How has the role of horizontal gene transfer (HGT) in evolution changed over time?

How has the role of horizontal gene transfer (HGT) in evolution changed over time?
 
  a. Early life-forms were fixed entities and the only way to adapt was through HGT. HGT no longer occurs.
  b. Early life-forms had modular genomes for which HGT played a large role in adaptation. This role has decreased over time.
  c. Early life-forms had modular genomes for which HGT played a large role in adaptation. This role has increased over time.
  d. HGT was irrelevant for early life-forms. HGT is a more recent phenomenon and is important for current adaptations.