California condor embryos can develop without fertilization
Shutterstock/Barbara Ash
on the origin of sex
Lixing Sun, Profile Books
As children, many of us learn the facts of life from examples in the natural world. The thinking is that picturing the “interaction” with images of birds laying eggs and bees pollinating flowers is more straightforward (and certainly less uncomfortable) than focusing on humans or, God forbid, their parents.
But this is a comfortable fantasy, as evolutionary and behavioral biologist Lixing Sun points out in his fascinating and often mind-boggling book, On the Origins of Sex: The Weird and Wonderful Science of How Our Planet Is Populated. The “birds and the bees” analogy does not capture the diversity, drama, and strangeness of animal reproduction.
Even animals that seem dull to us have surprising and frequently cut-throat approaches to reproduction: barnyard chickens can expel most of the sperm after mating with low-ranking males; Male ducks have evolved a corkscrew-shaped penis; And female ducks have evolved vaginas that spiral in the opposite direction, giving them greater control over paternity.
on the origin of sex Full of examples that bring the biological nuts and bolts to life. The limits of reproduction make for the most adventurous of human activities, but, as the title suggests, the scope of the Sun is far more ambitious. Instead, he grapples with the big questions that plague us about reproduction, starting with why there is such a thing as sex at all.
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The strict male-female binary begins to seem not only limiting, but downright outdated.
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To readers who are not well versed in evolutionary biology, this may seem a strange line of inquiry, 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 producing female gametes. At first glance, asexual reproduction is far 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 species of vertebrates, including the California condor, bonnethead sharks and Komodo dragons.
So why did the “two-gender system” start? Sun devotes the first section of the book to that question, with a quick tour through key ideas, such as Muller’s Ratchet, which was proposed by geneticist Hermann Muller in 1964. This suggests 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 fight tooth and nail just to keep up with their rivals and stay in the game”, Sun writes. This effectively reinforces the stakes of sex as a matter of life and death.
A distinguished research professor at Central Washington University, Sun is well-connected with many of the scientists whose work he cites. on the origin of sex This is his fourth book for a general audience, following works on fraud and fairness in the natural world – and the natural history of beavers.
By Surya’s own admission, on the origin of sex is a “fast, selective ride”, but its intended readership is not always clear. He can acquire considerable knowledge of evolutionary biology. At other times, his asides and flourishes point toward more accessible titles. But the book remains interesting because the many examples of strange animal sex make it worth a read, even if you have to grapple 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, is also infectious. Her chapters on gender adaptation and gender diversity are particularly refreshing. Sun argues that we need to take a more expansive view of gender, writing that our current definition is “hopelessly homophobic, disconnected from our evolutionary ties”. He uses the term to describe an animal’s roles and functions beyond gamete production, and his patient, step-by-step explanation of the science underpinning both sex and gender not only clarifies, but highlights just how much humans have invested in the subject.
He writes that “In popular discourse, gender is often treated as a social construct. What is missing, however, is a more basic understanding of what gender is.” organic Creation – something evolutionary biology can help highlight. This is where I come.”
He is careful not to wade into the culture wars and emphasizes that his concerns are evolutionary biology and mostly non-human animals. But it becomes clear that nature is not binary. Sun writes, “Biological sex is not simply a matter of dividing the world into sperm-producers and egg-makers.” For example, swamp eels, clownfish, wrasse, and other fish naturally change sex. Even among birds and mammals (including humans), there is wide variation, including intersex individuals.
Gender, too – not just reproductive roles but behavior and social dynamics – is more fluid among animals than we thought, and it’s related to sex, too. For example, hyenas do not live exclusively in matriarchal herds; Females also have a “pseudo-penis” (and a “pseudo-scrotal sac”), which has evolved to give them an edge in competition for food after the kill, but complicates pregnancy.
Sun points to this kaleidoscopic range to argue for new thinking on sex and gender, so that differences can be included as “natural, adaptive parts of the diversity of life.” As he concludes, “the strict male-female binary begins to seem not only limiting, but downright outdated”.
Elsewhere, Sun has spoken of his initial ambition to on the origin of sex Science had to be included in discussions of diversity and social justice. He succeeds by sticking to science and giving examples from nature. Indeed, the baffling diversity among nonhuman life and the many questions about sex and reproduction that still await answers should challenge us to accept complexity and keep an open mind, he suggests.
We may think we know the facts of life, but Sun also concludes his book by being surprised by “how little we really understand.”
elle hunt is a writer based in Norwich, UK
Three more great books on sex and evolution
The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley
Check out this 1993 pop-sci classic written by author Matt Ridley. Lixing Sun named it for how it shaped our understanding of gender and sex – and for its “sharp wit and impressive prose”.
Sexual evolution: how 500 million years of sex, gender and orgasm shape modern relationships By Nathan H. Lents
Filled with examples of animal sexual diversity, this entertaining and witty book by Nathan H. Lent makes light work of his rigorous research and also makes important points about sexual diversity in humans.
Intimate Animals: The Science of Love, Loyalty, and Bonding by Justin Garcia
This 2026 book sheds light on the science of human attraction and connection. Justin Garcia is superqualified as an evolutionary biologist, executive director of the Kinsey Institute, and scientific advisor to the dating site match.com.
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