Why Do We Actually Have Sex?

Why do we have sex? This science documentary on the evolution of sex reveals a biology paradox Darwin never solved: Natural selection should have erased sex from every species on Earth, and it didn't. A snail in a New Zealand lake dies for being too common. A water flea in Belgian mud loses the same arms race for forty years and never falls behind. A worm in a dish of red bacteria lives only because it had a partner, while its self-fertilizing neighbors go extinct in thirty generations. And a woman smells a stranger's t-shirt and picks the immune system least like her own. In this video we trace one of the oldest unsolved problems in biology: the two-fold cost of sex. In nineteen seventy-eight, John Maynard Smith showed mathematically that a female who cloned herself would out-reproduce a sexual female two to one, doubling her lineage every generation until sex vanished. Over ninety-nine point nine percent of complex life does it anyway, and there is still no agreed answer why. We cover the billion-year-old fossil where sex first appears in the rock record; Muller's ratchet, and why cloning is a countdown, not a strategy; the argument that meiosis evolved to repair DNA, not to mix it; the Red Queen hypothesis and the Indiana lab that watched worms go extinct without sex; the all-female fish that beat the odds; and the two hundred thirty-seven reasons people gave for having sex, with reproduction nowhere near the top. Why would evolution keep the most expensive way to reproduce ever invented? Why does your body deliberately shatter its own DNA in hundreds of places to build one sperm or one egg? And if the fertile window is only six days a month, what is all the rest of it for? Drop a comment: If you could clone yourself instead - no partner, no shuffling, a perfect copy every time - would you actually want to? ━━━━━━━━━━━━━━━━━━━━━━━━━━━━ SOURCES ━━━━━━━━━━━━━━━━━━━━━━━━━━━━ THE PARADOX ▸ Maynard Smith, J. (1978). The Evolution of Sex. Cambridge University Press. ▸ Gibson, A. K., Delph, L. F., & Lively, C. M. (2017). "The two-fold cost of sex: experimental evidence from a natural system." Evolution Letters, 1(1): 6–15. WHERE SEX BEGAN ▸ Butterfield, N. J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes." Paleobiology, 26(3): 386–404. ▸ Gibson, T. M., et al. (2018). "Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis." Geology, 46(2): 135–138. MUTATIONS AND REPAIR ▸ Muller, H. J. (1964). "The relation of recombination to mutational advance." Mutation Research, 1(1): 2–9. ▸ Paland, S., & Lynch, M. (2006). "Transitions to asexuality result in excess amino acid substitutions." Science, 311(5763): 990–992. ▸ Bernstein, H., & Bernstein, C. (2010). "Evolutionary origin of recombination during meiosis." BioScience, 60(7): 498–505. ▸ Mirzaghaderi, G., & Hörandl, E. (2016). "The evolution of meiotic sex and its alternatives." Proc. R. Soc. B, 283(1838): 20161221. ▸ Bell, A. D., et al. (2020). "Insights into variation in meiosis from 31,228 human sperm genomes." Nature, 583(7815): 259–264. THE RED QUEEN ▸ Lively, C. M. (1987). "Evidence from a New Zealand snail for the maintenance of sex by parasitism." Nature, 328(6130): 519–521. ▸ Decaestecker, E., et al. (2007). "Host–parasite 'Red Queen' dynamics archived in pond sediment." Nature, 450(7171): 870–873. ▸ Jokela, J., Dybdahl, M. F., & Lively, C. M. (2009). "The maintenance of sex, clonal dynamics, and host-parasite coevolution in a mixed population of sexual and asexual snails." The American Naturalist, 174(S1): S43–S53. ▸ Morran, L. T., et al. (2011). "Running with the Red Queen: host-parasite coevolution selects for biparental sex." Science, 333(6039): 216–218. THE EXCEPTIONS ▸ Vakhrusheva, O. A., et al. (2020). "Genomic signatures of recombination in a natural population of the bdelloid rotifer Adineta vaga." Nature Communications, 11: 6421. ▸ Ricemeyer, E. S., et al. (2026). "Gene conversion empowers natural selection in a clonal fish species." Nature. doi:10.1038/s41586-026-10180-9 WHY HUMANS DO IT ▸ Wedekind, C., et al. (1995). "MHC-dependent mate preferences in humans." Proc. R. Soc. B, 260(1359): 245–249. ▸ Wilcox, A. J., Weinberg, C. R., & Baird, D. D. (1995). "Timing of sexual intercourse in relation to ovulation." NEJM, 333(23): 1517–1521. ▸ Meston, C. M., & Buss, D. M. (2007). "Why humans have sex." Archives of Sexual Behavior, 36(4): 477–507. ▸ Terao, M., et al. (2022). "Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9." PNAS, 119(49): e2211574119. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━ For business inquiries: [email protected] ━━━━━━━━━━━━━━━━━━━━━━━━━━━━ #WhyDoWeHaveSex #EvolutionOfSex #Evolution #Biology #ScienceDocumentary #NaturalSelection #Genetics #HumanEvolution #Darwin