Happy Nice Mall

Mr. Xia brought the find to his superiors Itai Yanai and Jef Boeke to see what they thought. Dr. “I almost fell out of my chair because it was just a stunning result,” Yanai recalled.

Mr. Xia and colleagues genetically modified mice with the TBXT mutation, carried by humans, to test the idea that the mutation plays a role in the loss of our tail. When these embryos developed, most animals failed to develop tails. Others only grew a short one.

Mr. Xia and his colleagues suggest that this mutation randomly struck an ape about 20 million years ago, causing it to grow only one tail or no tail. Yet the tailless animal survived and even thrived, passing 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 wasn’t the only reason we grew the coccyx instead of the tail. While mice produced a number of altered tails in their experiments, our tailbone is almost always the same from person to person. There must be other genes that mutate later and help create a uniform anatomy.

Even as geneticists are starting to explain how our tails disappeared, why is still the question that baffles scientists.

The first monkeys were larger than monkeys, and their increased size could have made it easier for them to fall from branches, and these falls were more likely to be fatal. It’s hard to explain why apes, who don’t have tails to help them balance, didn’t experience a significant evolutionary disadvantage.

And losing a tail could bring other dangers. Mr. Xia and colleagues found that the TBXT mutation not only shortens the tails, but also sometimes causes spinal cord defects. Yet somehow, losing a tail provided a huge evolutionary advantage.

“It’s very confusing why they lost their tails,” 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 would be the advantage?”