Enormous strides have been made recently in understanding Charles Darwin. The latest evidence suggests that Darwin's anti-slavery beliefs helped to shape his theory of evolution. He became an evolutionist in 1837, after the Beagle voyage, but did not publish On the Origin of Species until 1859. The unique theory that he devised after stepping ashore rested on the “common descent” of all animals and plants - an approach that spawned the “tree of life” image that was Darwin's distinctive way of looking at nature.

Historians have wondered why he adopted such a genealogical perspective with its joined bloodlines. The answer, it now seems, is to be sought in his anti-slavery heritage. Darwin's grandfather was Josiah Wedgwood, founder of the 250-year-old chinaware company that collapsed only weeks ago. Wedgwood's cameo, depicting a kneeling slave begging “Am I Not a Man and a Brother?” is a highly recognisable icon. It suggests the very “brotherhood” image of race relations that may have influenced Darwin's thinking on “common descent”. If black and white people can look so different yet share the same umpteenth grandparent, perhaps all animals could be similarly related.

To assess the Darwin family's commitment to anti-slavery, Professor James Moore, from the Open University, has burrowed into the Wedgwood archives. He discovered an abolitionist obsession. Darwin's aunt, Sarah Wedgwood, gave more to the anti-slavery movement than any other woman in Britain. Darwin's mother and wife were Wedgwoods and anti-slavery was what Darwin called a “sacred cause”. He was taught to see the oppressed black as a “brother”. This explains why, when he went to Edinburgh University at 16, he could apprentice himself to a freed Guyanese slave to learn the art of bird preservation without thinking it infra dig. That former slave became an “intimate” friend.

Nowhere was Darwin more outraged by slavery than in South America. During the Beagle voyage he saw the aftermath of slave revolts and the instruments of torture, and heard of a planter who threatened to sell the children of recalcitrant slaves. “It makes one's blood boil, yet heart tremble,” he wrote. Slave trading was ubiquitous here. State documents show that, on her previous journey, even Beagle's supply ship was a former slaver - and after being sold it returned to slave-smuggling while Darwin was in South America.

Times Archive, 1887: The life of Darwin

It has been said with truth that we must go back to Newton before we meet with Darwin's peer

• The Times obituary: Charles Robert Darwin

• The Descent of Man

• The Descent of Man 2

Related Links

• Attenborough on Charles Darwin

White masters considered slaves subhuman. They were assumed to be another species. It is no coincidence that Darwin, fresh off the Beagle, took an opposite tack. In his first evolution notes he railed against this view and extended the Darwin-Wedgwood motto, making the black person a “Man and a Brother”. He joined the races by giving them a common ancestor, uniting all “animals, our fellow brethren in pain, disease death & suffering ... our slaves in the most laborious work” by means of trillions of “common descents”. Each animal and plant had a pedigree that ultimately united it with every other one.

The “common descent” image is so common now that we have lost sight of its racial roots. Those who execrate Darwin may be staggered to learn that humanitarianism lay behind his profoundest achievement.

Darwin's Sacred Cause: Race, Slavery and the Quest for Human Origins, by Adrian Desmond and James Moore, is published by Allen Lane on January 29, £25

Times recommends

• Energy-saving ideas from switched-on professor
• Rooks are as clever with tools as chimpanzees
• Burping of the lambs blows roast off menu