Science historians Adrian Desmond and James Moore have compiled compelling new evidence which reveals Darwin was passionately opposed to slavery and this was the moral impetus behind his work.
Private notes and letters uncovered by the pair reveal that Darwin's opinions on slavery were far stronger than had previously been believed.
Notebooks from his five year voyage on HMS Beagle, during which Darwin first began to form his famous theories on natural selection, detail his revulsion at the slavery he witnessed in South America.
The historians have also discovered letters written by Darwin's sisters, cousins and aunts that reveal the family as highly active abolitionists. Darwin's grandfather and uncles were also key members of the anti-slavery movement.
The pair claim in a new book that Darwin partly chose to highlight the common descent of man from apes to show that all races were equal, as a rebuttal to those who insisted black people were a different, and inferior, species from those with white skin.
They say Darwin attempted to show that his theory of sexual selection, where traits seen as desirable but which give no competitive advantage to a species are passed down through generations, was responsible for differences in appearance between races of both animals and humans.
Professor James Moore, from the department of history of science at the Open University, said that Darwin originally shied away from tackling the origins of humans in his book On the Origin of Species, which was published in 1859, as it was a controversial subject.
"We are not trying to explain away all of Darwin's work as being due to his passion for emancipation, but our argument is that his passion for racial unity is what drove him to touch this untouchable and treacherous subject," he said.
"Darwin was finally goaded into starting his work on the origins of man in 1865 by a rising tide of scientific belief that the races were separate species."
The new book, called Darwin's Sacred Cause, examines notes that Darwin made during his voyage on the Beagle. After visiting Brazil he wrote of his disgust at the slavery he saw in the country.
From an entry on July 3 1832, just one year before the Slavery Abolition Act was passed in Great Britain, he said: "The state of the enormous slave population must interest everyone who enters the Brazils... I hope the day will come when they will assert their own rights & forget to avenge these wrongs."
In notebooks he used while drawing up his theory of natural selection, he also made references to slavery. He wrote: "Do not slave holders wish to make the black man other kind?... from our origin in one common ancestor we may be all netted together."
Darwin also describes the brutality of slavery in his best-selling journal about his Beagle voyage and recalls staying opposite an old lady near Rio de Janeiro who kept thumbscrews to crush the fingers of her female slaves. He also tells of how a young boy was whipped "thrice" for handing him a glass that was not clean.
Correspondence between Darwin and a Jamaican magistrate Richard Hill, a former slave who went on to oversee disputes between former slave owners and emancipated slaves, also reveals some of the naturalist's views.
He writes to Hill just a few months before publishing On the Origin of Species to congratulate him on his work for the "sacred cause of humanity".
Professor Moore claims that Darwin's family were instrumental in helping the biologist form his opinions on slavery. His grandfather was Josiah Wedgwood, who founded the famous china factory and was an active anti-slavery campaigner.
Darwin's uncles included Josiah Wedgwood II, an abolitionist MP, while his aunts, cousins and sisters wrote many letters and donated money to the cause.
Professor Moore added: "Darwin's mother died when he was eight years old, so his sisters brought him up with help from their Wedgewood cousins. He was under the influence of these highly principled and liberal thinking ladies who taught him about anti-cruelty and the sin of slavery."
Next month will mark the 200th anniversary since Darwin's birth, while in November scientists will celebrate 150 years since his seminal work On the Origin of Species was published.
Many supporters of Darwin's work have used his theories to argue against the existence of God and the need for religion, while the controversy that followed the publication of his work is now seen to have mainly been on religious grounds.
In fact, Darwin was a religious man until relatively late in his career, often shying away from speaking publicly about the controversy his research had provoked.
Professor Moore and his co-author Adrian Desmond will present their new theory at a lecture and book launch at Imperial College London on Monday 9 February.
Mr Desmond, an honorary research fellow at University College London, said: "Darwin doesn't overtly refer to slavery and racism as his motivation for writing Descent of Man and On the Origin of Species, but it is there lurking in the background.
"I don't think anyone has really looked at how strong his belief in anti-slavery was, and this could be why it has been overlooked.
"What he was saying was that if you accept evolution, then you don't accept the view that black people are a separate species. It is clear that he believed the same as his grandfathers – that slaves were men and brothers."
Professor Armand Leroi, an evolutionary biologist at Imperial College London who is presenting a documentary What Darwin Didn't Know on BBC 4 on Monday, said although the new theories outlined in the book did not change Darwin's achievements, it gave a fresh insight into his motivations.
He said: "We as evolutionary biologists tend to view Darwin as being very much motivated by the things that motivate us, which is the explanation for the diversity of all the things in the world.
"Origin of the Species is a classic scientific work as it has no obvious social agenda, although when you read the journal of his voyage on the Beagle it becomes clear he was horrified by the slavery he saw and how it weighs upon him."
The Darwin's Sacred Cause book launch takes place at 6pm on Monday 9 February 2009 in the Great Hall on Imperial College London's South Kensington campus.
Entry is by advance free ticket only. Tickets can be reserved by emailing firstname.lastname@example.org