Two studies, in the journal Cell, found a mutation increased the virus' ability to infect human cells fourfold.
Scientists have argued the mutation may have been "pivotal" in the outbreak becoming the largest in recorded history.
There were 28,616 Ebola cases in Guinea, Liberia and Sierra Leone.
And 11,310 people died during the outbreak.
Researchers at the University of Nottingham and the University of Massachusetts analysed the genetic code of nearly 2,000 Ebola virus samples.