@pete evolution works not so much top down as it works laterally. Viruses mostly and also bacteria could have played a much bigger key role in evolution than many thought. Imagine this:
Stable biome -> catastrophic change -> stress induces endogenic virus to wake up from dormant state -> viruses engulfing genetic fragments from old host infect new hosts -> evolution takes place pretty quickly

It explains why we see stratification and not all the intermediate steps in evolution.