August 6, 2003
The emergence of life on Earth
Earth is estimated to have formed about 4.5 billion years ago. Life first appeared less than a billion years later, in the form of primitive aquatic organisms much like modern-day bacteria. These organisms set off processes such as photosynthesis and vegetable decomposition that had a profound impact on their environment.
It took another 2 to 3 billion years for enough atmospheric carbon dioxide to become trapped in the rocks and used by plants to make oxygen, leading to the development of a new, nitrogen- and oxygen-rich atmosphere. As ozone formed, blocking out ultraviolet rays from the Sun, more complex organisms started to evolve.
From the time that life emerged on terra ferma about 500 million years ago, plant and animal evolution became a land-based affair. The first non-aquatic plants appeared about 430 million years ago, followed by the dinosaurs some 225 million years later. Man finally evolved less than a million years ago.
Earth’s ability to support life is thanks to its ideal position relative to the Sun—neither too near nor too far—and to the protective shield provided by its atmosphere. This creates a natural greenhouse effect, keeping the average surface temperature at 15°C when it would otherwise be around –18°C. This is in sharp contrast to both Venus and Mars, respectively too hot and too cold from either retaining or losing too much heat.
|When you are on Earth…|
…gravity keeps you on the ground;
…oxygen in the atmosphere enables you to breathe;
…the Sun keeps you warm in the daytime;
…you can see changes in atmospheric composition and pressure in the form of cloud movements, rainfall and lightning;
…you can observe interactions between solar particles and Earth’s magnetic field by watching an aurora borealis such as the Northern Lights;
…you can look out through Earth’s atmosphere to see the stars in the night sky.