Reductionist theory must assume a primary or uncaused cause. The Big Bang is an example. The origin of life is another. That, before which, nothing can be connected. Dark matter is a third.
Uncaused events are happening all around you, every minute of every day. Every time a radioisotopic atom decays, it does so spontaneously, uncaused and unbidden. What's one more uncaused event, among a nearly infinite number of such occurrences daily?