Sentence:  When we examined the detailed history of the evolution, we fond large gaps of time in which little happened at all. Then we saw the sudden appearance of a key circuit (an enabling technology) and quick use of this for further technologies. A full adder circuit might appear after say 32,000 steps; and 2-, 3-, and 4-bit adders fairly quickly after that. In other words, we found periods of quiescence, followed by miniature "<WSD>Cambrian explosions</WSD>" of rapid evolution. 
Meaning of the word marked with <WSD> tags:  The rapid appearance of anything.