The ancient Chinese YIN-YANG scholars thought the universe was divided into three parts. The first part is Heaven, the second part is Earth and the third one is for all living species. Because humans are in charge of all the living species, the third one is named Human.
For example, trees have their own growth cycle in between Heaven and Earth. Humans must have life cycles similar to those of trees. The seasonal changes affect the growth of trees. Trees grow faster in the spring and slower in the fall. The environmental changes also affect the life of trees. Without adequate sunshine, trees grow too slowly. With too much heat, trees will be dried out. Without water, trees cannot grow. With too much water, trees will be uprooted and afloat. With insufficient earth, trees will not grow tall....
The human life cycle is also affected by the same seasonal and environmental changes. The seasonal changes come from the Sun and Moon which are the clock for the calendars. The Chinese calendar is designed from the Sun and the Moon plus the Stem-Branch (concept from trees) cycle. Chinese YIN-YANG scholars have for thousands of years applied the Five Elements on the Chinese Stem-Branch calendar. This way humans can get their Five Element weights using their birthdays and times.
When the seasons and environment change, the Five Elements have certain responses. Humans respond in a similar way. The YIN-YANG scholars made predictions on the human life cycle, from birth to death, by using this natural phenomenon.