On the Origin of Sex: The Weird and Wonderful Science of How Our Planet is Populated

by Lixing Sun

A NEW SCIENTIST BOOK PICK FOR 2026

'Fascinating ... this book will make you rethink what you thought you knew about sex' CAT BOHANNON

'A thrilling biological detective story. Brilliant told and relentlessly entertaining' CHARLES FOSTER

Explore the spectacular science of reproduction - from frogs and funghi to seahorses, sparrows and everything in between

Let's talk about sex. Not boring, human sex, but the endlessly fascinating, varied and complex forms of reproduction that can be found in the rest of the natural world.

Whiptail lizards and California condors are capable of immaculate conception, while strawberries are reproductive flip-floppers. Clownfish can permanently change sex, while bearded dragons undergo temperature-dependent sex reversal. For the humble slime mould, sex can involve hundreds of mating types - a veritable bonanza.

Lixing Sun has spent decades expanding the bounds of what we know about reproduction, in all its many flavours. And as it turns out, sex is far more complex and eye-opening than we ever could have imagined.

Genres: Nonfiction, Science, Evolution, Animals

368 pages, Kindle Edition

Published March 5, 2026

About the author

Author also writes under L.Sun

Reviews

[Lixing Sun]

Dear Reader,

I'm the author of this book, and I just wanted to say how grateful I am that you've taken the time to read it. Hope you've enjoyed it. Your thoughts and insights truly mean a great deal to me. They help the ideas grow beyond the page.

I'd greately appreciate any feedback or suggestions you might have. I’m always learning, and your perspective is critially important for me to improve in my future projects.

I’ve also started a YouTube channel connected to the book (https://www.youtube.com/@WondersofLif...), where I share short videos on related topics. If you happen to visit, I would love to hear your thoughts there as well.

Thank you again for your time, your curiosity, and your generosity in sharing your reflections.

Warm regards,

Lixing

[Tutankhamun18 · Rating 4 out of 5]

//4.5 stars//

Sex is one of biology's biggest paradoxes. Asexual reproduction is faster, simpler, and allows an individual to pass on 100% of its genes, yet sexual reproduction dominates much of life on Earth. This book explores why sex evolved, why it persists, and how it has shaped everything from genomes and chromosomes to behaviour, gender, and parental care.

The book argues that sex survives because it constantly reshuffles genes, helping organisms keep pace in an evolutionary arms race against parasites and environmental change. Along the way, it examines the origins of sperm and eggs, the evolution of males and females, the strange fate of sex chromosomes, and why some species switch between sexual and asexual reproduction.

The story then expands into the ongoing conflict and cooperation between the sexes. Males and females often have different evolutionary interests, leading to battles over reproduction, offspring investment, growth, and even gene expression itself. From genomic imprinting and sperm competition to parental care and sex-ratio manipulation, reproduction emerges as a complex strategic game played at every biological level.

Ultimately, the book presents sex not as a simple biological fact but as a dynamic evolutionary process that influences genetics, behaviour, development, and even our understanding of gender. The result is a fascinating account of how the struggle to reproduce has shaped life itself.

I enjoyed this book alot, many of the chapters were truly fantastic (chapters 1-4 and 9), while others could have been shorter/ were slightly less interesting to me (5,6,7,8). At times the authors writing style was brilliant with explantory power and his enthusiasm shining through, at times points felt slightly scattered and I wondered why there were included. Recommend reading this!

QUOTES

on the origin of sex

Chapter 1:
* Virgin Birth (parthogenesis)
* Muller's ratcet→sex persists
* inefficiency of sexual reproduction
* Small genomes nulnerable due to gene nappins, evidence found in mess left behind -> nuclear genomes
* mitochondria (oxidative stress -> nucleus) trepped in cell
* Asexual strategies:
1. good mutations
2. small genomes
3. gene transfer
• Transduction
•conjugation
•transformation
* some plants, fungi and small animals flip flop between sexual and asexual reproduction

Chapter 2:
* Law of constant extinction
* Red Queen Hypothesis
-> sex reschuddles genes -> rare. (win over parasites)
-> inbreeding fucks this up inbreeding depression
* r vs. k strategy
* r = lots
* k = sturdy

Chapter 3:
* sperm ~ 70-100 mitochondria, egg ~ 100’000 mitochondria
* “the germ line is immortal, preserving and relaying genetic information from one generation to another, while the body is transient, playing no role in inheritance.”
* mating types -> 2 mating types. makes sense to stay small/latge if the other is small/large cost-benefit analysis
* egg is diploid and sperm is haploid

Chapter 4:
* bird chromosome. male is ZZ and female is ZW
* ways to lower social cost of meiosis:
* 1. haplo-diploid sex determination “ant way” aka eusociality
* 2. inbreeding
* X chromosome recombines -> all genes used (100 working genes)
* Y chromosome doesnt -> 71 working genes in mates
* -> at least 10 spp where Y has disappered

Chapter 5:
* “Although biologists commonly use the term 'sex-role reversal" to describe this phenomenon, this book will instead use 'gender reversal' to emphasize the distinction between sex and gender. The rationale for this choice will become evident later in this chapter.”
* Gender is a continuous trait
* Books references Augstin Fuentes Sex is a Spectrum & Performance All The Way Down

Chapter 6:
* “Expectedly, the more promiscuous the females, the tougher the competition, and the more males have to invest in larger testes to keep up with the demand."
Chapter 7:
* none

Chapter 8:
* “Certainty about paternity - knowing for sure those kids are yours - is a major incentive for dads to roll up their sleeves and get to work. That's why paternal care shines brightest in species where fertilization happens out in the open, like bony fish and amphibians.”
* “The simple fact that males can see their sperm connect with the eggs drives their paternal instincts in species with external fertilization.”
* “Sexual conflict doesn't just play out via sneaky behaviour -and clever strategies. It digs much deeper - right down to the genes. Consider this: a newborn foal can weigh around 100 pounds, while the mother pony herself might only tip the scales at a few hundred pounds. Now, what happens if a tiny pony mom is mated with a much larger stallion?You'd probably expect her to have some serious birthing issues, right? But as it turns out, pony moms are armed with a genetic ace up their sleeves, a little trick called 'genomic imprinting - inactivating dads' genes by adding a methyl group on the DNA. This clever genetic manoeuvre stops the baby from ballooning to a size that would spell disaster.* Here, the male-female showdown doesn't just stay in the ring of behaviour; it goes 'nucleic', battling it out in the DNA itself.”
* “Let's return to the curious case of crossing a small pony with a large horse. Strangely enough, it's not horses but rodents that offer a surprising clue to how this odd couple manages to make it work. The poster child for this genetic tug-of-war between males and females is a gene called insulin-like growth factor 2, or Igf2. Both parents pass on a copy of this gene, but only the one from Dad is active in the embryo." This isn't just a growth boost for the baby - it's a strategic push from Dad's genes to draw more resources from Mom. The trade-off is that this extra growth can come at a cost to the mother's health, draining her ability to invest in future offspring. So she fights back, producing a counteracting gene, 1gf2r (the receptor), which can silence that pushy 1g? with a chemical tag called a methyl group. This methylation is actually the addition of a methyl group (-CH) to DNA, a tiny chemical tweak that can shut a gene down. If you remove this silencing tag in female mice, the babies can balloon in size - proof that this parental arms race is written into the genome. But why all this genetic warfare between the sexes?”
* “We know that males, with their tiny gametes with little energy and nutrient storage, are basically reproductive parasites. They can only achieve their Darwinian fitness through females.”
* “Enter calico cats. In females, one of the two X chromosomes is randomly shut down in each cell during early development - a process called X-inactivation. This means that in some cells, the X from Mom stays active; in others, it's Dad's. Since the genes for fur colour - particularly orange and black pigments - are located on the X chromosome, this random inactivation creates a patchwork effect. The result is a feline masterpiece of orange and black spots, as if an abstract artist spilled paint on an exceptionally cute canvas.”
* “In the rock pigeon, for instance, when a mother's testosterone levels spike, she's more likely to produce sons (ZZ). But if stress hormones are running high, she tends to have more daughters (ZW)." In some species of birds, females, tuned in to these same hormonal cues, don't treat all chicks equally If the father is a handsome, high-stepping show-off, the mother will feed his chicks more generously than those sired by a less impressive mate.2 As we brought up in Chapter 8, when a female mates with a flashy, genetically desirable male, she'll actually load her eggs with extra testosterone - giving her sons a hormonal jump-start towards being just as dazzling as their dad. But if she ends up with a dud, the eggs get no such boost. No VIP perks for average genes.”
* “It's not just testosterone getting in on the action - other hormones tied to social status can also tip the scales in favour of sons. Consider progesterone. It's linked to female dominance, and surprise, surprise: moms with higher levels of it tend to pop "out more boys." But, as with birds, when there's a crowd off males around, female mammals - like lab mice - get flooded with stress hormones and suddenly the balance shifts towards daughters." It's as if the moms are thinking, 'Hmm, too many boys in the room, better bring in some more girls!" In humans, stress hormones may have a similar effect, potentially causing male foetuses to be lost, leaving a higher ratio of daughters. We all know that nutrition is key to a female's health, but how does a mom's diet impact the sex ratio of her offspring? It seems there's a middleman' - blood glucose - involved but the story's a bit more complicated than you'd think. High glucose levels, which signal that a mom is well fed and thriv-can be harmful for early-stage female embryos in Wout ing, can be harmful for early stage female embryos in wond mammals.* So when there's plenty of glucose in the system, male embryos are more likely to thrive.? And when glucose teams up with testosterone, the result, you guessed it, is more sons. They're the ultimate chemical power couple when it comes to producing boys. It's not too surprising that females have ways to steer the sex ratio of their offspring - but they're not the only ones. Males are getting in on the action too, and studies suggest that guys can tweak their sperm strategy depending on how attractive their mate is or how intense the competition is from rival males. In some species, males even adjust the proportion of X- versus Y-bearing sperm in their ejaculate, subtly stacking the odds. That's one reason dominant males across a wide range of species tend to sire more sons than daughters.? So yes, dads are pulling the strings as well, whether they're bugs, birds or mammals. In fact, males are active players in the game of manipulating sex ratios to score an evolutionary advantage.* The battle of the sexes rages on in ways that push not only the limits of biology but also our imaginations.”

* “In humans, the story is even more striking Genes passed down through men have evolved at roughly double the pace of those passed through women, as sen when comparing mitochondrial DNA (inherited from mont) with Y chromosome DNA (inherited from dads). Bur das rapid evolution isn't without a hefty price tag. As history shows, it comes with a much steeper rate of reproductive fal ure and lineage extinction for men compared to women."
* “Here, the 'rate of evolution' refers to how quickly genetic lineages accumulate changes that let us trace them back to a single common ancestor. And when we do that with theY chromosome - passed only from father to son - we can track all modern men back to a common male ancestor who lived about 20,000 years ago. But if we follow the maternal line using mitochondrial DNA - passed from mothers to all their children - we land much further back, around 150,000 years ago. Why the difference? It's a reflection of genetic survival. Over time, more maternal lines have persisted than paternal ones. That's likely because fewer males reproduced successfully each generation, while more females did - leaving more continuous maternal lineages and making the mitochondrial tree stretch further into the past. In short: more moms left a legacy, while most dads were evolutionary dead ends”

[Annemiek Heuvel · Rating 5 out of 5]

In April 2023, Lixing Sun published the fascinating book “Liars of Nature and the Nature of Liars”. That book changed the way I looked at deception in the natural (and human) world, so when I heard he was writing a new one, I eagerly awaited its release. His new book is just as good -and perhaps even better- than his previous book.
In a time where discussions about sex and its influence can have serious repercussions, Sun published a remarkably important book. As in the “Liars of Nature and the Nature of Liars” he shows how sexual behavior and genetic influence shape human behavior. Some chapters left me stunned, searching for even a grain of evidence to challenge what he wrote. All I could find was confirmation, making me wonder whether we should rethink the influence of genes and sex entirely- and whether our cultural and social habits need a fundamental update.
Sun begins by clarifying that there are two basic forms of reproduction: cloning and recombination (the latter achieved via sex). He describes protozoa that do not have two distinct female and male reproductive cells but instead possess seven mating types. He also discusses mammals with unusual reproductive systems, such as the European mole, which has both ovaries and testes. In spring she takes on the maternal role, but once her pups are born her ovaries recede and her testes become dominant. She becomes very aggressive and defending her territory. So is the mole female, male or a hermaphrodite?
The book explores sex from a biological perspective across three dimensions, its origin, its development and its evolutionary path. In part 1 Sun takes the reader back two billion years to examine the origin of sex and how it became life’s preferred mode of reproduction. As anyone familiar with the fading quality of repeated photocopies knows, copying alone has its drawbacks. Sexual reproduction offers a major advantage: by combining the genes of two parents new genetic combinations arise, giving offspring a better chance of surviving environmental change. This is the engine behind life’s incredible diversity.
Part two of the book delves into the intricate biology of sex determination, exploring how different species create males and females and showing why intersex conditions are an adaptive strategy. Sun presents species that can switch sexes to gain evolutionary advantage overturning the idea of fixed sex. He also highlights animals that challenge human assumptions by bending gender roles to suit their need.

The final part tackles sexual selection, where members of the same sex compete for mates while the opposite sex make the choices. Sun reveals how mating strategies adapt to social and ecological pressure, reshaping old tropes about “coy females” and highlighting fierce female competition. With a wealth of evidence, he shows that both males and females have evolved their own strategies for reproductive success, breaking down biases and debunking any pseudoscientific basis for sexual stereotypes. In this section, he explains that the male body is fine-tuned for only one primary purpose mate competition - a relentless struggle for reproductive success. Even in humans this remains visible. Using these insights. Sun explains human mate competition across millennia and offers a surprising explanation for why modern societies show fewer extreme forms of this competition.
He concludes the book with a striking discovery. Across the animal kingdom (and plants as well) geneticist have uncovered a nearly universal pattern: females, in most species, shuffle their genes more than males do. The likely cause is sexual selection. Not all males reproduce - only a few do- so a mechanism is needed to maintain genetic diversity. This recombination bias is a long-term survival tactic , especially for the females. By shuffling genes each generation, recombination helps preserve genetic diversity and prevents a females lineage from being erased by a few overachieving males.

[Mandy · Rating 4 out of 5]

In On the Origin of Sex, Dr. Lixing Sun tackles one of the most fundamental and puzzling questions in evolutionary biology: why does sex exist at all? Given its apparent inefficiency compared to asexual reproduction, the persistence of sexual reproduction across so many species demands explanation. Sun’s book is an ambitious attempt to answer that question while also rethinking how we understand sex itself. Rather than offering a single, neat solution, Sun guides the reader through the major scientific theories that have tried to account for sex such as genetic variation, parasite resistance, mutation repair, and more. Sun is particularly good at showing how each theory explains part of the puzzle without fully resolving it, leaving readers with a sense of science as an evolving conversation rather than a settled body of knowledge. Sun writes in a clear, accessible style, avoiding excessive jargon while still engaging seriously with complex ideas. He uses examples from across the natural world -microbes, plants, animals - to illustrate how diverse and sometimes bizarre sexual systems can be. These case studies are among the book’s most engaging elements, grounding abstract theory in vivid biological reality. The book invites the reader to think more deeply about a subject often taken for granted. I found the book an engaging read, even if at times I found myself grappling with some of the more complex ideas and theories, but overall felt that at the end I had a new and deeper understanding of the subject.

[E.Reads · Rating 5 out of 5]

Amazing nonfiction book.

It's a perfect blend of hard science and easy to follow explanations with plenty of examples (and graphs!) to make it all make sense and stick.
The book went way deeper than I thought it would, each chapter leading you deeper and deeper into the whys and hows. It's not just about why sex evolved as part of evolution, but also about the why and how of genders, societal pressures and constructs, reproductive selections, etc etc...
There's a lot in this book. Now that I'm done, I know it's one I'll want and need to re-read because there's too much for me to remember it all in a single read through. Endlessly fascinating. And as a bonus, it's extremely quotable too. I underlined so many passages I knew I wanted to share with the world.

Highly recommend to anyone with a love of science and a curiosity about the world around (and in) us.

A copy of this book was provided to me by NetGalley. All opinions are my own.

[Keyi · Rating 4 out of 5]

Informative book that breaks down the evolution of sex, sexual differentiation/differences, from both a biological and social perspective. This book is packed with hard science, and isn’t akin to nature writing, so those looking for something more scientific than descriptive will find it here. I appreciated that the writer went into a good amount of detail, even listing specific transcription factors, and there were plenty of interesting anecdotes regarding animal behaviour.

Thank you to NetGalley and Basic Books for the ARC.

[Andrew Brassington · Rating 4 out of 5]

All I can say is, that after reading this book, I think garter snakes are truly foul.
If you're looking for an examination of why sex actually exists, then I would say that this is a really intriguing read. I'm not usually one for learning about evolutionary biology, but this was certainly very interesting and, at times, disgusting.