which point humankind physically started shaping the environment around him.

(see April 08 edition of Wired)