What to read this week: Lixing Sun’s ambitious book On the Origin of Sex

On the origin of sex
Lixing Sun, Profile Books

As children, many of us learn the facts of life through examples from the natural world. The idea is that it’s more straightforward (and certainly less awkward) to illustrate “talk” with pictures of birds laying eggs and bees pollinating flowers than to focus on humans or, God forbid, your parents.

But this is a comforting fiction, as evolutionary and behavioral biologist Lixing Sun states in his engaging and often stunning book, On the Origins of Sex: The Weird and Wonderful Science of How Our Planet Is Populated. The “birds and the bees” analogy just doesn’t capture the diversity, drama, and weirdness of animal reproduction.

Even animals that might strike us as obtuse have surprising and often shrewd approaches to reproduction: barn hens can eject most of their sperm after mating with a lowly male; male ducks have developed a corkscrew-shaped penis; and female ducks have evolved vaginas that spiral in the opposite direction, giving them more control over paternity.

On the origin of sex is packed with such examples, bringing biological nuts and bolts to life. The scale of reproduction makes the most adventurous human endeavors seem tame, but as the name suggests, the Sun’s scope is much more ambitious. Instead, it tackles the big questions that puzzle us about reproduction, starting with why there is such a thing as sex.

“
The strict male-female binary begins to feel not only limited, but downright outdated
“

This might seem strange to readers not well versed in evolutionary biology, but from an evolutionary perspective, sexual reproduction is more difficult and costly than asexual reproduction. This is because it requires two partners, one producing male gametes and the other female gametes. At first glance, asexual reproduction is much more efficient and is still used by many species such as bacteria and fungi. Parthenogenesis, a form of asexual reproduction in which an embryo develops without the need for fertilization, has been found in more than 80 vertebrate species, including California condors, bonnethead sharks, and Komodo dragons.

So why did the “two sex system” take off? Sun devotes the first part of the book to this question with a brisk tour of key ideas such as Muller’s ratchet, proposed in 1964 by geneticist Hermann Muller. This explains that the lack of genetic mixing in asexual reproduction leads to the gradual accumulation of irreversible “bad” mutations.

Then there’s the “Red Queen hypothesis,” which sees evolution as a perpetual arms race in which species “have to rush at full throttle just to keep up with their opponents and stay in the game,” Sun writes. This effectively reinforces the stakes of sex as literally a matter of life and death.

Sun, a distinguished research professor at Central Washington University, is well connected to many of the scientists whose work he cites. On the origin of sex is his fourth book for a lay audience, following works on cheating and justice in the natural world—and the natural history of the beaver.

By Sun’s own admission On the origin of sex is a “fast, selective ride,” but its intended reader is not always obvious. May assume considerable knowledge of evolutionary biology. At other times, his parties and achievements point to a more accessible title. But the book remains interesting because the many examples of geeky animal sex make it a leisurely read, even if you struggle with some of the more technical scientific discussions.

Sun’s enthusiasm for his subject and his wonder at the adaptive strategies of the animal kingdom are also infectious. His chapters on gender adaptations and sexual variations are particularly refreshing. Sun argues that we need to take a broader view of gender, writing that our current definition is “hopelessly homocentric, cut off from our evolutionary connections”. He uses the term to describe the roles and functions of animals beyond gamete production, and his patient, step-by-step explanation of the science behind sex and gender not only clarifies, but highlights how much people have invested in the subject.

She writes that “in popular discourse, gender is often treated as a social construct. What is missing, however, is a more fundamental understanding of gender as biological construct—something that evolutionary biology can help reveal. That’s where I come in.”

Careful not to wade into the culture wars, he emphasizes that his concern is evolutionary biology and mostly non-human animals. But it is clear that nature is not binary. “Biological sex is not just a matter of dividing the world into sperm and egg producers,” writes Sun. For example, mud eels, clownfish, wrasse and other fish change sex as a matter of course. Even in birds and mammals (including humans) there is wide variation, including intersex individuals.

Also, gender – embracing not only reproductive roles but also behavior and social dynamics – is much more variable among animals than we thought, and is also related to sex. For example, hyenas don’t just live in matriarchal packs; females also have a “pseudo-penis” (and a “pseudo-scrotum”) that has evolved to give them an advantage in the competition for food after the kill, but this complicates pregnancy.

Sun points to this kaleidoscopic range to argue for fresh thinking about sex and gender to embrace difference as “natural, adaptive parts of the diversity of life.” As he concludes, “the strict male-female binary is starting to feel not only limited but downright outdated.”

Elsewhere Sun told his early ambitions with On the origin of sex was to bring science into discussions of diversity and social justice. By sticking to science and giving examples from nature, he succeeds. Indeed, the dizzying variety of non-human life and the many questions about sex and reproduction that still await answers should challenge us to embrace complexity and keep an open mind, he suggests.

We might think we know the facts of life, but even Sun concludes his book marveling at “how little we really understand.”

Ella Hunt is a writer based in Norwich, UK

Three more great reads on sex and evolution

The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley

Check out this 1993 pop sci-fi classic from author Matt Ridley. Lixing Sun name-checks her for shaping our understanding of sex and gender—and for her “sharp wit and engaging prose.”

Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships by Nathan H. Lent

Filled with examples of sexual diversity in animals, this entertaining and witty book by Nathan H. Lents brings to light his painstaking research and makes important points about sexual diversity in humans.

The Intimate Animal: The Science of Love, Loyalty, and Connection by Justin Garcia

This 2026 book sheds light on the science of human attraction and connection. Justin Garcia is super-qualified as an evolutionary biologist, executive director of the Kinsey Institute, and dating science consultant Match.com.

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