One of the great puzzles of Darwin's Origin of Species is that one lone pregnant phrase: ‘Light will be thrown on the origin of man and his history.’ The Origin is all about primroses and pigeons; ‘man’ was kept well at bay. But the reviewers and critics certainly knew what lay in store. T.H. Huxley in his reviews swung it into view, kicking and screaming, followed by Lyell, and Huxley's own take on the difference between us and the nonhuman primates. Darwin himself waited patiently, until 1871's Descent of Man.

But, according to Adrian Desmond and James Moore's latest book, the problem of man – or, rather, the ‘family of man’ – concerned Darwin right from the beginning. This much we already knew. What is startling is the way that Desmond and Moore have filled out the familiar outline. Their claim is that Darwin's concern about slavery and the contemporary growth of natural philosophers' justification of man's inhumanity to man lay at the core of his concerns about evolution, species, classification, psychology and ethnography.

Anti-slavery was, Desmond and Moore show, a middle-class cause, arising smack in the midst of the jolting turn to full-scale industrialism, wage labour and colonialism. The issue of slavery exercised Whigs, Tories, radicals and Unitarians alike. Its importance only escalated as the nineteenth century progressed, provoking a deep moral and religious controversy. And a scientific one, too. Darwin came from a long line of anti-slavery activists, as did his wife (and first cousin) Emma. The issue hovered over his domestic, social and scientific concerns.

Darwin clearly abhorred the institution, and he was never loath to express his disgust at the practice. But what was not entirely clear, before the new detailed researches of Desmond and Moore, was how central this was to Darwin's fight for a new view of life. They present us with oodles of detail, gleaned from the original research for their award-winning biography of Darwin (1991) and augmented by their recent explorations into the place of anti-slavery in British science and society. They offer us one of the best accounts now available of the degree to which the slavery issue played into the moral–scientific debates of Darwin's time.

Where their biography would (somewhat breathlessly) situate Darwin in the midst of Chartist riots, glancing over his shoulder at artisanal atheists, Larmackian chain-of-being gangs and Edinburgh materialists, here he is embroiled with – or at least damned near to – radical abolitionists to the left and right, and their vociferous debates with pro-slavery advocates. For the early part of the story, much seems to lie on interpretation and ‘innocence by association’. We read, for example, that Darwin ‘learned much’ from a black taxidermist at Edinburgh – but it is not entirely clear that this made Darwin any more accepting of the unity-of-man position. He hated the Fuegians and could hardly imagine that he was related to them – and clearly hoped not. It is this very dual (or plural) nature of Darwin's reaction to difference and proximity that provides much flexibility of interpretation.

Darwin's Sacred Cause, like the biography, works very hard at settling the case by context and juxtaposition. And – again as in the biography – the reader may hesitate at some of the juxtapositions standing in for causal explanations. I counted at least one per page until the sixth chapter (and then gave up). Here are some examples, randomly chosen: in 1831 the Cambridge Union Society scheduled its 15 February debate on the ‘early and entire abolition of Slavery in the British Colonies’ … ‘We don't know whether Darwin attended’, Desmond and Moore write, ‘but he did tell a cousin that he was going out that evening. Nothing meant more to him, as fellow students testified’ (p. 65). Or Darwin's ‘hectic movements in these months [1831] matched the nation's mood’ (for political reform – p. 72 – it turns out he was madly running around preparing for his trip on the Beagle). Or ‘the speed with which Darwin adopted evolution hints at a gestation on the voyage. Those shocking events around the world – the brutality, black slavery, segregation and degradation – tested his moral resilience on racial unity’ (p. 114). Sometimes the proximity is based wholly on a ‘stone's throw’ (p. 125), or a woven metaphor: ‘The strands of twenty years' research were coming together. They were like the yarn spun from slave cotton in Lancashire's mills, fibres wound tight enough in the new theory to withstand being torn apart by its critics’ (p. 280).

There is a certain monomania in the approach, and Desmond and Moore push their case to the limits. Yet we also see that Darwin was no egalitarian. Indeed, his own favoured evolutionary mechanism, natural selection, could easily be given a pro-slavery or pro-colonial twist. He may have demurred over the slogan ‘survival of the fittest’, but it proved fit for all kinds of illiberal scenarios.

We should, of course, be careful to remember that Darwin's own evolutionary theory acknowledged several mechanisms. The one that he especially emphasized when examining the question of human races in the Descent was sexual selection. Here much more needs to be done, and Desmond and Moore's fine research opens up new horizons in the understanding of the importance of ‘non-fitness’ characters in the making up of difference. Darwin's own understanding of aesthetics and, especially, his reading of Edmund Burke's works on beauty and the sublime need to be further examined – Darwin's aesthetic theory of selection looks decidedly pluralist and almost postmodern.

Darwin's Sacred Cause is a most helpful and definitive antidote to the kind of gymnastic dimwittery of those who see a kind of direct destiny between Darwin, racism and even national socialism, found in such books as Ken Ham and A. Charles Ware's Darwin's Plantation: Evolution's Racist Roots (2007), Richard Weikart's From Darwin to Hitler (2004) and more than enough debate on blogs and listserves. But, still and gnawingly, Darwin was ever an elitist, and the authors of this fascinating and sometimes aggravating book are not ashamed to examine that context too.