EARTH IS THE ONLY PLANET in the solar system that is capable of supporting advanced life. Its unique combination of liquid water, a rich oxygen- and nitrogen-based atmosphere, and dynamic weather patterns provide the basic elements for a diverse distribution of plant and animal life. Over millions of years, landforms and oceans have been constantly changing, mountains have been raised up and eroded away, and continental plates have drifted across Earth. The atmosphere acts like a blanket, evening out temperature extremes and keeping warmth in. Without this" greenhouse effect" (p.47), Earth would be about 60°F (33°C) cooler on average. Over the last few decades, scientists have measured a gradual increase in Earth's temperature. Glaciers and polar ice caps have begun to shrink. It is feared that human activity is causing this rapid change by increasing the amount of carbon dioxide and other "greenhouse gases" in the atmosphere.

Earth's crust is made up of a number of plates that are constantly moving because of currents that rise and fall from the molten Sahara iron core at the center of the planet. Where Desert the plates collide, they can lift the rocky landscape upward to create mountain ranges that are then eroded into craggy shapes like the Andes in Patagonia. The tensions caused by these movements sometimes result in earthquakes and volcanic activity.

The first life on Earth was primitive plants that took carbon dioxide from the air and released oxygen during photosynthesis. Animals evolved when there was enough oxygen in the atmosphere to sustain them. Knowledge about evolving life forms comes in the form of fossils in the rocks (left). However, life forms survive only if environmental conditions on Earth are suitable for them. The dinosaurs, for example, though perfectly adapted to their age, became extinct abou t 65 million years ago.

THE SPHERICAL EARTH: As early as the 5th century BCE the Greek philosophers had proposed that Earth is spherical, and by the 3rd century BCE they had worked out a series of experiments to prove it. But it was not until the first satellites were launched in the late 1950s that humans saw what their planet looks like from space. The one feature that makes Earth unique is the great abundance of liquid water; more than two thirds of the surface is covered with water. Water makes Earth a dynamic place. Erosion, tides, weather patterns, and plentiful forms of life are all tied to the presence of water. There is more water in the Sahara Desert than there is on Venus.