Scientists turn the clock back 350 million years to show how humans lost their tails. Twice We might not wag our tails anymore but humans still bear vestigial traces of one. Inside the uterus, human embryos start off with a tail that gradually disappears and once we come into this world, there’s a tailbone to remind us that we haven’t gone that far. Strikingly, our early ancestors lost their tails not once, but twice, say scientists who analyzed 350-million-year-old fossils. To get to the bottom of things, researchers at the University of Pennsylvania analyzed the fossilized hatchlings of the Aetheretmon, a jawed fish and ancestor to terrestrial animals. The Aetheretmon used to have both a fleshy tail and a flexible tail fin, one sitting atop the other, the analysis showed.