In Jain belief, time is endless and runs in cycles. The present era (Avasarpini, the descending half of the cycle) began with Adinath (Rishabhanatha), who is regarded as the first Tirthankar of this half-cycle. Before his appearance at the start of the current Avasarpini, the world existed in earlier ages of the cycle, when people were very tall, long-lived, and dharma was flourishing. Adinath then taught the basic codes of conduct and laid the foundations for Jain practice, including the five great vows, after which society gradually moved into the later stages of this era. This idea—that Adinath marks the beginning of the present time cycle and that the world existed in a very different, more prosperous state before him—is described in Jain cosmology. You can read more here