Organisms first evolved in water, which gave them support against the pull of the earth gravity. The air, which is much less dense than water, could not provide a similar support against gravitational pull for land organisms. Thus, to be a little more than a non motile, jelly like mass of cells, a land organisms had to have a support system. Such a system was even more essential for a complex land animal which had to actively move from place to place in search of food and escape from danger. This is because support and body movement are closely linked. Body movement in a complex animal is brought about by muscles which work by pulling on some king of support. Even in water, an active multicellular animal needs a support system to succeed its hunt for food. As a result, most complex animals have evolved supporting structures or skeletons which provide support against gravity, and yet are flexible enough to allow movement.
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