Explore the Origin of Sex
Lixing Sun, Profile Books

The natural world often teaches us about life through vivid examples. Many kids learn about reproduction through birds laying eggs and bees pollinating flowers instead of focusing on human examples, which can be more challenging to convey.

However, as evolutionary and behavioral biologist Heung-sun Lee discusses in his thought-provoking book On the Origins of Sex: The Weird and Wonderful Science of How Our Planet is Populated, the “birds and bees” analogy is far too simplistic. It overlooks the vivid diversity and intricacies involved in animal reproduction.

Even species that might appear unremarkable have fascinating reproductive practices. For instance, barnyard hens can expel sperm from less dominant males, while male ducks sport corkscrew-shaped penises. Interestingly, female ducks have evolved opposite-spinning vaginas, enabling them to exert control over paternity.

On the Origins of Sex presents numerous examples to illustrate fundamental biological concepts. The wide range of breeding methods observed makes many human practices seem mundane. However, Sun’s ambitious goal is to delve deeper into the critical questions surrounding reproduction, especially the very existence of sexual reproduction.

This question might baffle readers unfamiliar with evolutionary biology, but from an evolutionary standpoint, sexual reproduction can be far more complex and resource-intensive than asexual reproduction. Achieving sexual reproduction requires two partners—one generating male gametes and the other producing female gametes. At first glance, asexual reproduction appears more efficient, a method still employed by numerous species, including bacteria and fungi. Parthenogenesis, a form of asexual reproduction where embryos develop without fertilization, is noted in over 80 vertebrate species, encompassing California condors, booby sharks, and Komodo dragons.

So why did sexual reproduction become prevalent? Sun dedicates the book’s first section to addressing this question, succinctly introducing essential concepts like the Müller ratchet, proposed in 1964 by geneticist Hermann Müller. This theory describes how the absence of genetic mixing in asexual reproduction leads to the gradual accumulation of detrimental mutations.

Additionally, the Red Queen hypothesis likens evolution to a continuing arms race, suggesting that species must evolve continuously to survive and outpace their competitors, underscoring the inherent risks of sexual reproduction.

Sun, a distinguished research professor at Central Washington University, draws from his connections with numerous scientists whose work he references throughout the book. On the Origins of Sex is his fourth work aimed at a general readership, following his explorations into fairness in nature and the history of beavers.

Despite Sun’s admission that On the Origins of Sex is a selective narrative, the intended audience isn’t always clear. While readers might need some background knowledge in evolutionary biology, the book remains captivating, enriched with examples of exotic animal reproduction that are well-paced.

Sun’s passion for his subject is infectious, particularly evident in his discussions on sexual adaptation and variation. He advocates for a broader understanding of gender, arguing that current definitions are “hopelessly homocentric and disconnected from evolutionary realities.” His detailed, step-by-step explanations of the sciences underpinning sex and gender reveal just how much human society relies on these biological concepts.

He articulates, “In popular discussions, gender is often perceived as a social construct. What is often overlooked is a deeper understanding of gender as a biological construct—something evolutionary biology can elucidate.”

Though careful not to engage in cultural arguments, Sun primarily focuses on evolutionary biology in non-human animals. It becomes evident that nature’s binary classification is an oversimplification. “Biological sex is not merely about distinguishing species that produce sperm from those that produce eggs,” he asserts. Consider fish like eels, clownfish, and wrasse, which can naturally change sex. Within birds and mammals (including humans), numerous variations exist, including intersex individuals.

Gender fluidity is far more prevalent among animals than traditionally understood, affecting not just reproductive roles but also behaviors and social structures. For example, hyenas, known for their matrilineal society, exhibit females possessing a “pseudopenis,” which aids in food competition and complicates pregnancy.

By highlighting this diversity, Sun urges a paradigm shift regarding sex and gender, advocating for a perspective that embraces differences as “natural and adaptive aspects of life’s continuum.” He concludes: “The rigid male/female binary is increasingly recognized as limited and outdated.”

In previous discussions, Sun has expressed a desire to infuse scientific insights into debates about diversity and social justice. He effectively meets this goal through a science-focused approach, enriched with examples from the natural world. Indeed, he posits that the immense diversity of non-human life and the numerous unanswered questions about reproduction should inspire us to welcome complexity and openness.

We may presume to understand the fundamentals of life, yet even Sun closes the book in awe of “how little we truly grasp.”

Elle Hunt – Writer based in Norwich, UK.

3 Great Reads on Sex and Evolution

The Red Queen: Sex and the Evolution of Humanity
Written by Matt Ridley

Check out this essential 1993 favorite, praised by Lixing Sun for shaping contemporary views on sex and gender with its “sharp wit and engaging prose.”

The Evolution of Sex: How 500 Million Years of Sex, Gender, and Mating Have Shaped Modern Human Relationships Written by Nathan H. Lentz

This witty and insightful book showcases the remarkable diversity of sexual behaviors in animals while providing crucial lessons about human sexual diversity.

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

This 2026 publication explores human attraction and emotional bonds. Justin Garcia, an esteemed evolutionary biologist and executive director of the Kinsey Institute, brings his expertise to the science of dating.