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The Earth formed about 4.6 billion years ago, along with the other seven planets orbiting Sol, our Sun. Earth is the largest of the terrestrial planets and the fifth-largest planet overall (see Table of Solar Planets for physical and orbital data).

Unique among the objects in the Solar System, our planet has an oxygen-rich atmosphere and a solid surface sustaining large bodies of liquid water. About 71% of the Earth is covered by water, to an average depth of 3.8 kilometers (2.4 miles).

Our planet arrived at its present physical, chemical, and geological configuration by means of an evolutionary process that began with its initial formation out of colliding planetesimals. However, only the last billion years of this process are understood in much detail. The shapes and properties of the oceans and continents have undergone continuous change since permanent bodies of water were first established on Earth. Yet the earliest period for which we can reconstruct the overall appearance of the globe is only about 750 million years ago.

Single-celled organisms may have appeared in watery environments about 4 billion years ago. Some of them eventually developed photosynthesis. Such microscopic species dominated the primitive biosphere for the next 2 billion years. Then, around 1 billion years ago, multicellular organisms emerged, first in the form of algae and later as seaweeds. At the same time, the oxygen content of the Earth's atmosphere reached about 1% and continued growing (Reid et al. 2007).

Algae and seaweeds remained the most advanced organisms on Earth until the remarkable period known as the Cambrian explosion, approximately 540 million years ago. In The Panda’s Thumb, the biologist Stephen Jay Gould has characterized this epoch as the simultaneous appearance of “almost all complex organic designs” known to evolutionary biology.

Still more recent than the Cambrian explosion is the age of dinosaurs, the most remote period of evolutionary history that is widely represented in popular culture. Dinosaurs began their evolutionary dominance about 250 million years ago, in the Mesozoic Era, and suffered widespread extinction only 65 million years ago. The ancestors of our own species emerged only a few million years ago.

When we speculate on the nature of extrasolar planets, we need to bear in mind that our own planet presented a radically different appearance, both geologically and biologically, just a few billion years ago. Thus the age and evolutionary history of a given exoplanetary system is a key factor in our interpretation of the conditions prevailing on its planets.

All text is copyright Raymond Harris 2006-2009. Image credits appear in the accompanying caption.