Last week marked Charles Darwin’s 210th birthday. Of the many writings about his work and enormously influential legacy, there is one topic that deserves more attention: Darwin’s strong anti-slavery sentiments and their influence on his creative output and position.

The case is explored in the 2009 book Darwin’s Sacred Cause: Race, Slavery and the Quest for Human Origins, by Adrian Desmond and James Moore, who argue that Darwin’s repugnance to slavery was a trigger to his ideas and writings unifying all of humankind under a single species.

To my personal sadness, it was during his exploration of Brazil—my native country—while on the HMS Beagle (1831-1836) that Darwin had his first direct contact with the horrors of slavery. Reminiscing in 1845, he wrote in his Voyage of the Beagle:

Near Rio de Janeiro I lived opposite to an old lady, who kept screws to crush the fingers of her female slaves. I have stayed in a house where a young household mulatto, daily and hourly, was reviled, beaten, and persecuted enough to break the spirit of the lowest animal …. I will not even allude to the many heart-sickening atrocities which I authentically heard of … nor would I have mentioned the above revolting details, had I not met with several people … [who] speak of slavery as a tolerable evil.

In The Origin of Species and The Descent of Man, Darwin argues for a common origin of all life. As such, there is a common root to all humans, making slavery into a heinous crime. He made his views clear in the second edition of Descent (1874):

Slavery, although in some ways beneficial during ancient times, is a great crime; yet it was not so regarded until quite recently, even by the most civilized nations. And this was especially the case, because the slaves belonged in general to a race different from that of their masters. As barbarians do not regard the opinion of their women, wives are commonly treated like slaves.

Darwin’s grandfather, the famous doctor and poet Erasmus Darwin, was a devoted abolitionist, and a close friend of Josiah Wedgwood, a name famously associated with beautiful porcelain tableware. Wedgwood used his kilns to create a medallion with the image of a chained slave and the caption: “Am I Not a Man and a Brother?” The medallion became a symbol of abolitionism and a fashionable way to disclose abolitionist views. This was the environment where Darwin grew up.

The Darwin and Wedgwood families were united by several marriages. Charles himself married Emma Wedgwood, his first-degree cousin. (A curious choice for the father of the theory of evolution. Of their ten children, two died in childhood. Every time one would become ill, Darwin would worry about the closeness of his family roots.) Given the two families and their views, abolition was surely a common discussion topic.

When Darwin studied theology in Cambridge, he met members of the Anglican Church that radically opposed slavery, learning early that his feelings were shared. However, he wanted more than moral arguments. He wanted scientific arguments.

When he proposed the evolution of the species by natural selection, Darwin included humans in life’s wide fold. Not surprisingly, the dominant reaction at the time was one of disgust, especially in white Victorian England and the Empire’s current and past white-dominated colonies: Are we the descendants of orangutans? Cousins of Africans, of the Chinese, of the Aboriginals of Australia? Impossible! It was also not surprising that many of Darwin’s critics were Bible-thumping whites, who conveniently chose to misinterpret certain teachings of their chosen book to serve their racial prejudices. For example, Acts 17:26 (“From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands”) has sometimes been misinterpreted to support segregation. But others have interpreted it to support racial equality, noting that the verse implies that all humans, no matter their race, are descended from one.

To Darwin, life came from a single seed and bifurcated in myriad ways, fighting for survival along the way, as it morphed and adapted to changing conditions. Humans were simply part of a larger picture, as he wrote in The Origin of Species:

Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

That we now know that we share some 99 percent of our genes with chimpanzees and bonobos only serves to prove his point. Darwin’s science aligned deeply with his moral values, a lesson many still need to learn today, more than 150 years after his Origin of Species.

Gleiser is a professor of natural philosophy, physics and astronomy at Dartmouth College.