There is also 'cross pollination' across history when teachers learn from other traditions and sometimes very similar styles are developed independently. For example, Trekcho practice in Dzogchen is very similar to Zen training of shikantaza or 'silent illumination'. On the other hand it's very fundamental practice just like following the breath, so it can be traced back to the beginning of Buddhism.

Jnana-vichara (self-enquiry method) taught by hindu guru Bhagavan Sri Ramana Maharshi is very similar to Zen koan practice. The question of "Who am I" is often used as first koan in Zen.