There's long been suspicion that Charles Darwin revised his ideas on natural selection after receiving a letter from fellow naturalist Alfred Wallace.
He hung on to the letter for two weeks, goes the theory, allowing him to change certain elements before jointly announcing the theory of evolution by natural selection in July 1858.
However, two researchers from the National University of Singapore say they've been able to clear his name, by reconstructing the route the letter took - and finding that Wallace sent the letter a month later than historians had always assumed.
Wallace discovered evolution by natural selection independently of Charles Darwin, while living on the island of Ternate in the Moluccas, now Indonesia.
He wrote up his ideas in an essay which he sent to Charles Darwin in 1858, for him to pass on to geologist Charles Lyell. Darwin’s accusers claim that he lied about the date he received it, giving himself two weeks to revise his own ideas in the light of Wallace’s.
“It occurred to me that we really have no contemporary evidence of when Wallace sent the essay to Darwin, only his much later recollection that he sent it by the next post after writing it in February. That suggested that the essay was sent in March 1858," says Dr John van Wyhe.
"But this recollection from years later seemed to me not very reliable as evidence of what really happened at the time. The other evidence that Darwin received it on 18 June 1858 seemed more likely."
So van Wyhe and collaborator Dr Kees Rookmaaker decided to try and track the letter from Darwin’s end - and found that a steamer had arrived in England the day before Darwin received the letter.
Using contemporary newspapers, they traced back steamer connections across the globe.
"Eventually our mail itinerary was completed all the way back to Ternate and we were astonished to find that there was an unbroken series of mail connections to Ternate - not in March as all other writers before had assumed, but in April 1858!" says van Wyhe.
In addition, says van Whye, Wallace was replying to an earlier letter from Darwin that arrived in Ternate on the March steamer. And surviving correspondence shows that he never replied to a letter on the same steamer which delivered it, as the turnover time was too short.
"Therefore this is an additional reason to doubt that Wallace could have sent the famous letter to Darwin in March as so long assumed,” he says.