The hiragana symbols used today evolved from the early practice of using Chinese characters to represent the sounds of the Japanese language. Unlike the angular shapes of katakana, the smooth, flowing forms of hiragana came from writing those Chinese characters in a cursive style.
Below are three examples of the Chinese characters that were originally used to write the sounds "su," "ke," and "chi" in Japanese. Place your mouse over the character to see how the modern hiragana symbol for that sound evolved.