How Ancient Humans Lost Their Tails?

Reach behind you right now. Run your hand down to the very bottom of your spine. You can feel it — a small, hard, pointed bone. That's the end of a tail. At four weeks after conception, every human embryo grows a tail with up to twelve vertebrae. By week eight, your body has eaten it back. What stays behind is the coccyx — three to five fused vertebrae at the base of your spine. For over a hundred years, biologists knew tail loss happened around twenty-five million years ago. Nobody could find the switch. In 2024, a graduate student named Bo Xia did. It was a single piece of jumping DNA — an Alu element — that inserted itself into a gene called TBXT. The same gene that builds tails in every other vertebrate on Earth. But evolution doesn't give you anything for free. Mice engineered with the same mutation lose their tails. They are also born with higher rates of spina bifida. We may have traded a tail for a slightly buggier spine — and gained the ability to walk upright. Primary source for this video is Xia et al. (2024), "On the genetic basis of tail-loss evolution in humans and apes," Nature 626, 1042–1048. Embryology details draw on O'Rahilly & Müller's work on human neural development. Alu element biology draws on Deininger (2011). You didn't lose your tail. You just stopped finishing it. ━ PRIMARY GENETIC RESEARCH Xia, Bo et al. (2024). "On the genetic basis of tail-loss evolution in humans and apes." Nature 626, 1042–1048. Primary source for the AluY insertion in TBXT intron 6, the mouse model with truncated TBXT, and the elevated neural tube defects observed in those mice. EMBRYOLOGY OF THE HUMAN TAIL O'Rahilly, Ronan & Müller, Fabiola (2008). "The development of the neural crest in the human." Journal of Anatomy, 211(3). Source for the appearance and regression of the embryonic tail between weeks 4 and 8. VESTIGIAL TAIL CASE LITERATURE Bar-Maor, J. A. et al. (1980). "Human tails." Journal of Bone and Joint Surgery, 62-B(4). Source for the clinical prevalence and surgical treatment of true vestigial tails in newborns. ALU ELEMENT BIOLOGY Deininger, Prescott (2011). "Alu elements: know the SINEs." Genome Biology, 12(12), 236. Source for the count of Alu elements in the human genome and their behavior as retrotransposons. PRIMATE TAIL EVOLUTION Larson, Susan G. (2018). "Nonhuman primate locomotion." American Journal of Physical Anthropology, 165(4). Source for the timing of tail loss at the ape–Old-World-monkey divergence around 25 million years ago. NEURAL TUBE DEFECT EPIDEMIOLOGY Mitchell, Laura E. (2005). "Epidemiology of neural tube defects." American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 135C(1). Source for the approximate global incidence of spina bifida. BIPEDALISM CONTEXT Cartmill, Matt & Smith, Fred H. (2009). The Human Lineage. Wiley-Blackwell. Source for the relationship between tail loss, pelvic geometry, and the evolution of upright walking. #HumanEvolution #Genetics #Anthropology #Biology #ScienceExplained