Charles Darwin’s On the Origin of Species rattled Victorian readers in 1859, even though it said almost nothing about how the idea of evolution applied to human beings. A dozen years later, in 1871, he tackled that subject head-on. In The Descent of Man , and Selection in Relation to Sex , published 150 years ago this month, Darwin argued forcefully that all creatures were subject to the same natural laws, and that humans had evolved over countless eons, just as other animals had. “Man,” he wrote, “still bears in his bodily frame the indelible stamp of his lowly origin.”

In Descent , Darwin details a theory that he calls “sexual selection”—the idea that, in many species, males battle other males for access to females, while in other species females choose the biggest or most attractive males to bond with. The male-combat theory would explain, for example, the development of a bull’s horns, or a moose’s antlers, while the quintessential example of “female choice” is seen in peahens, which, Darwin argued, prefer to mate with peacocks having the biggest, most colorful tails. For Darwin, sexual selection was just as important as natural selection, which he had outlined in Origin —the idea that organisms with favorable traits are more likely to reproduce, thus passing on those traits to their offspring. Both mechanisms helped to explain how species evolved over time.

“I think for Darwin, sexual selection was what connected humans with non-human animals,” says Ian Hesketh, a historian of science at the University of Queensland in Australia. “It provided the continuity in Darwin’s system, from animals to humans.”

In Descent , Darwin illustrates this continuity by noting the similarities of the human body to those of our primate cousins and to other mammals, focusing on anatomical structure—such as the similarity of their skeletons—and also on embryology—the embryos of related animals can be almost indistinguishable.

Descent , like Origin , became a huge bestseller. As writer Cyril Aydon put it in A Brief Guide to Charles Darwin: His Life and Times : “With Darwin’s name on the cover, and monkeys and sex on the inside pages, it was a publisher’s dream.” Descent is still seen as a landmark in the history of the life sciences—though, inevitably, some passages strike modern readers as offensive, especially where Darwin speculates on issues of race and on gender roles. He also tackled difficult problems that continue to spark debate today, such as the evolution of minds and of moral beliefs.

Many aspects of sexual selection seemed implausible to Darwin’s contemporaries. For example, the theory attempted to explain the development of so-called secondary sexual characteristics, such as the peacock’s tail or other traits that made a male animal more appealing to a female. If these traits are selected by the female, they can develop to extremes over the course of time—at which point they may hamper, rather than aid in, survival. For example, an overly-colorful tail could attract predators. Darwin’s argument also seemed to suggest that animals possessed a sophisticated ability to rate the attractiveness of each potential mate with a kind of check-list of criteria.

“The most contentious aspect [of the book] has to do with how it relates to the development of coloration and what he called ‘charms’—anything that had to do with wooing the female,” says Hesketh, “No one seemed to be on board with that, because it suggested that animals had an aesthetic sense, and that they were making mate-choices based on really miniscule observations.”

The two aspects of sexual selection were not equally well received: The male-combat idea, which casts males as aggressive and females as passive, seemed plausible enough to Darwin’s contemporaries, as it meshed with the prevailing prejudices of the time. But the other part of the theory, in which females appear to have the power of choice by selecting from among an array of prospective males, struck many as a radical notion. For humans, however, Darwin switched it up; in our own species, he argued, it was the male that did the choosing.

“The argument here is that males have ‘seized the power of selection’ from females, because they’re more powerful, in body and mind, than women,” says Evelleen Richards, a historian at the University of Sydney and the author of Darwin and the Making of Sexual Selection . In Descent , Darwin writes of “man attaining a higher eminence in whatever he takes up than woman can attain—whether requiring deep thought, reason or imagination, or merely the use of the senses and hands.” He added, “Thus man has ultimately become superior to woman.”

Passages like that reveal Darwin’s “androcentric bias,” as Richards puts it, noting that his views on sex and sex differences were very much derived from a male perspective and were a product of Victorian society. To complicate matters, Darwin’s views about sex were intimately tied up with his theories about race. A much-debated question in Darwin’s time was the puzzling diversity of humanity. Did the various races emerge independently of one another? That view, known as “polygenism,” was popular among members of the Anthropological Society of London, which Richards describes as an “out-and-out racist” organization. The Society supported the Confederacy in the U.S. Civil War, and its leader, a speech therapist named James Hunt, declared that we “know that the Races of Europe have now much in their mental and moral nature which the races of Africa have not got.” Others, including Darwin, argued that all races shared a common origin, a view known as “monogenism.” But monogenists still had to explain what caused the diversity seen today. This is where sexual selection comes in. Darwin argued that differing judgements of attractiveness held the key; he believed that men of one tribe or group were naturally most attracted to members of their own tribe. He wrote that “the differences between the tribes, at first very slight, would gradually and inevitably be increased to a greater and greater degree.” Few of Darwin’s readers found this plausible, says Richards, because they imagined European ideals of beauty to be universal; they simply couldn’t imagine, for example, “that black skin could be attractive to anyone,” she says.

All of this, Richards says, highlights the complexity of Darwin’s views on race. In contrast to many of his contemporaries, he believed in “the brotherhood of man,” as Richards puts it, and found slavery repugnant—and yet he believed, as most Victorians did, in a racial hierarchy with Europeans at the top. Even so, some of his ideas—like the notion of Africans being attracted to Africans—struck his contemporaries as too radical.

Perhaps the most difficult puzzle for Darwin was the remarkable cognitive power of humans, and, especially, the human capacity for moral reasoning. Some of Darwin’s contemporaries, notably Alfred Russel Wallace, saw the human mind as evidence that a divine intelligence was guiding evolution. Wallace, who co-discovered natural selection, came to embrace spiritualism in his later years. Historians see Descent largely as a rebuttal to Wallace, that is, as an attempt to set forth a purely naturalistic explanation for the mind and for moral behavior. While the details were far from clear, Darwin saw minds and morals as rooted, ultimately, in biology. For example, he argues that a primitive kind of moral sense can be seen in certain animals—those “endowed with the social instincts” and which “take pleasure in one another’s company, warn one another of danger, defend and aid one another in many ways.” As such instinctive behavior is “highly beneficial to the species, they have in all probability been acquired through natural selection.”

Unlike Origin , which was immediately hailed as a groundbreaking scientific work, Descent has had a checkered history. The idea of sexual selection, in particular, languished in the decades following its publication. That’s partly because of lingering doubts over animals’ aesthetic sense and the idea of female choice, and partly because Darwin was never able to convince his old allies—people like Wallace and also Thomas Henry Huxley—that sexual selection was an important facet of evolution. Others, meanwhile, were uncomfortable with naturalistic accounts of minds and morals. “By the turn of the century, sexual selection, for all intents and purposes, is basically dead,” says Henry-James Meiring, a PhD student working with Hesketh at Queensland.

In the 20th century, however, it began to make a comeback. Biologists absorbed many of the ideas in Descent into the so-called modern synthesis that combined Darwin’s theory of evolution with the new science of genetics; later, aspects of sexual selection received support from evolutionary theories of social behavior. By the 1970s, sexual selection “starts making a comeback in modern science, and in some form has continued ever since,” says Meiring. Evelleen Richards adds that sexual selection has only recently “come back on the agenda as a scientific fact in its own right.”

On the bigger picture—the unity of all living things—Darwin was on the right track. That unity, he reasoned, applies not just to bodies but also to minds. True, scientists continue to debate the question of exactly how the brain (a biological organ) gives rise to a mind (with its intangible mental processes), but it is clear that brains are what make minds possible, and these evolved just as our bodies did. In this sense we’re no different from our primate cousins; Darwin argued that the cognitive powers of human beings differ from those of apes in degree, not in kind. Darwin’s thinking on these problems today “enjoys broad support in disciplines such as neuroscience and evolutionary psychology,” says Meiring.

Other aspects of Darwin’s thinking in Descent continue to spark debate. Some scholars have criticized attempts to explain social behavior in terms of biology as overly reductionist, and many facets of evolutionary psychology, in particular, have faced skepticism in recent years. For example, some anthropologists argue that we don’t know enough about the environment in which early humans lived, or the advantages that particular behavioral traits conferred, to be certain that behaviors observed today are the result of early conditions. And puzzles persist over the origins of language, music and religion.

“Darwin, like any other scientific figure of the past, got some things right and got a lot of things wrong,” says Meiring. “And his own biases around gender and race had an impact on the way that he theorized and thought about science.” In Descent , he says, Darwin grappled with “things that we are still arguing about, and that we have still not resolved. I think that’s possibly its greatest legacy.”