The disconnect means that the two tails went on their own evolutionary paths. Fish lost the fleshy tail and kept the flexible one to improve their swimming. Having just the back fin, she explained, "allows for more refined movements, which a muscular tail (originally present for power swimming) would disrupt."
Fish that evolved to become semi-aquatic and then land-dwelling animals lost the flexible back fin, but kept the fleshier one that over time became the familiar appendage we now see on dogs, cats, cows and many other animals. As dogs show, tails are useful for visual communication, slapping away flying insects and other functions.
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Adult apes, including human ancestors, took the tail loss process a step further, Sallan said, "losing the remaining bony tail for better upright movement. Like fish, the remnants of an embryonic bony tail are buried in our lower backs-the coccyx or tailbone-stunted by a loss of molecular signals that would otherwise cause it to grow out like an arm or leg. Thus, humans and fish embryos share mechanisms for controlling tail form."
The fossil record for early apes is not great, but since apes lack tails, she thinks our primate ancestors lost them when they first started to walk on two legs. Monkeys that often walk this way have stunted tails, further proving that tails can get in the way of moving around while upright.
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