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Mr. Xia took the discovery to his supervisors, Itai Yanai and Jef Boeke, to see what they thought. “I almost fell off my chair because it’s just a mind-blowing result,” recalls Dr. Yanai.
To test the idea that the mutation was involved in the disappearance of our tail, Xia and his colleagues genetically engineered mice with the TBXT mutation that is carried by humans. When these embryos developed, many animals failed to develop a tail. Others only made a small one.
Mr Xia and his colleagues suggest that this mutation struck a random monkey around 20 million years ago, causing it to grow just a tail stump, or none at all. Yet the tailless 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.
Scientists have said that the TBXT mutation is not the only reason we grow a tailbone instead of a tail. While the mice in their experiments produced a range of altered tails, our tailbone is almost always the same from person to person. There must be other genes that mutated later, helping to produce a uniform anatomy.
Even though geneticists are starting to explain how our tails disappeared, the question of why still puzzles scientists.
The first monkeys were larger than monkeys, and their increased size would have made it easier for them to fall from branches, and more likely to be fatal. It is difficult to explain why the monkeys without a tail to help them balance would not have suffered a significant evolutionary disadvantage.
And losing a tail could have entailed other dangers. Mr Xia and his colleagues found that the TBXT mutation not only shortens the tails, but also sometimes causes abnormalities in the spinal cord. And yet, one way or another, losing a tail has proven to be a major evolutionary benefit.
“It’s very confusing why they lost their tails,” said Gabrielle Russo, an evolutionary morphologist at Stony Brook University in New York City who was not involved in the study. “That’s the next open question: what would be the benefit? “
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