Plate tectonic theory explained some most important aspects of the Earth's geophysical features. It also allowed the evolution of life to be better understood in the context of the state of the Earth in geological history.
The continental land and oceans, after all, provide the habitats of almost all plants, animals and microbes, and so the dynamics of plate movement and its effects, such as volcanoes, earthquakes, mountain formation, ocean renewal and destruction, clearly will have had a tremendous influence on the evolution of life on Earth.
The evolution of life into today's diverse species also depended upon the appearance of oxygen in the atmosphere. Oxygen accounts for a fifth of the volume of the air that we breathe. Without it we cannot convert the energy of our food into the metabolic energy required for muscular activity, growth and other bodily functions.
In combination with hydrogen, oxygen produces water, H20, the medium in which life on Earth probably evolved and upon which it depends. Oxygen, alone or in combination with other elements, is the most abundant element in the Earth's crust and it is the fourth most abundant element in the Cosmos, behind hydrogen, helium and neon.
In the upper layers of the atmosphere, oxygen gets converted to ozone by the Sun's ultraviolet rays. This ozone forms a blanket that protects life on the Earth's surface because it absorbs dangerous ultraviolet light.
Knowledge of oxygen and what factors determine its rate of conversion to ozone is fundamental and essential for us to appreciate what can be done to protect the precious ozone layer from destruction by human beings.
When substances burn (or when metals tarnish), they combine with oxygen in the air. Yet, even though fire must have been one of humankind's earliest discoveries, and despite the fact that oxygen is the essential component of air that gives us life, it is a mere two hundred years since oxygen was discovered.
That discovery was an important step forward because it explained how things burn and was a turning point in the study of chemistry. It also helped to extend chemistry into the realms of biology, marking an important stage in the development of modern biochemistry, the study of the chemistry of life. Indeed, it has been said that the history of oxygen is the history of life.
Remarkably, the significance of the discovery of oxygen was incompletely appreciated by some of the scientists involved and it took the genius of a brilliant French chemist, Antoine Laurent Lavoisier (1743-1794), to explain properly the available experimental observations.
Lavoisier's work on oxygen and its extension into other areas of chemistry had such important implications that he is now known as the Father of Modern Chemistry.
The importance of oxygen for life on Earth is taken for granted by most people today. Oxygen is also widely used industrially, for example in the manufacture of steel, and medically, in oxygen tents, oxygen inhalators and incubators for newborn and premature babies. As a rocket fuel, liquid oxygen is helping humankind to explore worlds beyond Earth.
The discovery of oxygen represents an excellent example of how existing ideas in science must be relinquished for new ones when experimental findings no longer agree with old theories. It also demonstrates how important it was for chemists, towards the end of the eighteenth century, to develop ways of measuring the amounts of substances before and after chemical reactions.
Precise measurement of the weights of chemicals and how they change during chemical processes was one of Lavoisier's major contributions to science.
Perhaps more significant, however, is that the discovery of oxygen and its implications for modern chemistry, medicine and technology originated in the curiosity of scientists and in their quest for an understanding of the natural world.