Mr. Xia brought the finding to his supervisors, Itai Yanai and Jef Boeke, to see what they thought. “I nearly fell off my chair, because it is just a stunning result,” Dr. Yanai recalled.
To test the idea that the mutation was involved in the disappearance of our tail, Mr. Xia and his colleagues genetically engineered mice with the TBXT mutation that is carried by humans. When these embryos developed, many of the animals failed to develop a tail. Others only grew a short one.
Mr. Xia and his colleagues propose that this mutation randomly struck an ape some 20 million years ago, causing it to grow just a stump of a tail, or none at all. Yet the tail-less animal survived and even thrived, passing on the mutation to its offspring. Eventually, the mutant form of TBXT became the norm in living apes and humans.
The scientists said that the TBXT mutation is not the sole reason that we grow a coccyx instead of a tail. While the mice in their experiments produced a range of altered tails, our coccyx is almost always identical from person to person. There must be other genes that mutated later, helping to produce a uniform anatomy.
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.
And losing a tail could have brought other dangers, too. Mr. Xia and his colleagues found that the TBXT mutation doesn’t just shorten tails but also sometimes causes spinal cord defects. And yet, somehow, losing a tail proved a major evolutionary advantage.
“It’s very confusing why they lost their tail,” said Gabrielle Russo, an evolutionary morphologist at Stony Brook University in New York who was not involved in the study. “That’s the next outstanding question: What on earth would the advantage be?”