The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships

THE SEXUAL EVOLUTION: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships by Nathan H. Lents. Mariner Books, 2025. 336 pages. Hardcover; 32. 00. ISBN: 9780063375444. *Biologist Nathan Lents's newest book on the sex lives of animals, The Sexual Evolution, is neither written from a Christian perspective nor written to a Christian audience. Nevertheless, this book offers a convicting call for Christians to join a rapidly growing boundary-crossing conversation: What does nature reveal about morality? *Lents is an accomplished scientist and thoughtful writer who recognizes the unsteady ground on which he treads: "Believe it or not, this book is not about values; it is about biology" (p. 11). However, I don't believe it. There are multitudes of fascinating topics in biology worth writing about, yet Lents has chosen a topic that inevitably flows from biology to ethics. Lents's primary message can be summarized in his repeated is-ought phrase: "Nature loves diversity. We should too" (pp. 40, 233). By placing human sexual behavior in an evolutionary context of living things, vertebrates, mammals, primates, and great apes, Lents proposes that we ought to accept a more inclusive concept of human sexual ethics, arguing for the moral equivalence of heterosexual and homosexual behavior, sex within marriage and sex without, and sex with one person and sex with many. Lents is, in his words, "forcefully pulling up a chair" (p. 4) for biology to join the discourse on human sexual ethics. In my opinion, this important interdisciplinary conversation is long overdue. *As biologists, Lents and I agree on many things in general. Sex is a biological category, not a social one. Lents helpfully uses the term "gametic sex" to refer to the sperm and egg producers of life and "biological sex" to mean the other aspects of reproductive biology beyond simply what gametes one makes (things such as internal and external anatomy, hormones, and hormone receptors). And while sex is based in biology, gender--how one chooses to present their sexual identity to the world--is a social construct. Too often Christians treat the words "sex" and "gender" as synonyms, which is neither linguistically accurate nor helpful when trying to understand the complexities of human sexuality. I also support Lents's compassion for marginalized people, specifically those with disorders of sex development, people whose anatomy does not easily fit into rigid categories of "male" and "female" and whose existence and intrinsic value ought to be affirmed more often in religious conversations about sexual ethics. *Ultimately, Lents provides a well-evidenced argument that (1) homosexual behavior is natural (i. e. , found throughout the animal kingdom among normal populations), (2) homosexual behavior is adaptive, meaning it persists in animals because it provides some biological benefit, and (3) sexual behaviors are about far more than reproduction--animals have sex to strengthen social bonds, establish hierarchies, and just because it feels good. Each of these points is convincingly made and each one counters a common myth believed by many Christian thinkers. We have been caught relying on outdated and incorrect scientific facts when we argue that same-sex behavior et cetera is wrong because it is unnatural or maladaptive. *Lents is careful to describe animal behavior according to our best current understanding. However, in one case, he gets the facts wrong--and wrong in a way that reveals how dangerous his project can be if his logic and arguments are correct. In his exploration of sexual monogamy, Lents calls our attention to the many socially monogamous species that are sexually promiscuous. In doing so, he is making the implicit point that fidelity and promiscuity are morally equivalent because both are natural and adaptive. One of his examples is the jackdaw, a highly intelligent bird that forms lifelong pair bonds between mates. Lents suggests that pair-bonded females willingly seek out extra-pair copulations with neighboring males. However, according to the article Lents himself cites, this is not what happens. 1 Male jackdaws watch the nests of their neighbors and when the resident male leaves, they will invade and attempt to copulate with the vulnerable female. Importantly, the female resists the interloper. Their violent struggle can result in significant injuries to the female and sometimes the destruction of her eggs. 2 Studying animal behavior reveals the disturbing biological fact that pursuing sex through violence is also natural and adaptive. Nature loves diversity. We should too? *To Lents's credit, he seems aware of this potential critique and offers a solution: human sexual ethics should be decided first by Nature and then Society. When using nature as a guide, he finds that same-sex behavior and gender fluidity are (1) taxonomically widespread, (2) quantifiably beneficial to a species, and (3) biologically influenced. Therefore, Lents infers that these behaviors are morally acceptable. However, in nature we also find that sexual assault, incest, and sexual contact with minors are (1) taxonomically widespread, (2) quantifiably beneficial to a species, and (3) biologically influenced. If his reasoning is valid, one could advocate for the moral acceptability of these behaviors as well. *Lents avoids this unpalatable consequence by suggesting that it is then up to society to determine what natural inclinations are acceptable or not. He surveys human cultural diversity, finding that same-sex behavior and gender fluidity have been normalized and encouraged in various societies throughout human history. His logical conclusion then is that these behaviors are morally acceptable; they are natural and socially accepted. However, as he does for the darker parts of nature, Lents ignores or glosses over the fact that sexual assault (e. g. , marital rape among the Gusii people of Kenya3), incest (e. g. , brother-sister marriages in ancient Egypt4), and sexual contact with minors (e. g. , the coming-of-age rituals of the Simbari and Mangaia peoples Lents, p. 188) have also been normalized and encouraged in various cultures throughout human history. These behaviors too are natural and at times socially accepted. *It is clear that neither nature nor society provide the robust standard of morality that Lents, indeed all of us, are searching for. Nature tells us that virtually all behaviors and inclinations are permissible, while morality according to society is subjective and ephemeral. *The worrisome evolutionary ethic undergirding The Sexual Evolution demands an effective response. Christians need to present an objective sexual ethic grounded in the character of God and affirming that all humans are made in God's image while also accurately describing our biology. Who we were and who we are by nature really does matter. Crafting this response will require careful, compassionate effort across academic disciplines. Will you join me? *Notes *1Lisa F. Gill, Jaap van Schaik, Auguste M. P. von Bayern, Manfred L. Gahr, "Genetic Monogamy Despite Frequent Extrapair Copulations in ‘Strictly Monogamous' Wild Jackdaws, " Behavioral Ecology 31, no. 1 (2020): 247-60, https: //doi. org/10. 1093/beheco/arz185. *2Rebecca Hooper, Kathryn Maher, Karen Moore, Guillam McIvor, David Hosken, and Alex Thornton, "Ultimate Drivers of Forced Extra-Pair Copulations in Birds Lacking a Penis: Jackdaws as a Case-Study, " Royal Society Open Science 11, no. 3 (2024): 231226, https: //doi. org/10. 1098/rsos. 231226. *3Peggy Reeves Sanday, "The Socio-Cultural Context of Rape: A Cross-Cultural Study, " Journal of Social Issues 37, no. 4 (1981): 5-27, https: //doi. org/10. 1111/j. 1540-4560. 1981. tb01068. x. *4Russel Middleton, "Brother-Sister and Father-Daughter Marriage in Ancient Egypt, " American Sociological Review 27, no. 5 (1962): 603-11, https: //doi. org/10. 2307/2089618. *Reviewed by Jeremy D. Blaschke, associate professor of biology, Union University, Jackson, TN 38305.