Mr. Shea brought the result to his supervisors, Itai Yanai and Jeff Boeke, for their opinion. “I almost fell off a chair, because it’s an amazing result,” Dr. Yanai recalls.
To test the idea that the mutation was involved in the disappearance of our tail, Mr. Shea and colleagues genetically engineered mice with the TBXT mutation carried by humans. When these embryos developed, many animals failed to develop a tail. Others only grew for a short time.
Mr. Shea and his colleagues suggest that this mutation randomly infected a monkey about 20 million years ago, causing it to grow only a stem of the tail, or not at all. However, the tail-less animal survived and even thrived, passing the mutation on to its offspring. Eventually, the mutant form of TBXT became the norm in living apes and humans.
The scientists said the TBXT mutation isn’t the only reason the coccyx has grown in place of the tail. While the rats in their experiments produced a set of variable tails, our coccyx is almost always identical from person to person. There must have been other genes that were later mutated, helping to produce a uniform anatomy.
Even if geneticists begin to explain how our tails disappear, the question of why our tails disappear continues to baffle scientists.
The first apes were larger than the apes, and their increased size would have made it easier for the branches to fall off, and such falls were likely fatal. It’s hard to explain why monkeys without tails to help them balance would not suffer much evolutionary damage.
Losing the tail can lead to other risks as well. Mr. Shea and colleagues found that the TBXT mutation not only shortened the tail but sometimes also caused defects in the spinal cord. However, in a way, the loss of the tail proved a major evolutionary advantage.
“It’s very puzzling 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. “This is the next outstanding question: What would the advantage be on Earth?”