Charles Darwin was better connected and wealthier than Alfred Wallace
While the theory of evolution is most closely associated with Charles Darwin, some believe a Welsh scientist has been unfairly overlooked.
Alfred Russel Wallace wrote to Darwin in March 1858 setting out his thoughts on evolution while he was working in Indonesia.
The biologist's idea was strikingly similar to Darwin's own.
But Darwin took the plaudits after On The Origin of Species was published a year later when Wallace was still away.
Now historian and solicitor David Hallmark is championing Wallace's work on the 150th anniversary of the day Darwin received Monmouthshire-born Wallace's letter - 18 June, 1858.
He said: "The evidence is that when Darwin himself received the Wallace letter he wrote that same day to one of his big friends, the geologist Charles Lyell to say that his life's work was smashed.
"A few days later he was writing to say that Wallace had priority.
"These are all words that indicate that Darwin, who was clever enough to see what Wallace had written, recognised that Wallace had got there first.
"Whether there was dishonesty or not is part of a big debate which is carrying on at the moment and it is a little difficult to calculate until there is a proper analysis."
Mr Hallmark said he was adopting a scientific approach to finding out whether Darwin had plagiarised Wallace's work and was utilising specialised computer software and experts.
Wallace was born in 1823 in Llanbadoc the second youngest of nine children. His interest in natural history was cultivated when he moved to Neath at the age of 18 to became a land surveyor.
After meeting naturalist Henry Walter Bates while he worked as a teacher at Leicester Collegiate School, he sailed to South America where he began formulating his own theories on the origin of the species.
James D Williams, a lecturer in science education at the University of Sussex, said Wallace published an essay on evolution in 1855 which received praise from Darwin himself.
But it was not until Wallace was suffering from malaria in 1858 that he came up with the theory that only the fittest would survive. This is what he told Darwin in that key letter from 150 years ago.
When Wallace returned from south-east Asia Darwin's book had already captured the public's imagination, making him famous and starting the creationism versus Darwinism debate, which still rages today.
But according to Mr Hallmark, Wallace, who had been through much on his travels, was a modest man and this could be one of the reasons why he did not make a fuss about Darwin's book.
"There is also this theory that Darwin was much better connected," said Mr Hallmark. "He was wealthy. His father and grandfather were well known, whereas Wallace was not.
"Charles Darwin was living in a grand house with lots of servants and with great wealth. Whereas Wallace was wondering around in comparative poverty in the Spice Islands collecting what he saw and what he found.
"They were conducting their own lives in very different circumstances."