As a means of illustrating the influence of human activity on the environment in the recent post-industrial period, Professor McElroy begins by placing human influence in the largest possible geological context. According to McElroy, this introduction will allow for a greater understanding of the more complex problems of global climate change, our present environmental policy dilemma, and the Kyoto Protocol, and will offer some vision as to how we might move forward.

Formation of the Earth
McElroy states that the earth is 4.5 billion years old, formed from a spinning nebula of gas and dust. The evolution of the earth took place relatively rapidly over a few hundred millions of years. With comets and meteors adding more volatile materials to its composition, the early earth was probably ocean-covered; rocks appeared later as the interior of the earth turned over. Radioactive decay of elements at the interior of the earth caused early stages of continental drift, or plate tectonics. And as hot, molten material from the interior of the earth rose to the surface, it segregated: lighter elements tended to stay on top, and heavy elements sank back.

Early Forms of Life
Looking at the oldest rocks, it's clear that life was an early phenomenon on the earth. Simple forms of life, such as early eukaryotes — unicellular organisms that dominated the early history of the earth — were able to accomplish an incredible variety of chemical transformations. McElroy notes that in the mid-ocean ridges — vents of hot water coming out of the earth's interior —there exists a remarkable life system, energized by bacteria, that oxidizes sulfide to sulfate, produces energy, and feeds a complex set of organisms. Life may have begun in these isolated environments, states McElroy, existing and developing as these vents opened and closed, evolving from simple forms and developing complexity later on.

Evolution of Life
Early life evolved relatively slowly. Not until about 1.5 billion years ago, as recorded in Burgess Shale (a remarkably comprehensive fossil bed in the Canadian Rockies), did the earth begin to see an incredible profusion of life. And yet, in the earliest stages of the evolution of the earth, a climate dilemma existed: the sun was generating significantly less energy than it is today, yet at the same time, the earth was extremely warm. How is this dilemma resolved? One popular view is that this was an early manifestation of the greenhouse effect.

For the first couple of billion years of evolution, the earth progressed through a series of climate changes in which the environment influenced the path of evolution and vice versa, The image of earth as a living organism is readily defensible, states McElroy. Life and the physical environment are intimately connected on a global scale.

Arrival of Humans
In this context, think of the arrival of humans, or homo sapiens sapiens, who appeared on the scene 150,000 years ago. Compared to the age of the earth, that time frame is just a blink in time. As best one can tell, states McElroy, all humans have a close genetic affinity – a common maternal ancestor most probably originating in east tropical Africa.

When did humans first arrive in the Americas? Perhaps as recently as 50,000 years ago. During the middle of last Ice Age, the sea level was 350 feet lower than it is today, allowing for the crossing from Asia into North America over the Bering Strait. There is a common myth, notes McElroy, that early American inhabitants had a respect for nature. However, the historical record shows otherwise, indicating that a large number of mammals went extinct after the arrival of humans in America. Hence, a very small amount of people did a very considerable amount of damage to the environment at that particular time.