Originally there was nothing.
From nothingness came the distinction of yin and yang, dark and light, rest and motion. The period in which yin and yang are separate is known as "before heaven." When yin and yang are fused, i.e., the period of "after heaven", all the myriad things are created. In the period of before heaven, nothing is produced; nothing is destroyed. However, in the period of after heaven, all things are subjected to the dualistic law of yin and yang. Hence, Tai Chi is basically the perpetual interaction of yin and yang.
In the Sung Dynasty (960-1279 A.D.), a Taoist sage of Wu Tang Mountain, Chang San Feng, observed the special traits of twelve different animals. He matched their movements with the principles of yin-yang, the five elements (earth, fire, water, wood, and metal), and the Ba Gua (four corners and four squares). His effort resulted in the creation of the 64 movements of tai chi chuan.
In the Ching Dynasty (1644-1911 A.D.), Yang Lu Chan of Guang Ping city in Hobei province, became the first generation of the Yang Style Tai Chi. His son, Yang Ban-Hou, inherited the art. In just a few short years, Ban-Hou's martial arts abilities were unequaled, even surpassing his own father's. His name became famous throughout the country. At that time, the emperor wanted to enlist the best martial artist to teach his royal family and the imperial guards. And since Ban-Hou was the best martial artist of his time, he was ordered to serve as the instructor for the royal family. Ban-Hou considered himself a descendent of the Han dynasty and did not want to serve the emperor, who was a Manchu. However, refusing an imperial edict would mean decapitation. Hence, Ban-Hou acquiesced to the emperor's demand. Ban-Hou did not wish to reveal the secrets of his arts to the Manchu invaders, so he deliberately altered the movements into soft forms, later known as the Beijing Yang Style.
To preserve his art, Ban-Hou secretly taught a stable boy, Wang Jiao-Yu, the true system. Wang Jiao-Yu taught the system to Kuo Lien Ying and Wang Chih Chien, both of whom passed on the art to the fifth generation, Dr. Yun Chung Chiang. As a side note, the secrecy and the disparity in Yang Ban-Hou's Tai Chi were later confirmed by Pu Ru, the brother of the last Ching dynasty's emperor. Pu Ru was a renowned artist from whom Dr. Chiang learned Chinese calligraphy and Northern style painting.