Tricks of Evolution: Chasing after vestigial organs - How humans lost their tails

But then, roughly 25 million years ago, the tails disappeared. Charles Darwin first recognised this profound change in our ancient anatomy. But how and why it happened has remained a mystery. Now a team of scientists in New York say they have pinpointed the genetic mutation that may have erased our tails. When the scientists made this genetic tweak in mice, the animals didn’t grow tails, according to a new study that was posted online last week. 

This dramatic anatomical change had a profound impact on our evolution. Our ancestors’ tail muscles evolved into a hammock-like mesh across the pelvis. When the ancestors of humans stood up and walked on two legs a few million years ago, that muscular hammock was ready to support the weight of upright organs. Although it’s impossible to definitively prove that this mutation lopped off our ancestors’ tails, “it’s as close to a smoking gun as one could hope for,” said Cedric Feschotte, a geneticist at Cornell University who was not involved in the study. 

Darwin shocked his Victorian audiences by claiming that we descended from primates with tails. He noted that while humans and apes lack a visible tail, they share a tiny set of vertebrae that extend beyond the pelvis — a structure known as the coccyx. “I cannot doubt that it is a rudimentary tail,” he wrote. 

Since then, paleoanthropologists have found fossils that shed some light on this transformation. The oldest known primates, dating back 66 million years, had full-fledged tails that they likely used to keep their balance in trees. Today most living primates, such as lemurs and almost all monkeys, still have tails. But when apes appeared in the fossil record, about 20 million years ago, they had no tail at all. “This question — where’s my tail? — has been in my head since I was a kid,” said Bo Xia, a graduate student in stem cell biology at NYU Grossman School of Medicine. A bad Uber ride in 2019, in which Xia injured his coccyx, brought it back to his mind with fresh urgency. “It took me a year to recover, and that really stimulated me to think about the tailbone,” he said. 

To understand how apes and humans lost their tail, Xia looked at how the tail forms in other animals. In the early stages of an embryo’s development, a set of master genes switch on, orchestrating different parts of the spine to develop distinctive identities, such as the neck and the lumbar region. At the far end of the embryo, a tail bud emerges, inside of which a special chain of vertebrae, muscles and nerves develop. Researchers have identified more than 30 genes involved in the development of tails in various species, from an iguana’s long whip to the stub on a Manx cat. All of these genes are active in other parts of the developing embryo as well. Scientists are still learning how their unique activity at the end of an embryo gives rise to a tail. 

Even if geneticists are beginning to explain how our tail disappeared, the question of why still baffles scientists. The first apes were bigger than monkeys, and their increased size would have made it easier for them to fall off branches, and more likely for those falls to be fatal. It’s hard to explain why apes without tails to help them balance wouldn’t have suffered a significant evolutionary disadvantage.