Sentence:  It was around 2.9 billion years ago that <WSD>LUCA</WSD> split into the three domains of life: the single-celled bacteria and archaea, and the more complex eukaryotes that gave rise to animals and plants.... 
Meaning of the word marked with <WSD> tags:  (nodot=1); the hypothesised most recent primordial organism that is an ancestor to all organisms now living.