For thousands of year before 1200 BC the I Ching was more naturally linked to the cycles and rhythms of nature, from which much of the advice and many of the profound insights originated. This was the time of Early Heaven, and the earliest known teacher was the Emperor Fu Hsi from around 3300 BC.
Since 1200 BC the I Ching has evolved through a mathematical era started with the Chou-I by the Duke of Chou. This has become known as the world of Later Heaven. Magic squares and mathematical relationships inspired many of the insights into the hexagrams and lines.
The main distinction between the Early Heaven and the Later Heaven concepts was that Early Heaven was experiential via nature, and Later Heaven was experienced abstractly via mathematical deductions.