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What drove Charles Darwin to his extraordinary ideas on evolution and human origins? Adrian Desmond, with co-author James Moore, argue in a new book that the great scientist had a "sacred cause": the abolition of slavery.

"It makes one's blood boil," said Charles Darwin.

Not much outraged the gentle recluse, but the horrors of slavery could cost him a night's sleep.

He was thinking of the whipped house boy and the thumbscrews used by old ladies in South America, atrocities he had witnessed on the Beagle voyage.

The screams stayed with him for life, but how much did they influence his life's work?

Today you can still read of Darwin's "eureka" moment when he saw the Galapagos finches.

Alas, his conversion to evolution wasn't so simple, but it was much more interesting. It didn't occur in the Galapagos, but probably on his arrival home.

And new evidence suggests that Darwin's unique approach to evolution - relating all races and species by "common descent" - could have been fostered by his anti-slavery beliefs.

Family feelings

After circumnavigating the globe (1831-6), Darwin settled in London. Here in 1838 he formulated his theory of "natural selection", after which he became increasingly reclusive, particularly following his move to Down village in Kent.

He refrained from publishing a word on evolution until 1858 - not even a brief, priority-grabbing paper, as was his way with other projects. His hesitance is understandable. Evolution was execrable to his Cambridge friends.

One naturalist called it "abominable trash vomited" out by revolutionaries; and radicals did, indeed, deploy a self-sustaining evolution to undermine the creationist miracles on which Anglican power rested.

Darwin's gouty Cambridge professor, Adam Sedgwick, used "contempt, scorn, and ridicule" to trash one "filthy" evolution book in 1844. Darwin, sensitive about his reputation, wisely laid low.

So why devise such a beastly theory in the first place, if it threatened ignominy? Was there some integral moral gain?

Consider another question. Why was Darwin's evolution uniquely defined by common descent, the joining of races and species through shared ancestry? Darwin's common descent image is so obvious today that we forget to question where it came from.

'Man and brother'

Common descent in Darwin's younger day was ubiquitous in anti-slavery tracts. Consider the words of the famous cameo, depicting a kneeling slave asking "Am I Not a Man and a Brother?" That cameo was in fact the brainchild of the pottery-dynasty founder, Josiah Wedgwood, Darwin's grandfather.

New evidence shows how indebted Darwin was to this anti-slavery heritage.

Darwin's uncle Jos Wedgwood sold the firm's London showroom, and ploughed the proceeds into an anti-slavery society, and in the 1850s (with American slavery still flourishing) the Wedgwoods continued using labels showing the slave under Britannia's banner, which read "God Hath Made of One Blood All Nations of Men".

The anti-slavery agitator Thomas Clarkson - the man who rode 35,000 miles collecting statistics in the sea ports on the evil trade - was another bankrolled by Josiah Wedgwood.

With a Wedgwood wife and mother, Darwin saw abolition as a "sacred cause" too, and in his culminating work, the Descent of Man (1871), he placed Clarkson at the moral apex of humanity and called slavery a "great sin".

Such family feelings explain why, as a 16-year-old at Edinburgh University in 1826 (in a period often dismissed by historians), Darwin could spend 40 extra-curricular hours with a freed slave from Guyana studying taxidermy and become his "intimate" friend.

And this when many visiting Americans saw any black/white friendship as "revolting".

Torture accounts

Darwin witnessed slavery everywhere in South America. The Beagle's own supply ship on her previous trip had originally been a slaver, and, once sold, it reverted to slaving. While Darwin was on the continent, it was again disgorging chained Africans.

Darwin's journal of the voyage (1845) gives a damning account of the tortures he saw or heard of; but of all the "heart-sickening atrocities", the worst for him were the stories of masters threatening to sell the children of disobedient slaves.

As an outsider, he was "powerless as a child even to remonstrate". But within weeks of the Beagle's return, he developed a science which undercut the slave-master's notions.

Many plantation owners considered slaves a separate species, an animal to be exploited as such. Blacks and whites shared no joint ancestry.

Yet the Darwin-Wedgwood maxim made the slave a "Man and a Brother". Darwin opened his first evolution notebook in 1837, damned slave-holders for their separate species view, then pushed common parentage to the zoological limit.

Since species were only extended races, they too must share an ancestry. He moved from talking of the common "father" of mankind to an "opossum"-like fossil as the father of all mammals.

Human genealogy became the model for his famous "tree of life".

Fossil evidence

None of this minimizes the importance of Darwin's Galapagos and Pampas observations. The giant tortoises, mockingbirds and finches varied from island to island, and this became clearer to Darwin after London Zoo's bird expert John Gould analysed his finches in January 1837.

Then Richard Owen (the man shortly to give the world the "dinosaurs") diagnosed Darwin's fossils. Darwin thought that some were "rhinos" (Old World mammals), yet Owen showed that they were indigenous giant armadillos, sloths and anteaters.

So extinct animals were being succeeded by related living types. This evidence remains crucial, but it was the way Darwin marshalled it that concerns us. Assuming the tacit truth of racial "brotherhood" allowed him to join the bloodlines into a common descent configuration.

And he did so in 1837-8, just as the West Indies slaves were being released (technically freed in 1833, they were forced to serve an "apprenticeship" which effectively kept them in bondage till 1838).

This freedom filled Darwin with a sense of pride and he declared that "we... have made a greater sacrifice, than ever made by any nation, to expiate our sin". He certainly had.

All too clear

His common descent imagery was unknown elsewhere in natural history, beyond racially unifying works such as James Cowles Prichard's Researches into the Physical History of Mankind. That book traced animal races to common ancestors in order to prove that all humans could have descended from Adam.

Darwin, preparing to write the Origin of Species, scribbled inside his copy of Prichard: "How like my Book all this will be". It wasn't so. He remained a worried man and in the later 1850s dropped humans from his publishing plans because the subject was "so surrounded with prejudices".

But even though the Origin of Species (1859) skirted people, no one doubted that they remained at its core.

Darwin's "bulldog" T.H. Huxley, who took over the fight for human evolution, said that when it came to uniting black and white ancestries, he "was pleased to be able to show that Mr Darwin was for once on the side of orthodoxy".

Darwin could have wished for no more.

Adrian Desmond is an Honorary Research Fellow in the Biology Department at University College London. He is co-author with James Moore of Darwin's Sacred Cause (Allen Lane)