The intellectual origins of environmental history as a self-conscious domain of enquiry can be traced to the encounter of 17th and 18th century western Europeans with the startlingly unfamiliar environments of the tropics and the damage inflicted on these environments in the course of resource extraction by European empires. For nearly a century from the mid-19th to the mid-20th century, the discipline developed primarily in the form of "historical geography". A new phase of global environment history began with European decolonisation from the 1950s onwards. The imminence of a nuclear catastrophe and pesticide pollution stimulated the rise of a worldwide and populist environmental movement that reached full fruition in the 1970s. This article in two parts traces the beginnings and the shifting discourse of environmental history. Part I looks at the origins till the period of the 1930s. It was a time when the colonial encounter was becoming increasingly frayed, but environmental history benefited from the innovative convergence of writings on the part of geographers, archaeologists and ecologists, several of whom took an increasingly anxious and prescriptive view of human-environment interactions. Part II will appear next week.