INTRODUCTION

For nearly all of its four and a half billion years of existence, the Earth's environment has followed a path and pace of change affected much less by human activities than by other, much larger, natural forces. Humans began having an impact a few thousand years ago through such activities as the slash and burn gardens in the tropics and the burning of grasslands by hunter/gatherer societies prior to forest clearing for agriculture in temperate regions. In the past few hundred years, however, burgeoning human populations and their activities in all parts of the globe have greatly increased the scale of impact.

The goal of this chapter is to examine the period prior to the advent of widespread human effects on the Earth's environment, especially factors important to plants and the roles they play in the biosphere.

For the first half billion years or so after it began forming, the Earth was shaped and reshaped by physical and chemical forces alone. Around 3.8 billion years before the present (BP), the outer crust, which formed as the Earth cooled sufficiently to form a solid, unbroken surface layer, cracked into a number of massive plates floating on a deeper mantle of molten rock. Ever since, these tectonic plates on which the continents are carried have continued to move from place to place at the Earth's surface and, at their edges, to sink beneath one another into the hot mantle below, a process called subduction.