Yoga's history has many places of obscurity and uncertainty due to its oral transmission of sacred texts and the secretive nature of its teachings. The early writings on yoga were transcribed on fragile palm leaves that were easily damaged, destroyed or lost. The development of yoga can be traced back to over 5,000 years ago, but some researchers think that yoga may be up to 10,000 years old old. Yoga's long rich history can be divided into four main periods of innovation, practice and development.